Assessment of selected constituents in surface water of the upper Snake River basin, Idaho and western Wyoming, water years 1975-89

In 1991, a water-quality investigation of the upper Snake River Basin was initiated as part of the U.S. Geological Survey's National Water-Quality Assessment Program. The initial task of the assessment was to compile and analyze available nutrient, suspended sediment, and pesticide data collected in the basin. For analysis of nutrients and suspended sediment, data collected during water years 1980-89 were used. For pesticides, an additional 5 years of data were included for a total assessment period encompassing water years 1975-89. Nearly 9,000 analyses of nutrients and suspended sediment from more than 450 stations were retrieved from the U.S. Environmental Protection Agency STORET and U.S. Geological Survey WATSTORE data bases. Nineteen stations had sufficient analyses for quantitative assessment. Of the 19 stations analyzed, 4 are located on relatively unaffected stream reaches, 8 are at or near mouths of tributary basins affected by agricultural activities, and 7 are on the main stem of the Snake River. Data indicate that nitrite plus nitrate and total phosphorus concentrations generally increased in a downstream direction along the Snake River; concentrations were largest at the mouths of drainage basins tributary to the Snake River. Water-quality stations were categorized as unaffected or minimally affected, agriculturally affected, or main stem to compare nutrient concentrations between drainage basins of differing land use/land cover. Concentrations of nitrite plus nitrate, total nitrogen, dissolved orthophosphate, and total phosphorus were significantly (p