This chapter provides a description of the environmental consequences of the preferred alternative relative to the no action alternative on all elements of the affected environment as described in Chapter 3.0.
In general, the discussion for each of these elements of the affected environment includes an analysis of individual short-term and long-term affects for both the no-action and preferred alternative as described in Chapter 2.0. Additionally, planning for this project has identified a major mitigation feature located in the Scheuber Ditch SR-6 area (Figure 2.6). This feature is proposed to provide mitigation within the project area to address project impacts from several disciplines. Focusing on mitigation principally within the Scheuber ditch area is intended to consolidate multiple mitigation objectives to achieve added benefits through increased floodplain connectivity and improved interaction between wetland, fish and wildlife ecology. The following paragraphs give a brief overview of the components of this mitigation proposal.
Scheuber Ditch/SR6 Mitigation
The Scheuber floodplain actions would involve the creation of a pond/wetland complex on the west side of the Chehalis/Centralia reach of the mainstem Chehalis River, north and south of SR- 6. The purpose of the effort is to reconnect portions of the Chehalis River to the adjacent floodplain, providing functioning habitat for fish and wildlife as well as attenuation of flood flows. There are four elements of the project:
The oxbow immediately south of SR-6 would be reconnected to the Chehalis River by increasing to 400 feet the width of a low area between the oxbow and the river. This reconnection would provide surface flows to the oxbow at approximately the 1 or 2-year flood stage. Invasive plants would be removed and a 100-ft wide riparian zone would be planted around the oxbow. The area would be fenced off from livestock as needed. A shallow channel would also be excavated from the northern edge of the oxbow to SR-6. A 400-ft section of the State highway will be elevated onto a causeway with the shallow channel continuing beneath approximately 1000 feet to a new wetland area. The channel would carry flows at the 1 and 2-year flood stage. This would result in approximately 21 acres of new wetland habitat with 11 acres of riparian planting.
North of the highway, excavation and grading would allow distribution of flows through a series of interconnected ponds and wetlands. The existing topography and hydric soil distributions indicates that the wetlands are best located at the base of the slope on the west side of the floodplain near at the confluence of Bunker Creek and Scheuber ditch. The pond/wetland area would cover approximately 80 acres. The area would be configured as a high-interspersion habitat to increase productivity, refuge function, and hyporheic recharge. An existing unnamed creek would flow into the pond/wetland area and provide positive flow back through the shallow channel into the oxbow when Chehalis River flows are below approximately the 5-year flood stage. This would maintain water quality and increase function for salmonid rearing. Invasive plants would be removed from the area and shrubs and small trees such as alders, willows and dogwoods would be planted along the borders of the ponds and on the small islands within the complex to offer shading of aquatic habitat and foraging opportunities for terrestrial wildlife. An understory of shrubs and small trees as well as native grasses and forbs would be planted on the outer margins of the complex. LWD would be placed in the new channels and wetlands.
The connection between the pond/wetland complex and Scheuber ditch will include a sill so that flows below the 5-year flood stage will not flow north to the downstream end of the ditch, but rather would recede back through the pond/wetland complex. From the north end of the pond/wetland complex, a 400-ft wide riparian area (200 feet on each side) will be planted along both banks of Scheuber ditch to its downstream end, providing approximately 75 acres of riparian habitat. The ditch would be reconfigured to add meanders. This would provide primarily an aesthetic benefit, giving the ditch a more natural appearance.
A second wetland complex would be created at the north (downstream) end of Scheuber ditch.
This area would be excavated and graded with a configuration similar to the complex at the south end and designed to provide many of the same functions. The complex would receive inflow from Coal Creek and Scheuber ditch from the south. These would provide flow through the wetland into the Chehalis River. The wetland would also receive backwater from the Chehalis River at annual high flows, which would provide refuge from high velocities for coho, chum and chinook salmon. Vegetation planting and LWD placement would occur as described above for the south wetland complex. The north wetland complex would encompass approximately 46 acres.
The mitigation project will provide wildlife and fisheries habitat, floodplain function and vegetative diversity in the mid-Chehalis basin. The project will also provide hydraulic function.
Discussion of the relevance of each discipline to the conceptual mitigation project described above is included in the pertinent sections of this chapter.
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Under the No Action alternative, levees and other features of the preferred alternative would not be constructed. No structural changes would be made to the Skookumchuck Dam. Reservoir operations would not be modified for flood control, but would continue to be operated on a filland- spill basis with provision for a minimum 95 cfs outflow throughout the year. Minor and major flooding would continue to occur on the Chehalis River and tributaries, and the project area would continue to experience flood-related damages currently estimated at $9.5 million annually. Although no major projects with significant effects on hydrology and hydraulics have been approved in the area, other agencies or jurisdictions would continue to undertake flood hazard reduction measures. These efforts could include measures such as additional floodproofing or relocation of structures. Improvements to Interstate 5 to accommodate future traffic demands would require raising portions of the roadway in the vicinity of Centralia- Chehalis or other measures to provide flood protection. Widening and elevating the roadway would entail additional fill that would incrementally add to the loss of floodplain storage in the area. Although WSDOT has not completed detailed hydraulic studies, initial analysis indicated that this loss would have minor effects on 50-year and 100-year flood elevations (FHWA and WSDOT 1998). Widening of the freeway, construction of interchanges, and other improvements would increase the area of impervious surface and increase highway runoff. It is likely that bridges and culverts along I-5 would need to be modified to maintain adequate flow capacities.
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A significant effect on hydrology and hydraulics would occur if project construction and/or operation would:
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4.1.3.1 Short Term Effects
In locations where roadways would be modified as part of levee construction, flows in culverts or under bridges would be redirected for short periods during construction. Removal of vegetation, soil disturbance, and construction of temporary haul roads would temporarily increase runoff from construction areas. During levee construction, stormwater runoff would be routed to catch basins or other detention structures and treated in accordance with applicable criteria contained in the Stormwater Management Manual for Western Washington (Washington Department of Ecology 2001). With proper runoff controls in place, substantial increases in stormwater flows to local drainages are not expected.
Structural modifications to Skookumchuck Dam would be constructed during the late summer when the pool elevation in the reservoir is low. Since the modifications likely could not be completed in a single season, construction would be scheduled over two summers. During the interim fall and winter seasons, the reservoir would be allowed to fill and flows would be passed over the spillway as they are under existing operations. Since changes in the reservoir outflow regime would not occur until after construction of the dam modifications is complete, no shortterm effects on hydrology and hydraulics are expected.
4.1.3.2 Long Term Effects
The preferred alternative would alter flood hydrology and hydraulics in the project area. The levee system would result in some reduction of the floodplain area that is inundated and alter floodwater storage. However, because the levees would be set back a significant distance from the existing streambanks, there would be relatively little effect on the active floodplain. As a result, the active floodplain would generally function in a manner similar to existing conditions.
The areal extent of flooding would be only slightly modified during small floods. Notable changes in the areal extent of flooding would occur only during floods larger than the 2-year event. The areas that would be protected from flooding during these events are mostly urban areas east of I-5. The protected areas are, in general, not within the active floodway, but rather are backwater or temporary storage areas where short-duration flooding occurs with little flow velocity.
There would be a change in peak flood stage at several locations in the project area. There would be a decrease in the 100-year peak flood stage below RM 70.74 on the Chehalis River. The peak flood stage would increase between RM 70.74 and RM 78; the maximum increase would be 0.61 feet at RM 72.8. The 100-year peak flood stage would decrease by 0.18 feet at the Galvin Road Bridge, and by 0.24 feet at Grand Mound. Between Grand Mound and Porter, the peak flood stage decrease would vary between 0.07 feet and 0.49 feet.
Along the Skookumchuck River, the 100-year peak flood stage would decrease from 0.47 feet to 4.25 feet from RM 10 to the mouth. There would also be significant flood damage reduction to communities upstream; for example peak flood stages would be reduced by 3.22 feet at Bucoda.
During major floods, control boxes would isolate the flow of Dillenbaugh Creek and keep it from entering the Chehalis River to prevent backwater from flooding I-5 and Chehalis. Construction of the levee system may involve relocation of a channelized portion of Dillenbaugh Creek in the area between the Rice Road interchange and the BNRR tracks. Other reaches to the south may also need to be relocated. Detailed design of the channel relocation will be completed in future design phases.
The areas that would be protected from flooding do not represent significant sources of groundwater recharge to the Chehalis basin. The vast majority of groundwater recharge in the basin occurs as a result of the infiltration of direct precipitation. The infiltration of floodwaters in floodplain areas represents only a minor contribution to groundwater recharge. This occurs because of the limited areal extent of the floodplain, the relatively short duration of flooding, and the predominantly fine-grained soils in the area. The areas that would be protected from flooding are mostly developed and have a high percentage of impervious surfaces (e.g., roads, parking lots, roofs) that further limits recharge of groundwater resources.
Recharge in floodplain areas often constitutes bank storage that provides baseflow to the stream immediately following a flood, but provides little baseflow thereafter. Since flooding in the Chehalis River basin generally occurs from late October through March, baseflow contributions from bank storage to the river would be negligible during the low flow periods that typically occur in later July through early October. Overall, the levee system is predicted to have an undetectable and insignificant effect on groundwater recharge and baseflow contribution to the Chehalis River.
Modification of the Skookumchuck Dam would provide up to 20,000 acre-feet of flood storage and alter the timing and duration of flood flows from the reservoir. During non-flood periods, flows from the dam would be similar to those under the existing operation. Although a rule curve for managing high flow events has not been finalized, it is assumed that reservoir operations would be modified to allow drawdown of the pool to elevation 455 feet in late fall. Any inflows less than 3,000 cfs would be allowed to discharge directly though the new spillway outlet gates.
When inflows exceed 3,000 cfs, the flood control pool would be used to store most of the flood flow, except for a constant minimum outflow of 95 cfs. Because of the contributions of tributary streams below the dam, flows in the Skookumchuck River downstream from the dam would still be well above 95 cfs during floods. During a flood, outflows from the reservoir would be reduced to prevent Chehalis River flows at Pearl Street from exceeding 5,000 cfs. After the event passes, water stored in the reservoir would continue to be released at volumes high enough to reach but not exceed 5,000 cfs at Pearl Street. Discharge of flood flows from the dam would likely be limited to 3,000 cfs, which is essentially bankfull flow in the upper reaches of the Skookumchuck below the dam. As a result of the modified operations, flood peaks would be reduced, the timing of peak flows would be delayed, and the duration of bankfull flows would be extended. Depending on the event, bankfull discharges could last from a few hours for smaller floods up to approximately 5 days for extreme floods.
