Topography and Geology
In this subbasin, the Chehalis River meanders through a flat flood plain valley. The land is largely being cleared for agricultural purposes on the west side of the river. The east side of the river has been heavily developed, and is the location of Centralia, Chehalis, and Fords Prairie. The small community of Galvin is west of the river. Here, the Chehalis River averages about 50 yards in width, flowing through a cut bank area and consists mostly of pools with an occasional gravel riffle. The banks are primarily forested with deciduous timber and brush. As in adjacent subbasins, the uplands are in timber production. Residential areas are scattered throughout these forest and agricultural zones. Flooding is a serious problem in this area and in adjacent tributary stream valleys.
The average mean annual discharge of the Chehalis River, at the USGS gaging station near Grand Mound was 2836.8 cfs in the years between 1929 and 1979. The highest discharge recorded in these years was 49,200 cfs on January 1, 1972, but during the January, 1990 flood, flows were the highest on record, reaching 68,700 cfs on January 10, 1990. A low mean flow of 85 cfs was recorded for one day in 1968 (USGS, 1992).
This section of the Chehalis River drains 118 square miles. It consists of minor streams between the mouth of the Newaukum and the gaging station at Grand Mound.
The landscape consists of smoothly rounded hills and slopes that curve gently downward to valley floors. This is a typical ridge and ravine landscape of humid regions that support dense plant cover.
The upland area was carved by the process of erosion and its character has developed as a function of the vegetation that protects the soil from most erosive processes, except landslides and soil creep. Landslides are fairly common in the uplands because the region has a deep cover of wet soil that rests on weak bedrock. Elevations above sea level range from 100 to 1,800 feet.
The lowland area from Centralia northward to Grand Mound consists of Quaternary age outwash deposits of sand and gravel and is generally flat to very gently sloping. Elevations range generally up to 100 feet.
The sedimentary deposits of the uplands yield only small amounts of ground water, suited for limited domestic use. The outwash deposits, on the other hand, are very permeable and yield large quantities of high quality water. These aquifers are currently being tapped by the City of Centralia to augment their water supply. Minerals in this subbasin include sand, gravel, clay, and coal.
Major tributary streams on the east side of the river are Salzer, Coal, Dillenbaugh and Prairie Creeks, while Lincoln, Scammon, and Coal creeks enter from the west.
Prairie Creek enters the Chehalis River at Grand Mound, and is largely intermittent. The lower portion lies in the Chehalis River flood plain and is subject to seasonal flooding. The gradient increases somewhat in the middle portion and contains some gravel riffles. The upper portion lies east of I-5 and consists largely of swamps and beaver dams.
Dillenbaugh Creek also flows through the Chehalis River flood plain in its lower reaches, with the main creek valley being used for agricultural and residential purposes. Upland areas are in timber production. The upper portion of this stream is flanked by brush and deciduous trees and contains some gravel riffles. Several business areas are located along the lower part of the stream.
Scammon Creek and Coal Creek (WRIA 23-0877) enter the Chehalis River from the west and are similar in character. The lower reaches lie in the flood plain, and their valleys are being cleared for agricultural and residential uses. The upper reaches are in timber production. The streams have low gradients and have predominantly sandy bottoms.
Lincoln Creek flows into the Chehalis River near the town of Galvin. The lower reaches are in broad stream valleys and flood annually to at least RM 4.0. The soils in these lower valleys are a heavy clay alluvium. Stream gradients are very low with sand and silt being the primary bed material. The banks are low cut, with deciduous brush and trees adjacent to the water course. The upper reaches flow through narrower valleys and gravel is the predominant stream bed material. The adjacent slopes are in timber production. The Lincoln Creek watershed drains an area of 43.2 square miles and has a mean annual discharge of 123 cfs (Ecology, 1980).
Salzer Creek enters the Chehalis River between Centralia and Chehalis from the east. The stream is similar to Lincoln Creek in physical characteristics, but is only about three-fourths as long as is Lincoln Creek. The Salzer Creek watershed drains an area of 17.3 square miles and the mean annual discharge is 38 cfs (USGS, 1992).
The valley holding Salzer Creek is much less used for agricultural production, but has been highly developed for residential and commercial uses in the lower third of its length. Serious flood damage is incurred by increasing residential encroachment on the flood plain, both by reduction of valley flood storage and on the structures constructed in the floodplain.
