Nutrient Impaired 303(d) Streams for the Pacific Northwest

“Under section 303(d) of the 1972 Clean Water Act, states, territories, and authorized tribes are required to develop lists of impaired waters. These impaired waters do not meet water quality standards that states, territories, and authorized tribes have set for them, even after point sources of pollution have installed the minimum required levels of pollution control technology. The law requires that these jurisdictions establish priority rankings for waters on the lists and develop TMDLs for these waters” (U.S. Environmental Protection Agency, 2011). Waterways represented in this data set are a subset of these EPA 303 (d) listed streams that were classified as nutrient impaired. Nutrient impaired streams include those listed for nutrients, agriculture, crop sources, pH, dissolved oxygen, chlorophyll a, nitrate, phosphorus, and nitrogen. Total phosphorus and total nitrogen values attributed to stream reaches are based on EPA Level III Ecoregions aggregations for the National Nutrient Strategy values (U.S. Environmental Protection Agency, 2000). Values are based on 25th percentiles for aggregate nutrient reference conditions for both phosphorus and nitrogen. Data relate to a 2002 time period, with data collected from each of the state agencies as close to that period as possible. See processing steps for more detail on individual state data sets and time periods.