The reduction in overbank flooding on the Skookumchuck is not expected to significantly affect groundwater recharge in the Skookumchuck subbasin. The large volume of tributary input, frequency of local flooding, and location of adjacent wetlands indicate that the Skookumchuck subbasin approaches or exceeds groundwater saturation without the addition of water from winter and early spring floods.
Changes in flow velocities within the Chehalis and Skookumchuck Rivers are expected to have a negligible effect on bank erosion and would not increase damage to structures or risk to life.
Hydraulic modeling of the preferred alternative indicates that there would be little effect on flood stages downstream from Centralia. Because the levee system would cause relatively little change in the active floodplain, flood flows would be conveyed downstream in a manner similar to existing conditions. Improved flood storage at the Skookumchuck reservoir would result in flood stage reductions up to several tenths of a foot in areas downstream from the confluence with the Skookumchuck River. There would be negligible effect on the timing and duration of flooding downstream of Centralia.
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In summary, over the long term, implementation of the preferred alternative would change the hydrology and hydraulics of flood flows along the Chehalis and Skookumchuck rivers and tributaries but not substantially. The levee system would reduce the floodplain area that is inundated and alter floodwater storage characteristics, but these changes would be minimized by the levee setbacks and therefore no significant impacts. Significant reductions in groundwater recharge and baseflow (flow velocity within, downstream, or upstream of the project) support are not expected. There would be a slight reduction (no substantial increase) in flood stages downstream of Centralia.
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Mitigation actions in the SR 6/Scheuber floodplain area would reconnect portions of the Chehalis River to its floodplain. This would increase the frequency with which flood flows enter that area and allow distribution of flows through a series of interconnected ponds and wetlands.
These actions would be expected to enhance local groundwater recharge associated with minor (1 to 2-year) floods and provide some attenuation of flood flows from larger events.
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The alternatives considered include project components that could directly modify geomorphic features and alter geomorphic processes that form and maintain the river channel and its floodplain. Geomorphic features and processes documented within the project area are described in detail within Section 3.2. The assessment of potential environmental effects on river geomorphology describes the potential changes in geomorphic features and processes related to specific project elements for the No-action Alternative and the Preferred Alternative.
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Under the No Action Alternative, geomorphic processes would continue to operate under the current flow and sediment regime. Recent trends (last 50 years) indicate that channel change proceeds within the project area at a slow rate primarily through localized bank erosion, channel migration, and sediment accumulation within the channel and on the floodplain. The project area will continue to be a zone of sediment accumulation in which the rates of channel adjustment vary in response to temporal changes in sediment delivery.
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A significant effect on river geomorphology would occur if construction and/or operation activities would:
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The combined setback levee and Skookumchuck Dam modification alternative will alter flood hydrology and hydraulics within the project area. Since hydraulics drive the sediment transport and erosion processes that modify channel form, the assessment of environmental effects on river geomorphology evaluates the magnitude and location of changes to sediment transport capacity within the project area. Specific potential changes considered within the geomorphic analysis include the following:
4.2.1.1 Short Term Effects
These potential effects were analyzed by evaluating changes in shear stress and velocity values at cross sections within the Skookumchuck River and the Chehalis River. The details of the dam operation plan are not yet completed as part of the conceptual design. For the purpose of modeling flood hydraulics and dam operation, simplified hydrographs were developed to maximize the flood control benefits afforded by the reservoir by providing maximum flood storage and evacuating the reservoir as quickly as possible after the flood event. These dam management release hydrographs will need to be refined as the actual dam operation plan is developed. The evaluation of potential geomorphic effects is based on the flow hydraulics predicted by the current flood hydraulic model including the simplified dam management release hydrographs.
4.21.2 Long Term Effects
The evaluation of potential geomorphic changes showed that the anticipated hydrologic and hydraulic changes would have no effect on sediment transport capacity and potential for bank erosion within the Chehalis River except in a short (500 ft) reach of the river immediately upstream of the airport. Within this river reach floodwaters will be backwatered during extreme flood events reducing transport capacity locally and potentially encouraging additional sediment deposition within the channel in this area.
Within the Skookumchuck River, the potential for geomorphic change is greater than it is within the Chehalis River, but actual changes will depend strongly on the final dam operation plan for regulating flow releases. The simplified dam release hydrographs eliminated storm peaks, ramped from 95 cfs up to 3000 cfs very quickly, and maintained continuous, steady release rates between 2,000 and 3,000 cfs for approximately 5 days. The potential geomorphic effects of this hydrograph diminish progressively downstream because the flood wave attenuates as it propagates down the river. The steady release rate of 3,000 cfs is high enough to transport sediment and modify channel form. Since this elevated flow extends the duration of erosion and channel adjustment in comparison to the pre-modification hydrograph, there will be some impact on channel morphology. The specific locations and cumulative magnitude of potential channel changes have not been mapped out as part of the current analysis. This additional analysis should be performed in support of developing a refined plan for dam operation.
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In summary there would be no net loss in flushing flows or an increase in flood duration.
Erosion or degraded channel processes will not occur based on the ramping up of the flow as the requirement for fisheries habitat will dictate ramping procedures as discussed in Appendix B.
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The current analysis determined that significant effects on channel morphology are possible if dam operation maximizes flood mitigation benefits without accounting for potential geomorphic impacts. Potential geomorphic effects can be reduced at the expense of diminished flood control.
Mitigation needs will depend on the specific impacts associated with the refined operational guideline for Skookumchuck Dam and hydrologic reporting both anticipated to be completed in Planning, Engineering and Design phase of the proposed project. The relations between flow releases and geomorphic processes conceptually described above will guide the development of the operational guidelines to optimize the balance of flood management benefits and geomorphic impacts associated with dam operation.
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Under this alternative, no action would be taken by the Corps to change the current status of water quality in the project area. Therefore, water quality within the project area would not be affected by construction activities and would be expected to remain similar to existing conditions.
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A significant effect on water quality would occur if construction and/or operation activities would:
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4.3.3.1 Short Term Effects
Levee construction would involve disturbance of soil that could cause changes in water quality if sediments are introduced into streams. This could result in increases in turbidity, suspended solids, and biostimulatory nutrients within and downstream of the project area. Implementing pollution control plans throughout construction can minimize these potential impacts. For example, trench soils will be placed well above the streambank and protected with silt fences, hay bales, or other facilities that would reduce sediment runoff into the stream. Please see section 4.13.5 for measures that would be taken to minimize hazardous material spills.
4.3.3.2 Long Term Effects
Potential long-term consequences of the preferred alternative include increased levels of fine sediment and contaminants in stream corridor from nonpoint source runoff. However, long-term effects of the levee would also be minimized through the levee design; setting levees back away from river as far as possible, or a minimum 50-foot buffer zone from water edge to levee toe when setback is not possible will reduce levee-related adverse effects to non-significant levels.
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The construction and excavation of Skookumchuck Dam spillway sluices, intake structures, and other activities associated with this feature would be performed during summer months when water levels are low and water quality effects from short-term construction activities would, therefore, be minimized. Modifications to the Skookumchuck Dam have been designed to improve storage efficiency and should not result in changes to the seasonal water levels in the Skookumchuck River during the fall and winter months when flood storage would be required.
Since dam construction, sediments from the upper basin have likely been trapped by the reservoir. This sediment accumulation may provide a benefit by reducing the turbidity downstream during fill and spill operations. However, data are not available to further interpret the sediment supply and transport characteristics of the Skookumchuck River below the dam.
Potential long-term effects such as increased moderate flows may result in greater scour of substrate and increased sedimentation, however, whether sediment movement impacts will continue in the long term or become significant is difficult to predict. The geomorphic analysis of the proposed modifications to the Skookumchuck Dam suggests that the modifications would have a negligible effect on sediment transport capacity and potential for bank erosion (Section 4.2).
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In review of the information available, it appears that soil disturbance resulting from construction activities would be a temporary unavoidable impact. Construction-related sediment disturbances and potential effects from turbidity, suspended solids, hazardous construction materials would be minimized through the implementation of erosion and pollution control plans (refer to section 4.13). Long-term effects of operation and maintenance of the preferred alternative include increased runoff and increased sedimentation, which would be considered an unavoidable adverse effect. However, proposed mitigation and restoration activities have the potential to improve long-term water quality, and therefore minimize adverse water quality effects. There would be no contamination to the public water supply nor would contaminants be discharged into the river.
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Construction related disturbances and potential effects would be avoided or minimized through the levee design and implementation of pollution control plans. Design objectives include:
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4.4.1.1 Vegetation Impacts.
Short Term Effects
Existing practices for land use and development (including forestry and agriculture) will continue to be in place that can result in the modification, loss and/or adverse impacts to existing vegetation communities. These are not expected to change as a result of the No-Action alternative.
Long Term Effects
It is likely that changes to existing vegetation communities will continue to occur as the study area develops and population increases. Forestry and agricultural practices are likely to continue into the foreseeable future, although it is possible that more forest and agricultural land will be converted to residential and commercial uses.
4.4.1.2 Wetland and Riparian Area Impacts
Short Term Effects
The existing permitting process for wetland filling and alteration will continue to be in place as well as existing unregulated activities that adversely impact wetlands. These are not expected to change as a result of the No-Action alternative. There is very little regulatory authority covering riparian areas that are not wetlands, so existing activities that adversely impact riparian areas are expect to continue.
Long Term Effects
While no major projects with significant impacts have been approved in the project area, flood damage reductions measures would continue to be explored. The WSDOT is still likely to pursue improvements to I-5 to accommodate traffic demands, which may include flood-proofing measures such as raising the freeway or constructing levees. Given the extent of wetlands within the project area, loss and/or degradation of wetland resources would be expected with these
projects. Riparian areas are much more limited within the project area. Future impacts would need to be assessed on a case-by-case basis, but the Corps expects any major flood damage reduction measures would include impacts to riparian areas.
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The following criteria are used to determine if the preferred alternative has the potential to result in significant impacts to vegetation, wetlands and riparian areas. A significant effect on vegetation, wetlands and/or riparian areas would occur if construction and/or operation activities would cause:
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4.4.3.1 Vegetation Impacts.
