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Causes and sources of water quality impairment in the Upper Chehalis River, Washington

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  • Water quality problems have been identified in the Chehalis River in the vicinity of Centralia and Chehalis since the 1960's. In 1992, the Washington State Department of Ecology listed four Chehalis River segments, three segments of the Newaukum River, two segments of the Wynoochee River, and three other tributaries - Salzer, Dillenbaugh, and Wildcat Creeks - as Water Quality Limited. Of these twelve waterbody segments, seven either include, or are tributary to, the 9.4 mile stretch of the Chehalis River from Scammon Creek to the Newaukum River (River Mile LRMI 65.8 to RM 75.2), referred to as the Centralia reach. The Chehalis River displays the hydrologic characteristics typical of Western Washington rivers with minimal snowmelt input. Critical low flows occur in later summer, and high flows are almost exclusively associated with intense rainfall. Ambient water quality data show long-term and widespread problems from water quality impairment caused by low dissolved oxygen and high water temperatures during summer low flows, and elevated bacteria counts and turbidity during winter high flows. Since the passage of the Federal Water Pollution Control Act amendments of 1972, water pollution from point source discharges has decreased. This is true in the Chehalis River Basin, yet water quality in the Centralia reach remains badly impaired. Sources of persistent water quality impairment in the Centralia reach include "natural" conditions (the Centralia reach is deep, slow, and stratified); point sources discharges (waste water treatment plants. food-processing waste); storm water runoff from a variety of nonpoint sources; and poor quality water from the Chehalis River upstream and other tributaries of the Centralia reach. Water quality conditions in the Centralia reach resemble the nutrient-rich conditions of many eutrophic lakes in Western Washington, except that nitrogen rather than phosphorus limits algal growth. Data indicate that the Chehalis waste water treatment plant is probably the greatest contributing source of phosphorus loading to the Centralia reach.
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