Topography and Geology
The Skookumchuck River rises in the westernmost region of the Snoqualmie National Forest and flows in a northwesterly direction to the town of Bucoda, and from there to the confluence with the Chehalis River at Centralia. Including its tributaries, it drains 202 square miles. Much of the subbasin is coniferous forestland, but the river valley supports agriculture.
Elevations within the subbasin range from 150 feet at the mouth of the Skookumchuck River to over 3,000 feet at the headwaters. The gradient of the Skookumchuck River from its source to the town of Bucoda is steep, falling an average of 19 feet per mile. Then it flattens and falls about five feet per mile in the lower reach. Except for the extreme upstream portion, the Skookumchuck River flows through a winding channel in floodplains varying from a few hundred feet to 0.5 mile in width. The Skookumchuck River subbasin is 90% woodland; the remaining 10% consists largely of cropland. Annual mean precipitation on the drainage ranges from 40 to 80 inches (NOAA, 1991). The average annual discharge from the subbasin is estimated to be 247 cfs, with a maximum of 6,900 cfs recorded in December of 1972 and a minimum of 15.5 cfs recorded in November of 1952 at the U.S. Geological Survey (USGS) gaging station near Centralia. A major tributary of the Skookumchuck is Hanaford Creek with a drainage area of 58 square miles and an average annual flow of 85 cfs (USGS, 1992).
Sediment transport and other hydrologic characteristics of the Skookumchuck River are probably better understood when they are compared with those of the neighboring Newaukum River. Comparisons between the two subbasins are made frequently in the following discussion.
A comparison of runoff from the two subbasins is difficult because only the runoff from the upper one-third of the Skookumchuck drainage is gaged; however, comparison of drainage areas of the two subbasins and comparison of runoff at the Skookumchuck River near Centralia with that of the North Fork and South Fork Newaukum River suggests that the annual runoff of the Skookumchuck River is about 1.2 times that of the Newaukum River.
Most of the area is comprised of forest and agricultural land. Almost none of the area is served by sewers and the area within Lewis County has minimal regulation over development. Steep slopes with widespread clear-cutting contribute to increased sedimentation and siltation and increased run-off potential. Skookumchuck Dam and the Centralia Mine holding ponds have retention capabilities. Major water uses are irrigation, mines and gravel quarries,and domestic and livestock use.
Most soils in this subbasin are either too dense or too porous to allow effective use of septic systems. Only three major soil types exist over most of the watershed.
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Using 1990 U.S. Bureau of Census data and maps for the area, and relating census block numbers to the watershed boundary of the Skookumchuck Basin, the subbasin population appears to be 22,000.
Based upon an average for the entire Chehalis River Basin of 2.29 people per household, there are close to 9,600 households in this subbasin.
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The water use picture in the Skookumchuck is complicated by the storage reservoir and use of water for the Centralia thermoelectric plant in the Hanaford Creek watershed. The City of Centralia considers the Skookumchuck to be a possible future source of municipal and industrial water.
A reservoir for power production, with a capacity of 35,000 acre-feet, was constructed on the Skookumchuck River at RM 21.9 between 1968 and 1971 by Pacific Power and Light Company, with regulation of flow beginning in 1971. There are other minor diversions for domestic and irrigation uses. Irrigation water rights from surface water in the Skookumchuck drainage are for 893 acres. The actual land irrigated from surface water is estimated to be 400 acres. The 80 cfs water permit held by Pacific Power and Light Company is covered by reservoir storage and does not result in a total depletion of 80 cfs. The remaining rights are for 87.1 cfs, much of which could not be supplied by the natural flows in the river without the reservoir (Ecology, 1980).
Bucoda has the only community water system in this drainage area. Their water supply is from wells located within the community. Bucoda has surface water rights on the Skookumchuck River for 11.1 cfs which are presently unused.
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Forestry - 70%: The large areas of timber production create the potential for minimally controlled discharges of pollutants over much of the watershed.
Run-off from logging activities and road building on the very steep slopes (84% classed as "Severe Erosion Hazard" - USDA, 1975) lead to siltation as well as introduction of pesticides and fertilizers from forestry activities.
Agricultural - 20%: dairy farms, tree farms, grazing lands and crop lands.
Fecal coliform contamination from agricultural activities is a severe problem. Pesticide/herbicide, fertilizer and other chemicals must be applied carefully to prevent contamination. Grazing along streambanks causes disturbance of banks and foliage, and increases the incidence of animal waste entering waterbodies.
Residential - 5%: Contamination of ground and surface waters with compounds from septic systems and other domestic activities are difficult to regulate or monitor. Improperly sealed domestic wells facilitate migration of pollutants into underground aquifers.
Septic systems are difficult to maintain and require specific designs due to either too dense or too porous soils. Many systems in place do not meet today's standards. Very little of the subbasin is served by municipal sewers.
Increasing residential home site improvement and filling accounts for loss of wetlands, habitat and reduces flood plain storage potential. Storm water drainage and surface water run-off due to building and paving also create problems.
Surface Mining - 3%: Coal, sand and gravel extraction are present in the subbasin. The Centralia coal mine with its huge open pit on the upper reaches of Hanaford Creek is a significant operation. Siltation and the effect of gravel pits on ground water plus the contamination of old coal dumping and leaching from abandoned mine shafts are problems which are generating increased attention. Strip mining at Hanaford apparently has little adverse effects on water quality. The low sulfur coal which is mined contributes little sulfur to the water and coal washing waters are recirculated, thus preventing severe turbidity problems.
Commercial - 2%: The severe flooding resulting from the filling of flood plains and flood water storage areas is having a huge economic impact on developed areas within and adjacent to such areas. The potential for the introduction of dangerous toxic substances from production processes which are not adequately monitored escalate each year. The potential of a hazardous waste spill from a railroad or truck accident is increasing throughout the Chehalis Basin, as population and traffic grow. Additionally, the increase in storm water run-off in developed areas and the pollutants which migrate with it have not been thoroughly addressed, especially in Lewis County.
The Bucoda dump is closed but still used for illegal household refuse dumping. Leachate from the dump drains to the Skookumchuck River. A number of illicit dumps are located along the Centralia-Alpha road. Other illegal dumps exist throughout the Chehalis Basin, and most have not been officially identified (Beck and Associates, 1975).
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Based upon projections from the Washington State Office of Financial
Management, Forecasting Division (OFM, 1992), the Chehalis River Basin is experiencing growth. Using data from the three major counties (Thurston, Lewis, and Grays Harbor) this growth rate between now and the year 2000 could be 2.7% (low), 26% (high), or 11% (average).
Applying these figures to today's population and housing units results in the following estimates for the Skookumchuck Basin:
By the year 2010, the growth rate could be 6.1% (low), 53% (high), or 23% (average):
While this subbasin has not experienced high or even average growth rates, high rates of growth in nearby population centers are encouraging residential development in rural areas. This spillover effect could cause growth to exceed traditional norms.
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Water quality plans for this subbasin are reviewed in the basin-wide overview section of the Chehalis River Basin Characterization and the Water Quality Assessment.
Back to top or back to Action Plan home page or back to Action Plan Vol II Index Revised and corrected, April 2000 -
1990 Pop. in 2000 Units in 2000
Pop. Units low hi avg low hi avg
22000 9600 22594 27720 24420 9859 12096 10656
1990 Pop. in 2010 Units in 2010
Pop. Units low hi avg low hi avg
22000 9600 23342 33660 27060 10186 14688 11808