The assessment of vegetation was based on the review of existing resources on the vegetation characteristics of the study area (Franklin and Dyrness 1973) in addition the Washington State Department of Natural Resources databases on native plant communities. Other sources include the Lewis County and Thurston County soil surveys (UDSA 1991 and 1987, respectively) and site inspections of the study area.
Short Term Effects
Construction of the levees would result in the clearing and grading of approximately 45 acres of terrestrial (excluding wetland and riparian habitats, in Section 4.4.2) habitat. The majority of this area currently consists of farmland that is either in pasture grass production or corn. Other areas in the footprint of the preferred alternative include freeway right-of-way, Lewis County roads and road right-of-way, and Lewis County parklands. Very little native terrestrial vegetation, if any, is located within the preferred alternative footprint. Short-term effects would include disturbance and loss of existing vegetation.
Long Term Effects
The terrestrial vegetation communities that would be affected by the preferred alternative are widespread throughout the study area and the preferred alternative would have negligible impact on vegetation biodiversity and abundance. Most of the plants within the communities are either planted pasture grasses or crops and/or invasive non-native species such as Scot's broom and blackberries. The levees will be seeded with similar grasses to those currently in the project foot print and kept maintained (mowed and clear of large woody vegetation) for structural integrity.
Long-term effects to terrestrial vegetation would be minimal.
4.4.3.2 Wetland and Riparian Area Impacts The inventories for wetlands and riparian areas were prepared by the U.S. Fish and Wildlife Service and were based on 1998 color-infrared aerial photography. This level of effort was adequate to determine the extent of resources within the project area with the exception of determining the extent of emergent and/or farmed wetlands. This is due to the fact that it is difficult to determine the extent of emergent and farmed wetlands with the late-season photography typically used for inventory level wetland determinations; by the time the aerial photographs are taken all evidence of hydrology has usually disappeared.4
To account for some of the limitations of the wetland inventory maps in regards to seasonal and/or farmed wetlands the extent of mapped hydric soils was evaluated to determine the potential extent of wetlands with in the study area. This information was available from the soil surveys from Lewis and Thurston Counties as well as web-based data from the USDA web page (USDA 2002). A hydric soil is a soil that is saturated, ponded, or flooded long enough in the growing season to support anaerobic conditions in the upper part (NTCHS 2001). All wetlands have hydric soils. However, not all hydric soils support wetlands due to the fact that many hydric soils have been drained and/or converted for agriculture and no longer exhibit hydrologic characteristics typical of wetlands.
Between these two mapping and inventory conventions, forested wetlands, riparian areas, and scrub-shrub wetlands were reasonably well mapped and represent a reasonable estimate of extent. This is because the vegetation communities in these systems are persistent and distinct and therefore easy to interpret from aerial photography, regardless of the season. The emergent and/or farmed wetlands are likely underrepresented on the wetland maps and likely over represented on the hydric soils maps. As such, the wetland impact assessment for emergent wetlands is based on the soils maps because these maps represent the maximum possible extent of direct wetland impacts from the preferred alternative. The Corps also assumes that this maximum estimate can be reduced with further design modifications. Once a final design footprint is established, the Corps will complete a more precise wetland and riparian area delineation of the project area based on field data.
The preferred alternative would result in the direct loss of wetlands and riparian areas within the footprint of the levees. Table 4.4-1 provides a summary of the total of wetland and riparian impacts from the levee footprint by specific areas. The only wetland type within the project footprint is farmed wetlands and/or emergent wetlands. As such, the wetland impact acreage is based on extent of mapped hydric soils.
Table 4.4-1: Project Features, Linear Feet, Total Acreage of Project Footprint, and Total Impact Areas for Riparian Areas and Wetlands.
Total wetland loss is estimated to be 34 acres of wetlands over approximately 15 miles of levees and floodwalls. Approximately 14 miles of the preferred alternative consists of levees and 1 mile of floodwall.
Short term Effects
The levee alignment is in segments and located in areas that can be grouped based on specific locations and site characteristics within project area. The following discussion focuses on 5 areas of levees and the wetland and riparian impacts associated with them.
Chehalis reaches 1 and 2 are the northern most end of the preferred alternative set back well away from the river and follow high ground. There are no wetlands or riparian area within the impact footprint.
Chehalis reach 3 is immediately adjacent to the western edge of I-5 and to the east of the Chehalis River. The wetlands associated with this reach are emergent and/or farmed wetlands (pasture or grain crops). Functions associated with these wetlands include migratory waterfowl habitat, groundwater recharge and discharge, floodwater storage and low flow augmentation.
Any wetlands and riparian areas directly adjacent to the river may also provide a source of organic detritus (food chain support) to the river. Reach 3 provides abundant small mammal habitat and therefore support a large raptor population. These functions would be lost or adversely impacted due to project construction. Wetland impact acreage for this reach is 5 acres of farmed and/or emergent wetlands. No riparian habitat was mapped or noted during field inspections for this reach.
Total impact area for reaches 1, 2 and 3 is 17 acres of area (including 5 acres of wetland), 21,239 linear feet of levees and 1,400 linear feet of floodwalls.
Salzer Creek reaches of the preferred alternative are on the east side of the Chehalis River and I- 5, south of the City of Centralia and north of the City of Chehalis. Most of alignment surrounds areas of existing development, which resulted in crossing pastures and open fields. Most of the pastures and field have been mapped as either wetlands or as having hydric soils or both.
Functions associated with wetlands include small mammal habitat, sediment and nutrient trapping, flood water storage and low flow augmentation. These functions would be lost or adversely impacted under the preferred alternative. Total impact acreage is estimated to be 17.5 acres of wetlands. No riparian areas were mapped or noted during field inspections within the footprint of the preferred alternative.
Total impact area for these reaches is 17.6 acres of area (of which 17.5 are wetlands), 18,811 linear feet of levees and 1,034 linear feet of floodwalls.
The Dillenbaugh Reach sections of the preferred alternative are immediately adjacent to the west side of Interstate-5, south of Reaches 1, 2, and 3, east of the Chehalis River. The alignment is located within emergent and farmed wetlands. Functions associated with these wetlands include floodwater storage, sediment and nutrient trapping, low flow augmentation, small mammal habitat, and raptor habitat. These functions would be lost or adversely impacted to project construction. Total impact is estimated to be 11.5 acres of wetlands. No riparian areas were mapped or noted during field inspections for these sections of levees.
Total impact area for these reaches is 15 acres of area (of which 11.5 are wetlands), 17,129 linear feet of levees and 3,455 linear feet of floodwalls.
There are several smaller sections of levees that provide flood protection around the Skookumchuck River within the City of Centralia. In the Skookumchuck Reaches, levees are set back away from the river except for a narrow area where the flood plain in constricted upstream from the confluence with the Chehalis. The levees would transverse two riparian areas that are located adjacent to I-5 (Reaches 14 and 16). Although the inventory information indicates a loss of riparian areas at these Reaches, the impacts may be avoided through further refinement of project design. Functions associated with riparian areas include passerine bird habitat, source area of large woody debris to the Chehalis and Skookumchuck Rivers, shading for cooler water temperatures, aesthetics (adjacent to residential lakes), and passive recreation (bird watching).
These functions may be lost or adversely impacted due to project construction. Maximum total impacts would be the loss of 0.8 acres of riparian habitat.
Total impact area for these reaches is 12.6 acres of area (including 0.8 acres of riparian habitat), 15,534 linear feet of levees and 3,050 linear feet of floodwalls.
Long-Term Effects
The long term impacts associated with the potential loss of wetlands within the project area could include loss of animal and plant biodiversity and abundance (due to loss of habitat), decreased ability for flood water storage and low flow augmentation within the floodplain, and decreased connectivity between habitats. The levees may also result in additional ponding of water behind levees and/or a disruption of lateral surface and groundwater flow, which would change the hydrologic characteristics of existing wetlands. In the case of ponding, some areas of existing upland may develop into wetlands or existing areas of wetlands may be expanded. If there is substantial disruption of lateral surface and ground water flows that cutoff or diminish sources of water, then existing wetlands may be lost or adversely affected.
The maximum total wetland loss from the preferred alternative is approximately 3% of the estimated total wetlands within the study area (12,335 acres of mapped wetlands). Localized impacts could have synergistic and/or cumulative impacts to the entire area.
Indirect Effects
In addition to the possible short and long-term impacts of the preferred alternative within the study area, additional indirect impacts may also be associated with this alternative. The preferred alternative is located within the active floodplain of the Chehalis River. Floodplains are complex systems of ground and surface water flows that largely drive the characteristics of both wetlands and riparian areas. Changes in the floodplain that can change surface and groundwater interaction will likely have some affect on the existing wetland and riparian areas.
The levee and floodwall feature can trap water, which may expand the boundaries of existing wetlands and possibly create or restore wetlands in areas that do not currently exhibit wetland hydrology. Conversely the levee and floodwall alignment may disrupt some of the surface and ground water flow patterns, which may result in less water for existing wetlands. However, the complexity of the sources of water for the Chehalis floodplain wetlands and riparian areas (flooding, groundwater saturation, surface water ponding, and high precipitation) makes cause and effects relationships difficult to predict. The proposed location of the preferred alternative alignment, which is mostly setback from the river or located in areas that are already highly disturbed (along the freeway right-of-way), minimizes the potential for indirect impacts.
However, their maybe some development in areas that are better protected with the levee's in place unless local laws prevent development from occurring.
4.4.3.3 Skookumchuck Dam Modifications
Short-Term Effects
The Skookumchuck Dam modifications would not result in direct loss of wetlands and riparian areas, but may alter respective characteristics based on the operational plan for releases from the reservoir. No short-term impacts are associated with construction of the dam modifications.
Long-Term Effects
There are a limited number of wetlands and riparian areas along the Skookumchuck River.
There are no wetlands or regularly flooded riparian areas surrounding the reservoir. The existing wetlands and riparian areas are linear features likely influenced by annual high water periods.
The preferred alternative would change the duration and frequency of floods, especially between the 5- and 10-year floods; these floods would occur less frequently7. For example, the current 10-year event, after project construction, would have the characteristics of the current 50-year event.