Coal Creek (WRIA-23-0872) is a short stream that flows from the east, just north of Chehalis, and enters Salzer Creek between Chehalis and Centralia. The lower reaches of Coal Creek are heavily developed commercially. The stream bed is of low gradient and primarily has a silty, sandy bed. The upper reaches are in a narrow valley, bordered by rural homesites, with the adjacent slopes in timber production.
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The climate of the subbasin is characterized by generally mild temperatures year round. Precipitation is generally rain with some snowfall. Moderate accumulations occur at higher elevations. Average precipitation for this area ranges from 45 to 60 inches annually.
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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 Chehalis Grand Mound Basin, the subbasin population appears to be 18,000.
Based upon an average for the entire Chehalis River Basin of 2.29 people per household, there are close to 7,900 households in this subbasin.
Back to top or back to Action Plan home page or back to Action Plan Vol II Index Beneficial Uses of Water
Surface water uses in this reach of the Chehalis are typical of those of the basin as a whole, and include domestic, industrial, and agricultural water supplies, aesthetic enjoyment, fishing, and other water-based recreation. Fish and wildlife habitat is considered a beneficial use of water, but is impaired in this portion of the river, especially in the Centralia-Chehalis area, due to seasonal low flows, high temperature, pollution, and low dissolved oxygen levels.
Back to top or back to Action Plan home page or back to Action Plan Vol II Index Existing Land Use Patterns
Land use in this subbasin is very diverse and includes a large urban area and associated rural populations around Centralia, Chehalis, and Fords Prairie. The population is concentrated along the I-5 corridor but has been expanding into adjacent forested uplands and valleys.
Urban expansion is somewhat restricted at this time because of moratoriums on construction due to ground water contamination or the threat of contamination.
Industrial uses in the area include a cannery and frozen food processing plant, and a wood products site. Development of the Port of Centralia and Port of Chehalis is increasing industrial use in the area.
Agriculture in the subbasin occurs in the form of large commercial poultry, dairy and beef operations, a meat processing plant and a variety of associated cropping such as hay, feed grains, and peas. Commercial berry production is an important agricultural industry.
Commercial forestry is an important and major land use in the subbasin and includes operations on small private, industrial and state ownerships. The majority of forestry operations occur in the forested uplands lying east and west of the I-5 corridor and extend to the headwaters of all of the major tributaries of the Chehalis River.
Numerous parks and recreation areas around the Centralia/Chehalis area provide extensive recreational opportunities throughout the year. Dispersed recreation in the form of hunting, fishing, bird watching and all terrain vehicle trails occur throughout the subbasin on both state and private forest lands.
There are no Native American groups residing in the subbasin. The Chehalis Indian reservation, however, is immediately downstream and all of the activities, including residential, industrial, agricultural and forestry have a direct effect on water quality and fisheries on which these people depend. Although little or no information is available regarding Native American cultural sites in this subbasin, it does not preclude their eventual discovery.
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The streams, sloughs, and small lakes in this subbasin provide needed habitat for many species of fish including trout and salmon as well as spiny-rays such as bass, perch and sunfish. The lower reaches of the tributaries, due to their silty or sandy bed material, provide suitable summer rearing as well as escapement during times of flood. The upper reaches, having more incidence of gravel and being colder, serve as anadromous spawning areas. Shellfish, notably fresh water mussels, are common in the main Chehalis River.
The upland areas in the subbasin are forested primarily with young to mature stands of Douglas fir and associated conifers such as western red cedar and hemlock. Stands of red alder occupy the wetter sites and riparian areas.
The low lying areas are predominantly in agricultural crops and native grasses in the extremely wet valley soils of the Salzer Creek watershed and lower Lincoln Creek. In some of the wet valleys, noxious plants such as purple loose strife are becoming established.
The variety of habitat in this subbasin is an important factor in the diversity of wildlife found here. A wide range of forest age classes in the forested areas provide cover for big game species such as deer, bear, and elk as well as many upland bird species and non-game animals. Fur bearing animals common to other Chehalis subbasins can be found here as well as many types of migratory fowl.
Back to top or back to Action Plan home page or back to Action Plan Vol II Index Anticipated Population and Land Use
Based upon projections from the Washington State Office of Financial Management, Forecasting Division (OFM, 1992), the entire 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 Middle Chehalis Basin (above Grand Mound):
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
18000 7900 18486 22680 19980 8113 9954 8769
1990 Pop. in 2010 Units in 2010
Pop. Units low hi avg low hi avg
18000 7900 19098 27540 22140 8382 12087 9717