The change in these flood events are not likely have any noticeable effect on wetlands within the preferred alterative footprint because these systems are much more influenced by frequency of events rather than by magnitude. Riparian areas, however, rely on higher magnitude events for recruitment of seed sources, scouring of weedy vegetation, and input of sediment and organic materials. The degree to which riparian areas on the Skookumchuck River rely on these events is currently unknown, but the preferred alternative identifies riparian impacts on the Skookumchuck as the subject of study during the design process. Should impacts to riparian areas be indicated as a result of these studies, operational procedures at the dam will be evaluated to offset potential impacts. Mitigation would also be included as part of the preferred alternative should any unavoidable impacts be identified.
Indirect Effects
The Skookumchuck is an incised river within a terrace. The riparian areas and wetlands that are directly associated with the Skookumchuck River would be directly impacted, rather that indirectly from implementation of the preferred alternative. There are other wetlands within the study area surrounding the Skookumchuck, but their position in the landscape (in depressions or low-lying areas well away from the river floodplain) indicated that they are more influenced by ground and surface water rather than flood events. The preferred alternative would likely have minimal impact on these systems.
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In review of the information available and provided for above, the proposed project is determined not to significantly impact vegetation. Direct short- and long-term effects to wetlands caused by the preferred alternative could be significant due to the 34-acre wetland 7 Yearly floods and 2-years floods will occur with the same frequency and duration as the current condition under the preferred alternative.
impact; however, measures would be taken in design to further avoid and/or minimize those impacts. Proposed mitigation would offset the possibility of significant impacts to wetlands by restoring some of the historic function to the Chehalis River. No significant impacts are expected to occur to riparian areas. Measures would be taken in design to further avoid and/or minimize impacts to the 0.8-acre area that is currently within the preferred alternative footprint.
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Throughout the development of the preferred alternative, impacts to sensitive areas was included as a component of the alignment. Care was taken to stay close to developed areas, keep the alignment setback as far possible from the Chehalis River, tributary streams, wetlands, and riparian areas. The design also incorporated areas of existing levees or tied into an existing levee system. Lastly, floodwalls were incorporated into the design where levees would have encroached upon the river.
The Corps will continue to evaluate measures during the design process methods to avoid direct impacts to vegetation, wetlands, and riparian areas. These measures may include:
Measures that would avoid and or reduce potential indirect impacts include:
4.4.6.1 Levee and Floodwall Alignment
Under the preferred alternative, unavoidable impacts include disturbance to approximately 45 acres of existing upland vegetation (although this consists of disturbed communities and/or farmed areas), 34 acres of wetland impacts (based on the extent of hydric soils) and .8 acres of riparian area impacts. These numbers may be reduced through design, but the following mitigation scenario is intended to offset impacts to the full impact area. Functions associated with theses impacts include migratory and passerine bird habitat, small mammal habitat, floodwater retention and detention, low flow augmentation, habitat biodiversity, groundwater recharge, nutrient and sediment removal, nutrient and organic input to other aquatic ecosystem (streams, rivers), passive recreation and aesthetics.
Mitigation for unavoidable impacts to vegetation, wetlands and riparian areas from the preferred alternative will be incorporated into the Scheuber Floodplain plan as describe in Section 4 (Introduction). This includes:
The proposed mitigation area contains farmed areas that are both upland and wetland. Due to the difficulties in inventorying farmed wetlands, on-site delineation of the mitigation would occur during design to determine the extent of restored wetland versus created wetlands. On-site wetland and riparian delineations will also be done for the preferred alternative footprint to determine the exact extent of impact area. If more mitigation acreage is needed after this evaluation, this proposed area of mitigation will likely be able to incorporate additional acreage of created and/or restored wetland and riparian areas.
Additional areas at Dillenbaugh Creek are also being evaluated as potential mitigation sites, if necessary. This would include relocation of Dillenbaugh Creek away from I-5 and the enhancement of wetland areas that are currently in pasture grass production.
The intent of the proposed mitigation is to restore some of the historic function of the Chehalis River and the associated wetland and riparian areas. These functions include low flow augmentation, organic input, passerine and migratory bird habitat, fishery support habitat, sediment and nutrient trapping (water quality improvement), groundwater recharge, habitat biodiversity, and floodwater retention and detention. It also is in a fairly visible area from Interstate 5, SR-6, and Scheuber Road, which would increase the aesthetic values of the area and provide passive recreation opportunities.
4.4.6.2 Skookumchuck Dam Modifications
In summary potential long-term impacts to Skookumchuck River riparian areas remain largely unknown, although wetland impacts are not expected to occur. If design analysis indicates substantial impacts are expected to occur to either wetland or riparian areas (or both), reevaluation of the proposed dam operation would occur. The significance of potential impacts and the ability of mitigation to offset the impacts would also be re-evaluated. Mitigation would be incorporated for any unavoidable impacts to wetlands and/or riparian areas.
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Under this alternative, the Corps would take no action that would result in changes to the existing conditions. There would remain concerns over the historical loss of riparian habitat, wetlands, and connectivity. Assuming WSDOT implements plans for improving I-5, floodproofing measures such as raising portions of the roadway or constructing levees would likely result in loss of wetlands that provide habitat for wildlife. Other development in the area would continue to occur in accordance with local comprehensive plans and would likely result in some losses of wetlands, riparian areas, and uplands that are used by wildlife species.
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The following criteria were used to evaluate the significance of effects on Wildlife within the project area. Construction and operation activities would result in significant effects if they were to:
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4.5.3.1 Short Term Effects
The noise and activity associated with levee construction would cause disturbance of songbirds, waterfowl, raptors, small mammals, deer, and other wildlife that use habitats within or near the construction area. This disturbance would be temporary and displaced animals would be expected to use other nearby habitats while construction of the levee system is underway.
Because the species that inhabit these areas are generally tolerant of human activity, this temporary disturbance is not expected to have significant adverse effects on wildlife.
4.5.3.2 Long Term Effects
Because the levees are set back from the mainstem of the Chehalis and the Skookumchuck Rivers, effects on riparian habitat and habitat connectivity will be minimized. Within the levee footprint, areas of riparian forest and wetland would be permanently converted to upland habitat.
Areas to be converted currently provide habitat for a variety of wildlife, including some game species, small mammals, reptiles and amphibians, and songbirds. The maximum area of impact to wetland habitats is estimated to be approximately 34 acres. Levee segments adjacent to the western edge of I-5 would affect emergent wetlands that provide habitat for these animals as well as for migratory waterfowl. In upland areas within the levee footprint, topography and vegetation communities would be permanently modified by levee construction. . Following construction, the levees would provide some wildlife habitat, although the quality of this habitat would be limited by the need to perform periodic maintenance and by type of vegetation (mainly grass and shrub cover) that would be allowed to colonize the levee slopes.
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4.5.4.1 Short Term Effects
Short term effects to wildlife from construction of the Skookumchuck Dam outlet facility is limited to temporary disturbance at the site and along associated transportation corridors. Noise associated with construction and vehicular traffic may cause some wildlife species to relocate to less disturbed areas.
4.5.4.2 Long Term Effects
Dam modifications should not have impacts to wildlife habitat as long as the high pool does not exceed five consecutive days of storage. Exceeding high pool events may cause stress to the reservoir vegetation community. Frequency of events requiring storage above 477 feet should not occur more often than every other year to avoid impacts to the habitats surrounding the reservoir.
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The mainstem of the Chehalis has poor wildlife habitat because of inadequate off-channel habitat and wetlands. Riparian habitat along the mainstem of the Chehalis River has been largely lost through various land use practices or processes resulting in a limitation of riparian habitat to narrow bands along tributaries and around confluences By 1938, significant changes had already occurred due to conversion to agricultural fields making it difficult to determine what habitats have been lost. Along the South Fork of the Chehalis River riparian vegetation is rated poor for 70% of the sub-basin and is particularly sparse in the mid and lower reaches. This lack of riparian vegetation compounds the forest connectivity loss that is present along the entire length of the Chehalis River within the project area (Appendix A). The smaller tributaries such as Salzer Creek, China Creek, Big Hannaford Creek, and Elk Creek are also lacking in riparian vegetation. It has been suggested that before development of this area into agriculture farmlands and residential development, the area consisted of predominately bottomland hardwood forest, indicative of a floodplain for this area. No mature forests exist within the project area.
By combining the levee construction, and dam modifications there will be no additional impacts to the marginal wildlife habitat that currently exist in the project area other than wetland impacts.
However, there is potential to have a positive effect on wildlife by creating additional wetlands, connectivity, and additional riparian zones. There will be no loss in remaining habitat connectivity, riparian habitat or remaining functional wetlands. Impacts to wetlands will be on the marginal converted wetlands that are now used for farming.
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The preferred alternative will impact approximately 34 acres of wetlands and the mitigation for that impact will occur in the SR-6, Scheuber Ditch, and Oxbow area. That mitigation will consist of creating a riparian zone of 200 feet in width on either side of the ditch. It is anticipated that the mitigation for the wetlands may also offset the impacts to wildlife habitat for the preferred alternative by mitigating for impacted habitat connectivity, wooded wetlands, and riparian zones.
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Threatened and endangered species include those named by the (USFWS) as appearing on the List of Endangered and Threatened Wildlife and Plants, as authorized by the Endangered Species Act of 1973, as well as species listed by the state of Washington.
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Under the No Action alternative, the Corps would take no action that would result in changes to the existing conditions. There would remain concerns over the historical loss of riparian habitat, wetlands, and connectivity. Assuming WSDOT implements plans for improving I-5, floodproofing measures such as raising portions of the roadway or constructing levees would likely result in loss of wetlands that provide habitat for wildlife. Other development in the area would continue to occur in accordance with local comprehensive plans and would likely result in some losses of wetlands, riparian areas, and uplands that are used by threatened and endangered species.
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The following criteria were used to evaluate the significance of effects on threatened and endangered species within the project area. Construction and operation activities would result in significant effects if they were to result in:
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The project is not expected to result in significant effects to sensitive species. Less than 1 acre of riparian habitat, and between 18 and 34 acres of wetland habitats (all of them pastures) would be covered or otherwise destroyed by project construction. None of these riparian areas and wetlands is known to provide habitat to sensitive species, based on searches of the PHS and Natural Heritage databases. No nest or roost trees of bald eagles would be affected by the project, and project construction activities are not expected to significantly affect bald eagle nesting. Following are brief discussions of the general nature of the project's effects on specific habitat types.
4.6.3.1 Wetland Species
The levee would cause a relatively significant reduction in the areal extent of flooding in the lower Skookumchuck River valley in the Centralia area. However, increases in the peak stage within the Skookumchuck River channel would occur as a result of the levees keeping a higher proportion of the flow confined to a smaller floodplain area (i.e., less out of channel flow entering the floodplain due to the levees).
The alteration in duration and frequency of flooding in the floodplain results in adverse effects to the floodplain habitats, including the riparian areas and adjacent wetlands. These effects can result in loss of native plant seed dispersal and seedling desiccation, as well as allow invasion of exotic invasive species (Duncan, 1993; Nilsson, 1982; Meffe, 1984; Moyle, 1986-all in Poff, 1997). A lack of seedling recruitment would result in a long-term decay and turnover to a less desirable plant community (Bren, 1992). Aquatic species that have adapted to the natural variation of a free-flowing river will lose their ecological advantage over less variable-tolerant species, and result in a change in the overall dynamic of the aquatic ecology (Cushman, 1985; Petts, 1984; Travnichek, et al, 1995). Wetlands may not be maintained if not frequently flooded.
Having acknowledged these potential effects from altered hydrologic regimes, it must be said that these effects will scarcely be noticeable in the Chehalis and Skookumchuck Basins. The levee system is designed to contain the 100-year flood. While this means that lesser flood flows will be contained within the levees and not be allowed to enter the larger floodplain, for the most part very little habitat would be affected. The levee system will remove flooding from built-up areas. It is designed to have minimal environmental effects, by virtue of being set back as far as possible in every location. What this means is that the levee would be built against road shoulders; along fence lines; along residential back yards. In a few cases, structures will be sacrificed in order to preserve additional habitat. The primary area of impact to wetland habitat would be along the lower two miles of Salzer Creek; the levee in this location would cut across approximately twenty acres of wet pasture, removing about half of it from flooding. However, investigations of groundwater in the basin appear to show that it is fed and recharged primarily by rainfall, rather than flooding. Thus the wetlands in this area (and elsewhere in the project area where wetlands would be cut off from flooding) are expected to remain functional (COE, 2002).
While these wetlands are considered to have limited functional value for aquatic life because they are all wet pasturelands, and well removed from the streams due to the levees being set back (though certainly some loss of function will result by removing floods-seed dispersal, organic/detrital inputs, flood storage, water quality improvement, wildlife habitat (refuge during floods), and perhaps others). On a basin-wide scale, these minor losses of function are considered to be insignificant. This is especially so when the mitigation and restoration plans are factored in, which are expected to fully compensate for the lost function caused by levee construction.
4.6.3.2 Riparian Species
While the project will affect riparian habitats in subtle ways, no direct losses of riparian trees would occur, with the exception of three short stretches along the Chehalis River. One of these locations is approximately 1/8 mile in length; just a few trees would be lost from another site (near Mellen Street); and a number of trees would be lost from the SR-6 restoration effort to reconnect the oxbow to the river. It is likely that some nest trees for all of these species could be lost (buffleheads and wood ducks are cavity nesters; ospreys place a nest in the top of a large, broken-top snag or on top of a transmission line pole; great blue herons build nests in colonies in various kinds of trees-however, no great blue heron colonies are found within the action area (WDFW, 2002). The loss of these few trees is not expected to be significant to these species.
On the Skookumchuck River, flood events under the proposed project will represent a change from the current condition. Existing operations do not mitigate for downstream flooding which results from overbank flows upon reaching full pool. The proposed project will store the peak of flood flows behind the reservoir and release water to maintain flows of less than 5,000 cfs at Pearl Street in Centralia. The result will be the elimination of large overtopping events (greater than 2 years) as they are replaced with smaller events of greater frequency and duration. Refer to Section 2.4 for sediment routing and timing effects.
In most urbanized and altered river systems, overbank flooding does not allow for adequate return pathways to the river. The result is often juvenile and adult salmon stranded in pasturelands, roadside ditches and suburban neighborhoods with no mechanism for reentry to the river. Under natural conditions, where reaches have good connectivity by way of small tributaries, extensive side channel habitats or wetlands, overtopping events represent rearing opportunities for juvenile salmon and provide refuge for adults to escape the turbulent, debris filled mainstem. Field investigations along the Skookumchuck found most tributaries had good connectivity to the mainstem. In contrast, there were few mainstem reaches with connected side channel habitat and floodplain connectivity (PIE, 2000).
Potential effects to river channel vegetation are primarily due to the lack of overbank flooding at flows above 3,000 cfs at the dam and 5,000 cfs at the mouth. Vegetation reliant upon flows under these levels should continue without harm and may even increase as high flows are moderated during floods. Thus the primary impact of the project is the loss of the larger overbank events and their effects on future woody debris recruitment. Much of the river length would still be overtopped by a 2-year event (Appendix B). Therefore, riparian habitat recruitment is expected to continue, helping to maintain a supply of LWD, and other organic materials to the river.
4.6.3.3 Prairies, Forests, and Other Habitats
This project would not affect prairie habitats, as the levee system will not be constructed in prairies, and the limits of flood control effects would not reach prairie habitats. No fast-moving, cold-water streams would be affected by the project, nor would off-channel, slow-moving, warm-water habitats. The potential exception to the latter is the mainstem Chehalis River itself, through which lampreys and anadromous fish migrate. The infrequent changes to river flows resulting from the project would be indistinguishable to these fish. Open spaces where peregrines typically hunt will remain intact with this project; furthermore, no nests of peregrines are known in the action area (WDFW, 2002). Finally, no talus slopes would be affected by the project.
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Endangered Species Act Section 7 consultation has been initiated with USFWS. A biological assessment has been transmitted to the USFWS for their review and is attached (Appendix E).
Determinations for bald eagle and bull trout are not likely to adversely effect. The remaining species determinations are no effect. There would be no substantial disruption, loss, or degradation of riparian, shoreline vegetation, connectivity, wetland hydrology or vegetation within the proposed project area.
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The preferred alternative will impact approximately 34 acres of wetlands and the mitigation for that impact will occur in the SR-6, Scheuber Ditch, and Oxbow area. That mitigation will consist of creating a riparian zone of 200 feet in width on either side of the ditch. It is anticipated that the mitigation for the wetlands will also offset the impacts to wildlife habitat for the preferred alternative, by mitigating for connectivity, wooded wetlands, and riparian zones.
Specific mitigation requirements for threatened and endangered species will be negotiated through the ESA Section 7 consultation process.
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4.7.1.1 Short-term Effects.
Short-term effects to fisheries from the no action alternative arise principally from urban development needs of the basin as well as construction related impacts from planned transportation projects. There are no anticipated short-term project related impacts to fish from agricultural practices under the no-action alternative. Existing agriculture conditions will remain unchanged without the preferred project. Fish impacts from levee construction would be avoided.
Short-term effects to fish from urban and infrastructure development include only those projects reasonably foreseeable in the absence of the preferred project and related to flood control. The relocation of the wastewater treatment plant is likely to have some short-term impacts by way of vegetation loss, elevated turbidity and potential for small-scale contaminant releases.
Modifications to Interstate 5 to reduce future flooding may also have some short-term impacts to fish, as the potential exists for elevated turbidity and short-term water quality impacts. Planning and construction methods and monitoring will determine significance of these short-term impacts.
4. 7.1.2 Long-term Effects:
Chehalis River and Tributaries
Long-term effects to the environment under the no action alternative from agricultural practices are limited to impacts from the continuance of existing farming and livestock practices as well as from natural sources within the project area. Activities in the project area will continue to provide high summer levels of fecal coliform and turbidity to the project area from natural processes, livestock, heavy vegetation management, irrigation and frequent disturbance of unprotected soils. Fish impacts are those related to loss of off-channel and wetland habitats; loss of riparian cover, food productivity and also from degraded water quality. Water quality impacts to fish are principally those of elevated pesticides, and turbidity. Low dissolved oxygen, altered pH and elevated levels of suspended solids, metals other contaminants may also continue to the detriment of fish resources.
Long-term urban impacts from industrial and residential sources under the no action alternative will be limited to existing impacts from development and industrial activities. Runoff from developed areas, sewer and wastewater outfalls will continue to enter the Chehalis River.
Additional wetland and riparian habitat loss will continue as natural habitats are converted through residential development, urban growth and associated infrastructure improvements. Fish impacts are those related to the continued loss of off-channel and wetland habitats, loss of riparian corridor and the associated loss of cover and food productivity. Sources of degraded water quality will also continue. Water quality impacts are principally those of elevated fecal coliform from upland runoff, pesticides, and elevated metal concentrations from industrial and residential practices as well as road runoff. Low dissolved oxygen, altered pH and elevated levels of suspended solids, metals other contaminants may also continue to the detriment of fish resources. Foreseeable long-term benefits are also possible. A planned relocation of the Chehalis Wastewater Treatment Plant from its current location on the Chehalis River may provide some opportunities to reduce fecal coliform and related water quality problems.
Long-term urban impacts from planned infrastructure improvements under the no action alternative will be limited to impacts from the raising of I-5 and multiple smaller improvements throughout the cities. Under current proposal is a project to raise portions of I-5 in the project area to protect against flooding and closure. The resulting impacts to fish may include a loss of some accessible wetlands and minor removal of vegetation within the project footprint.
Skookumchuck River
Skookumchuck reservoir will continue to remain at full pool between late winter and spring allowing for water levels to remain adjacent to the vegetated shorelines of the reservoir. When the reservoir begins to recede in the summer, access to upper benthic communities and terrestrial prey input for resident and anadromous fish will become restricted. Reservoir drawdown will continue to deny fish the shade, productivity and shoreline cover afforded by reservoir vegetation. Reservoir operations will continue to inundate some areas of potential steelhead and resident trout spawning habitat in the tributaries, notably Fall Creek, Turvey Creek and the Skookumchuck River.
The existing Skookumchuck dam trap and haul facility will continue to operate for adult migrating steelhead. The steelhead will continue to be collected at the downstream weir, and be transported above dam for release. The existing outmigration chute and flume bypass system on top of the dam will continue to pass the majority of juvenile steelhead. Sluice gate operation may continue to affect some portion of outmigrating fish.
Temperature control of the lower river by Skookumchuck Dam would remain unchanged. The manner in which the Skookumchuck Dam is operated affects fish spawning and rearing habitat will continue to result in summer water temperature at or below 50o F to 55øF. During the period April 1 through August 31, Skookumchuck Dam will continue to provide a maximum flow of 95 cfs or natural flow plus 50 cfs, whichever is less. It would continue to provide minimum instream flows of 140 cfs September 1 through October 31 for fish spawning and 95 cfs November 1 through March 31 for incubation.
The existing "fill-and-spill" operation will continue to provide a nearly natural winter hydrograph for spawning, incubating and rearing salmonids in the river. Negative flood impacts from existing operations on aquatic resources may continue in the form of excessive redd scour
in confined or steep sections of the river and as severe overtopping in areas of limited mainstem connectivity. Positive benefits associated with overtopping in areas of extensive mainstem connectivity may continue resulting in unrestricted access to accessible overwintering areas and improving overwintering success by passing natural freshets. Flooding will continue to be a source of water quality degradation from petrochemicals, fecal coliform, metals and possibly other chemicals that are mobilized during overbank flooding and potentially transported into the channel.
Drawdown will remain a natural event resulting from reduced inflows above Skookumchuck Dam during this period. The Skookumchuck River downstream of the dam will continue to mimic natural inflow. Fish impacts may include dewatering of late spawning salmonids and stranding of juvenile fish in off-channel ponds as the waters recede. However, the prolonged nature of drawdown in the Skookumchuck River should allow for fish to adjust to the receding hydrograph and minimize the occurrence of stranding.
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A determination on significance of impacts from the preferred alternative is based on the existing conditions briefly described below. Criteria for significance have been developed to determine the projects potential to add or detract from the existing environmental conditions of the Chehalis River study area. Through this planning process, opportunities to avoid and minimize negative impacts have been be identified and unavoidable adverse impacts flagged for further consideration. Additional information on impacts from flood control operations within the Skookumchuck River can be found in Appendix B. The potential for effects on fish resources are further defined as short-term and long-term effects. A significant effect on fish and fish habitat would occur if construction and/ or operation activities would result in:
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The preferred alternative is described in detail within section 2.0 and also within the Skookumchuck Dam Fisheries Impact Report (Appendix B). Briefly, the preferred alternative includes modifications to Skookumchuck Dam for flood control and a levee system consisting of 16 reaches.
4.7.3.1 Short-Term Effects
Short-term impacts to fish from the preferred alternative are likely to be construction-related effects or short-term water quality degradations likely to occur as the project area adapts to landscape changes (elevated turbidity, loss of riparian habitat). Short-term impacts to fish may result from modifications to Skookumchuck Dam in support of flood control and the construction of a new levee alignment. These impacts are primarily related to short-term riparian disruption and water quality problems.
Chehalis River and Tributaries
Project area short-term impacts from the construction of levees are most likely to be associated with a temporary reduction in water quality as soils are disrupted by clearing and filling and increased construction traffic. Even with runoff controls in place, elevated levels of turbidity are likely. Actual affects on fish resources will be dependant on the size and location of the levee alignment with fish effects increasing with proximity to the river.
Fish resource impacts specifically from levee construction are anticipated to be those associated with loss of riparian vegetation, loss of off-channel habitats and floodplain disconnection. Fish impacts will be directly related to the proximity of the levee prism to the riverbank and its location within the floodplain. Constructing levee systems outside the riparian buffer areas will provide opportunity for riparian development and reduced impacts to fish.
Requirements for adequate riparian buffers are dependent on the size of the river and regulatory statutes. Most existing requirements found to be currently accepted, recommend riparian buffer zone requirements between 50 feet (for water quality) and 300 feet (riparian function) from the rivers edge. Since larger rivers have larger riparian needs, levee prisms built within 300 feet from the river should be considered to have indirect effects on fish and levee construction within 100 feet of the river considered to have direct affects to fish habitat, which may result in a reduction in productivity.
Habitat changes from encroachment on a riparian buffer may range from simplifying or altering existing riparian vegetation and habitats to heavy alteration of the banks and underwater habitats.
Associated impacts to fish from these alterations are considered larger in cases where streambank habitats are lost or where off channel habitats and tributaries are made inaccessible.
If bank alterations or riparian alterations are large enough, the capacity of a river system to hold and support fish resources may be reduced.
The current levee alignment for the preferred project recommends a total of 80,000 linear feet of levee construction or upgrading, most of which is located away from fish bearing rivers and creeks. Of the total construction length, 15,350 feet or 20% of the overall length is located within 300 feet of the river. The length of levee closer than 100 feet and more likely to have direct affects on fish resources is 8700 feet or 10.8% of the overall length. Not all levees currently proposed closer than 100 feet of the river are located directly on the riverbank. Bank stabilization and direct impacts to aquatic habitat is limited to 1900 feet (2%) of the total levee alignment and dispersed among 6 distinct locations. Seventy-eight percent (6800 feet) of levee proposed within 100 feet of a fish-bearing stream will affect tributaries directly. The remaining 1900 feet (22%) of levee will be located adjacent to the Chehalis River.
The size and nature of levee construction is in keeping with typical levee construction and rehabilitation projects permitted elsewhere in the basin and throughout the State of Washington.
The proposed levee construction is a mixture of several cross sections and floodwalls with and average width of 33.4 feet but ranging between 15.5 and 49 feet wide Figure 4.7.3. Anticipated short-term impacts to fish resources from construction of each reach are described below according to levee reach.
Figure 4.7.3 Levee and Flood Wall Alignment
Reach 1- Fords Prairie (CH-1)
None Anticipated
Reach 2. Sewage Treatment Plant (CH-2)
In this reach, 300 feet of levee alignment will be constructed approximately 200 feet from the Chehalis River. The levee is located around and behind existing sewage treatment plant structures. Short-term impacts could include minor and temporary turbidity increases. The existing riparian cover is limited in this reach. Salmon spawning or rearing habitat should not be affected.
Reach 3. Mellon Street to Salzer Creek Bridge (CH-3)
A combination of floodwall and levee construction is proposed within this reach to limit impacts to the Chehalis River. Attempts to limit Chehalis River interaction with the alignment has resulted in only two locations closer than 300ft of the channel for a total length of 1450 feet.
Impacts from the levee proposed within 100 feet will be limited to 250 feet. A floodwall is proposed for all areas where the river is closer than 300 feet to the alignment. Short-term impacts to fish will be limited to potential increases in construction run-off and traffic. Salmon spawning or rearing habitat should not be impacted.
Reach 4. Salzer Creek Right Bank (Ch-5N)
Levee construction in this reach is designed to prevent floodwaters from the Chehalis River from backing up Salzer Creek. The levee alignment follows the right bank of Salzer Creek and will be constructed within 300 feet of the creek for approximately 2750 feet of which 1800 feet is proposed within 100 feet of the creek. An existing levee will be utilized along an additional 2150 feet of length but all work will be conducted landward of it's existing location. The end of this reach will tie into an existing levee. The alignment will not directly cross the confluence of Salzer Creek but the levee will require armor rock to be placed along 150 feet of the right bank bridge abutment of I-5. Short-term impacts will most likely be temporary increases in turbidity and construction related spills and debris. Armoring of the bridge abutment will be conducted outside the creek channel and carry no significant short-term impacts. Some riparian loss may occur if the existing levee needs to be cleaned prior to additional construction landward of its location.
Reach 5. Salzer Creek Left Bank (CH-5S)
As with reach 4, the intent of this levee section is to prevent Chehalis River flooding from entering the Salzer Creek basin. It does not propose to cross the Salzer Creek confluence but will require approximately 150 feet of armor rock along the left bank bridge abutment at I-5.
The remainder of the levee system ties into an existing railroad grade and then after turning and crossing Coal creek, it ties into an existing levee. The constructed levee section crossing the confluence of Coal Creek will receive a culvert and flap gate structure to maintain Coal Creek flow and connectivity. Short-term impacts are associated with levee construction and flap gate construction and are likely limited to temporary increases in turbidity to Salzer Creek and the potential for construction related spills. Riparian loss will be limited since the majority of this levee section parallels a railroad grade that is principally unvegetated. The bridge abutment armoring will be conducted outside the creek channel and carry no significant short-term impacts. Salmon spawning or rearing habitat should not be impacted.
Reach 6. Coal Creek. (CH-6)
None anticipated
Reach 7. Salzer Ck to Airport (CH-4)
This alignment follows along Airport Way with Airport Way located between the levee or floodwall construction and the Chehalis River until it connects to an existing airport levee. The existing levee will be widened from the landward side. None of the levee alignment is in proximity to the Chehalis River or other fish bearing streams except for approximately 250 feet near the Salzer Creek Bridge. In this reach, the creek may require armoring but the levee alignment does not propose construction within the Salzer Creek channel. An existing levee on the south side of Airport Way will be removed out of consideration for an environmentally sensitive area located on the south side of the road. Short-term impacts in this reach are limited to potential riparian loss in the area around the Salzer creek armoring and minor potential for construction related spills and turbidity. Some riparian loss may occur along 4650 feet of the existing airport levee if it is cleaned prior to landward levee construction. Vegetation along the existing levee to be removed may also be removed and temporarily lost.
Reach 8. SR-6 to Railroad Underpass (DB-2)
Levee construction in this reach will not come near fish bearing creeks except where it crosses Dillenbaugh Creek. The levee will cross Dillenbaugh Creek perpendicularly at the point where it exits a large wetland complex east of I-5. A control box with a flap gate will be installed at this crossing to prevent Chehalis River floodwaters from entering the wetland at extremely high floods (1 foot below the 100-year event) and from flooding residences. Short-term impacts may include temporary creek disturbance in the location of the levee crossing and minor riparian loss in proximity to Dillenbaugh Creek.
Reach 9. Dillenbaugh Creek (DB-3)
This levee reach is designed to protect I-5 from Chehalis River floodwaters. The levee alignment has been offset 78 feet to compensate for planned widening of I-5 in the project area.
Dillenbaugh Creek follows a straight ditch along this levee reach for 3100 feet and will require relocation to allow for the levee prism. An additional 1,150 feet of levee will be constructed within 100 feet of the creek, which may pose some direct impact but will not require channel relocation. Dillenbaugh Creek also enters the wetland complex east of I-5 in this reach. A control box with a sluice gate at the point where the creek enters the wetland will be necessary to prevent Chehalis floodwaters from flooding the wetland and residences at extreme high floods (1 feet below 100-year event). The short-term impacts of this reach include temporary riparian loss at the relocation reach and adjacent levee sections as well as increased potential for turbidity and construction related spills. Although the reaches of Dillenbaugh Creek in this levee section are channelized and disrupted, there is a potential for temporary loss to spawning and rearing habitat if construction is not timed appropriately. The establishment of accepted work windows and precautionary guidelines should ensure no significant short-term impacts occur during construction of this levee section.
Reach 10. Dillenbaugh South (FW-2)
Construction of this reach is necessary only if Rice Road is not raised prior to implementing the preferred project. If needed, the project proposes a short floodwall located East of Dillenbaugh Creek where it comes within 300 feet of the creek in two locations for a total of 1100 feet. Of that length, 750 feet appears closer than 100 feet from the creek. Potential short-term impacts are most likely to be temporary increases in turbidity to Dillenbaugh Creek from levee construction and an increased risk of construction related spills. Temporary riparian loss may be short-term impact for the levee portions closest to Dillenbaugh Creek but no construction is likely on the banks of the creek.
Reach 11. West Reynolds Ave. to BNRR (SK-1)
No effects anticipated.
Reach 12. Chehalis Western RR to Borst Park (SK-2)
This reach roughly follows the left bank of the Skookumchuck River for approximately 2050 feet all of which is closer than 300 feet to the river. Of that length, approximately 700 feet is closer than 100 feet but none is proposed for construction adjacent to the riverbank. Still, the orientation of the levee will require approximately 300 feet along the outside bend to be armored from flood related erosion. Some existing armor rock may need to be maintained in the vicinity of the Harrison Street right bank bridge abutment. Short-term impacts from this levee construction are anticipated to be loss of riparian vegetation mostly associated with the armoring of the outside bend and possible temporary increases in turbidity from construction.
Reach 13. Harrison Street Bridge to I-5 right bank (FW-3)
This levee reach follows Hayes Lake for the protection of residences near the Skookumchuck River confluence with the Chehalis River but the levee is not in close proximity to either river.
Reach 14. Left bank I-5 to Harrison Street Bridge (SK-3)
This reach follows the left bank of the Skookumchuck River between the Harrison Street Bridge and Interstate 5. The alignment is in proximity to the Skookumchuck River and approximately 1000 feet of the levee will be near or on the riverbank. Where possible a floodwall is proposed to limit riparian disturbance. The alignment near the river increases to 2100 feet when areas closer than 300 feet are included. Armor rock will be necessary in areas adjacent to the riverbank to prevent scour and erosion from floodwaters. Short-term impacts will be loss of riparian and possibly aquatic habitat from the construction of levee adjacent to the riverbank as well as potential increases in turbidity and risk of construction related spills. The channel adjacent to the levee area is considered marginal spawning habitat. There are no notable offchannel or other rearing habitats in the levee reach.
Reach 15. Harrison St Bridge to Chehalis Western. Railroad. (SK 4)
This reach follows existing road infrastructure and may come into proximity with the Skookumchuck River in two locations for a combined 950 feet but does not appear to come closer than 300 feet at either location. Short-term impacts include the potential for increase turbidity from construction.
Reach 16. Chehalis Western RR to BNRR (SK-5)
This reach ties into the existing levee system of the Skookumchuck River and comes within 300 feet of the river in one location for 500 feet. Of that length, 100 feet is closer than 100 feet of the river. The levee reach is setback from the river except where it is necessary to connect with the existing levee system. Short-term impacts from this levee reach may include temporary loss of riparian corridor and some aquatic habitat as the levee comes to the riverbank to join with the existing levee and a potential for increased turbidity and construction related spills. The riparian corridor in this area is narrow in the reach affected by the levee construction and riparian disturbance is not anticipated to be significant.
Skookumchuck River
Short-term impacts to fish anticipated from modifications to Skookumchuck Dam may be associated with outlet gate construction. More information on Skookumchuck modifications and impacts can be found in Appendix B. In summary, impact from structural modification is anticipated only for concrete work in and around the dam. However, concrete pouring, drilling and mixing can have impacts to fisheries and wildlife by raising pH levels or elevating water temperatures if conducted in small poorly circulated aquatic environments, if allowed to cure in large quantities underwater or if concrete dust and slurry is inappropriately managed. Since most of the construction is conducted upland and the Skookumchuck River is large enough to ameliorate the effects of curing small concrete projects, experiencing events of elevated pH, temperature or extended turbidity is not anticipated. Cofferdams may be employed to further minimize water quality degradation from construction activities at Skookumchuck Dam.
Similarly, construction activities may alter access to fish passage routes or temporarily reduce passage efficiency. Construction of spillway gates may cause some disturbance to outmigrating juvenile salmonids if not timed and conducted appropriately. Adherence to accepted construction windows and proper sediment and construction management should reduce opportunity for significant short-term effects from Skookumchuck Dam modifications.
4.7.3.2 Long-term Effects
Long-term effects to the environment from the preferred alternative will most likely include impacts from maintenance of the proposed levees and operation of the Skookumchuck Dam Flood Control project. Long-term impacts are anticipated principally from loss of riparian and aquatic habitat from portions of the proposed levee system; changes in adult and juvenile migratory pathways during major flood events, tributaries habitat/flow alterations and flow changes from the operation of Skookumchuck Dam as a flood control project.
Chehalis River and Tributaries
Long-term impacts from the preferred alternative are most likely to be associated with a permanent reduction in riparian corridor succession and loss of in stream habitat from bank stabilization and altered fish accessibility to tributaries during flood events. Actual affects on fish resources will be dependant on the size and location of the levee alignment as the effects on fish habitat increases with proximity of the levee to the river.
As stated earlier, the current levee alignment for the preferred project indicates a total of 80,000 linear feet of levee construction or upgrading, most of which is located away from fish bearing rivers and creeks. Of the total construction length, 15,350 feet or 20% of the overall length is located within 300 feet of the river. The amount of levee less than 100 feet and therefore more likely to have direct affects on fish resources is 8700 feet or 10.8% of the overall length. Not all levees currently proposed closer than 100 feet of the river are located directly on the riverbank.
Bank stabilization and direct aquatic impacts are limited to 1900 feet (2%) of the total levee alignment and further dispersed among 6 distinct locations. Approximately 6800 feet (78%) of the levees within 100 feet of a fish-bearing stream affect tributaries. The remaining 1900 feet (22%) of levee is located adjacent to the Chehalis River. General alignment and levee features can also be found in technical memorandum #4 (Corps 2001a, tech memo. 4).
Reach 1- Fords Prairie (CH-1)
None anticipated
Reach 2. Sewage Treatment Plant (CH-2)
Long-term impacts at this reach should be limited to indirect effects to fish resources from loss of floodplain connectivity along the stretch nearest the Chehalis River (300 feet). Direct impacts to instream habitat, rearing habitat and other habitat requirements for anadromous and resident fish are not anticipated.
Reach 3. Mellon Street to Salzer Creek Bridge (CH-3)
Long-term impacts at this reach will include indirect and direct impacts to fish resources from loss of floodplain connectivity along the 1450-foot reach of levee within 300 feet of the Chehalis River (at 300 feet). Direct impacts to instream habitat, rearing habitat are not anticipated but some long-term reduction in woody debris recruitment or drainage patterns may occur.
Reach 4. Salzer Creek Right Bank (Ch-5N)
This levee reach will require at least 1800 feet of levee construction in proximity to Salzer creek.
While the levee is not expected to be on the riverbank, some loss of riparian cover is likely and may result in reduced terrestrial prey production, canopy cover and woody debris recruitment from levee maintenance. There are no significant off channel habitats in the reach but some small reduction in mainstem rearing habitat may occur depending on final placement location. A 150- foot section of levee located near the Salzer Creek Bridge will require armoring for erosion protection and may result in direct loss of instream habitat if constructed below ordinary high water. Significant impacts to spawning and migration of adult and juvenile fish are not anticipated, as the levee alignment does not direct alter tributary connectivity with the Chehalis River.
Reach 5. Salzer Creek Left Bank (CH-5S)
As with reach 4, the intent of this levee section is to prevent Chehalis River flooding from within the Salzer Creek basin. Long-term impacts from this levee section are limited to possible direct impacts from armor rock placement near the Salzer Creek Bridge. A 150-foot section of levee located near the Salzer Creek Bridge will require armoring for erosion protection and may result in direct loss of instream habitat if constructed below ordinary high water. Significant impacts to spawning and migration of adult and juvenile fish are not anticipated, as the levee alignment does not direct alter tributary connectivity with the Chehalis River.
Reach 6. Coal Creek. (CH-6)
None anticipated
Reach 7. Salzer Ck to Airport (CH-4)
This alignment follows along Airport Way with Airport Way located between the levee, floodwall construction and the river until it connects to an existing levee around the airport. The existing levee will be widened from the landward side. None of the levee alignment is in proximity to the Chehalis River or other fish bearing streams except for approximately 250 feet near the Salzer Creek bridge which may require armoring but is not proposed for construction within the Salzer Creek channel. Direct effects from this armoring will be seen only at high water events and only for the length of levee armored. Removal of the old levee may increase floodplain connectivity to previously isolated habitats and wetlands. Long-term vegetation maintenance may result in some loss of riparian function but the setback nature of the levee alignment should result in minimal impacts to fish.
Reach 8. SR-6 to Railroad Underpass (DB-2)
Levee vegetation management along this levee reach could create long-term impact through maintained reduction riparian vegetation and canopy cover along the levee alignment. Impacts are most likely where the levee parallels a large wetland complex and at the site of the control box installation. Additionally, the control box structure may prevent adult access and juvenile passage when closed during large flood events. The frequency of control box operation is anticipated to be sporadic and not likely to cause significant limitations to fish use of the wetland. The wetland should also remain open at the control box site during the outmigration period. Salmon spawning or rearing habitat should not be affected.
Reach 9. Dillenbaugh Creek (DB-3)
This levee reach is designed to protect I-5 from Chehalis River floodwaters. The alignment has been offset 78 feet to make room for expected widening of I-5 in the project area. Dillenbaugh Creek in this reach is degraded and contained within a straight ditch for 3100 feet. The levee will require a portion of the creek to be relocated to allow room for the levee prism. This creek relocation may prove to be beneficial if opportunities to reestablish sinuosity, vegetation, overstory and aquatic habitats are incorporated into design. Long-term impacts will be negative and possibly significant if the creek is simply relocated without enhancement. An additional 1,150 feet of levee alignment will not require creek relocation but will be within 100 feet of the bank and may result in some maintained loss of canopy cover as a result of levee vegetation maintenance. Currently, there is no riparian vegetation within these reaches of Dillenbaugh Creek. Rearing and other aquatic habitats are likely to be affected as a result of the channel relocation. Properly designed, the relocated channel should provide better aquatic conditions than the current condition. Additionally, the control box structure may prevent adult access and juvenile passage during large flood events. The frequency of control box operation is anticipated to be sporadic and not likely to cause significant limitations to fish use of the wetland. The wetland should also remain open at the control box site during the outmigration period. There is no spawning habitat within these reaches of Dillenbaugh Creek.
Reach 10. Dillenbaugh South (FW-2)
Construction of this reach is necessary only if Rice Road is not raised prior to implementing the preferred project. The reach consists of a short floodwall located east of Dillenbaugh Creek where it comes within 300 feet of the creek in two locations for a total of 1100 feet. Of that length, 750 feet appears closer than 100 feet from the creek although not on the banks of Dillenbaugh creek. Potential long-term effects are a maintained lack of riparian canopy. The floodwall approach should reduce the footprint and allow for a reduced need to manage vegetation.
Reach 11. West Reynolds Ave. to BNRR (SK-1)
No adverse effects are anticipated.
Reach 12. Chehalis Western RR to Borst Park (SK-2)
Long-term effects from construction of this levee reach are associated with indirect fish habitat impacts from the 2050 feet of levee within 300 feet of the bank and some degree of direct impact from the 700 feet of levee construction within 100 feet of the riverbank. The most likely source of long-term fish effects may result from the 300 feet of levee that will require armor rock for erosion protection. This 300 feet may result in some long-term habitat impairment, vegetation loss and loss of riparian canopy. A reduction in woody debris recruitment may also occur. The conversion of approximately 700 feet of streambank habitat to levee could represent a permanent habitat loss if the construction is not planned and mitigated appropriately.
Reach 13. Harrison Street Bridge to I-5 right bank (FW-3)
None anticipated
Reach 14. Left bank I-5 to Harrison Street Bridge (SK-3)
Long-term impacts from construction of this levee reach are associated with indirect fish impacts from the 2100 feet of levee within 300 feet of the bank and some level of direct habitat or fish impact from the 1000 feet of levee construction within 100 feet of the riverbank. Within this reach, most of the 1000 feet proposed levee will be on or near the riverbank increasing the potential for riparian and aquatic habitat impacts. The reach is not generally considered highly productive spawning habitat. Further detailed planning is still necessary. The conversion of approximately 1000 feet of streambank habitat to levee could represent a permanent habitat loss if the construction is not planned and mitigated appropriately.
Reach 15. Harrison St Bridge to Chehalis W. RR. (SK 4)
None anticipated
Reach 16. Chehalis Western RR to BNRR (SK-5)
Long-term impacts from construction of this levee reach are associated with indirect fish impacts in one location totaling 500 feet of levee of the bank and some level of direct habitat or fish impact from the 100 feet of levee to be constructed within 100 feet of the riverbank increasing the potential for riparian and aquatic habitat impacts. The reach is not generally considered highly productive spawning habitat. Further detailed planning is still necessary. The riparian corridor in this area is narrow in the reach affected by the levee construction and riparian disturbance is not anticipated to be significant from conversion of approximately 100 feet of streambank habitat.
Skookumchuck River
Long-term impacts to fish anticipated from modifications to Skookumchuck Dam are most likely associated with the management of the Skookumchuck River system to accommodate flood control needs and can be separated into 2 geographic areas. First is the possible long-term affect of operating a flood control dam on the reservoir. Secondly is the possible long-term impact on downstream fish resources from flood control construction and dam operations. The following section is a summary of potential long-term impacts from operation at Skookumchuck Dam.
Additional information on Skookumchuck modifications and impacts can be found in Appendix B.
Reservoir operations will increase winter pool fluctuations and require fish-bearing tributaries around the reservoir to adapt to a winter varial zone. These difficulties are not likely on the Skookumchuck River due its size and winter flow volume but may occur on smaller tributaries.
Absent a flood event, the preferred project would strive to keep a winter pool at elevation 455 which will result in a reduction in availability of shoreline vegetation to fish until the February refill. In addition, the lower reservoir may increase the opportunity for cutthroat trout to prey upon juvenile steelhead as surface acreage becomes smaller and terrestrial food sources become less available. Fish passage issues may also become a long-term adverse impact if the tributaries cut through unconsolidated soils exposed by the lowered reservoir. Heavy tributary scour through the reservoir sediments can create fish passage difficulties for resident fishes during the winter. Changes to sedimentation, woody debris movement, channel movement and other channel maintenance processes are not anticipated.
At the dam, the creation of a flood control project will allow all flows to be passed through the new larger gates and it may influence fish migration pathways. Since flow is a primary factor for passage selection, some percentage of steelhead and resident trout moving through the dam between November and February would likely travel through the new outlets even if access to the spillway was available. In the winter however, fish passage would be limited to some adult steelhead that fall back after transfer, and resident trout. Steelhead out-migration generally occurs after the flood control season. If the new outlet structures are inefficient or otherwise compromise fish passage efficiency, it should be considered a long-term fish impact. Planning is ongoing to design outlets that minimize or avoid impacts to fish passage and consider its reconnections to the river.
The preferred project would not allow the reservoir to refill in winter as it does currently. Early winter freshets that are normally captured in the reservoir would be passed to maintain a reservoir elevation of 455. These freshets may have positive effects on fish migration as they can influence adult salmon movement. Adult salmon may use inflow pulses caused by fall rains to begin moving toward their spawning grounds where they may continue have benefits by assisting to initiate spawning. Juveniles rearing in the river may also use fall freshets to begin moving into side channels and other over wintering habitat prior to the onset of winter. It is presumed the reservoir will be managed to maintain the minimum pool in anticipation of floods, which would result in most freshets being passed quickly. If the flood control operation calls for capture of the freshets until the pool has filled to a critical point, than the benefits of the freshets would be diminished.
Downstream of the project, late fall flood flows could have long-term impacts to spawning salmon depending on the duration of high flow events. Freshets captured and then released over an extended timeframe, may artificially elevate river stage and put redds built by salmon on high gravel bars at risk of desiccation. This impact is most commonly a problem in spring when high flows are less likely to keep higher elevation redds watered and typically affects spawning steelhead most heavily. Conversely, a modified hydrologic regime that allows for extensive and frequent connections to side channel complexes and other off-channel rearing areas may benefit rearing juvenile salmon. Juvenile benefits from frequent and low level flooding of off-channel habitats are predicated upon having good habitat in reaches with good connectivity by way of small tributaries, extensive side channel habitats or wetlands. Overtopping events in these reaches may represent a long-term benefit by providing rearing opportunities for juvenile salmon and refuge for adults to escape the turbulent and debris filled mainstem. Areas of poor river connectivity would not realize these benefits and may even constitute a significant long-term risk of adult and juvenile salmon stranding. Most likely in the urbanized and altered sections of the river, overbank flooding in these areas would not allow for adequate return pathways to the river and possibly result in excessive juvenile and adult salmon stranding in pasturelands, roadside ditches and suburban neighborhoods with no mechanism for reentry to the river.
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4.7.4.1 Chehalis Levees
In general, areas directly impacted by the preferred alternatice include opportunities for further minimization of impacts. Minimization techniques will be necessary to avoid excessive impacts from long-term loss of instream habitat, juvenile cover and refuge habitats and reductions in riparian canopy.
Despite additional minimization techniques however, vegetation maintenance in these areas may still propagate a long-term reduction in riparian habitat by limiting overstory growth, associated prey resources, woody debris recruitment and cover habitat. Areas of indirect impact to fish
from levee construction (construction between 100 feet and 300ft from fish habitat) could also have long-term impacts to water quality, nutrient transport and runoff but the limited amount of this type of impact should not result in significant impacts to resident or anadromous fish.
Long-term impacts from the construction of these tributary levee systems and control structures are different for each tributary. Levee construction around the tributary confluences has associated long-term impacts similar to those of the Chehalis River levee. Potential impacts include a loss of riparian overstory and a related reduction in woody debris requirement, reduction in shading and terrestrial prey production. The degree of this impact is directly related to the proximity of the levees to the tributary.
Watershed-scale long-term impacts should not be appreciable as the amount of Chehalis River and tributary floodplain will be allowed to remain largely intact. Significant alterations to groundwater flow and sediment transport are not anticipated under the proposed alternative.
Allowing floodwater inundation over a large segment of he existing floodplain may minimize future human development and institutionalize the presence of necessary floodplain processes.
4.7.4.2 Skookumchuck Dam Modifications
Impacts most likely above the dam appear related to increased reservoir fluctuations, which may cause fish to adapt to water surface fluctuations, create potential fish blockages at tributary mouths and increase turbidity potential. Alterations to sedimentation, woody debris movement, channel movement and other reservoir maintenance processes are not anticipated.
Impacts to fish at the dam are not anticipated. Construction of the outlet facility should occur outside the fish migration window and done principally in the dry. When completed, the outlet should pass fish effectively or provide a full pool prior to the onset of outmigration. A full pool condition would allow for fish to outmigrate through the existing spillway.
Below the dam, the preferred project would allow for early winter freshets that are normally captured in the reservoir would be passed to maintain a reservoir elevation of 455 potentially benefiting juvenile and adult salmon. Flow fluctuations may provide an opportunity for annual high flow events but would eliminate large-scale flooding. Annual overbank flooding events may represent a long-term benefit by providing rearing opportunities for juvenile salmon and refuge for adults to escape the turbulent and debris filled mainstem. Areas of poor river connectivity would not realize these benefits and may even constitute a significant long-term risk of adult and juvenile salmon stranding.
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Proposed mitigation for fish impacts will be managed principally through avoidance and minimization techniques aimed at reducing the amount of direct and indirect impact to fish and fish habitat through construction timing, levee alignment and location. Fish impacts associated with changes to the Skookumchuck River hydraulic regime during the flood control season will require further study. During the PED phase, operational features of the dam will be defin