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Water-quality samples were collected at 10 sites in the Clark Fork-Pend Oreille and Spokane River Basins in water years 1999 – 2001 as part of the Northern Rockies Intermontane Basins (NROK) National Water-Quality Assessment (NAWQA) Program. Sampling sites were located in varied environments ranging from small streams and riv­ ers in forested, mountainous headwater areas to large rivers draining diverse landscapes. Two sam­ pling sites were located immediately downstream from the large lakes; five sites were located downstream from large-scale historical mining and oreprocessing areas, which are now the two largest “Superfund” (environmental remediation) sites in the Nation. Samples were collected during a wide range of streamflow conditions, more frequently during increasing and high streamflow and less frequently during receding and base-flow condi­ tions. Sample analyses emphasized major ions, nutrients, and selected trace elements. Streamflow during the study ranged from more than 130 percent of the long-term average in 1999 at some sites to 40 percent of the long-term average in 2001. River and stream water in the study area exhibited small values for specific con­ ductance, hardness, alkalinity, and dissolved solids. Dissolved oxygen concentrations in almost all samples were near saturation. Median total nitro­ gen and total phosphorus concentrations in sam­ ples from most sites were smaller than median concentrations reported for many national programs and other NAWQA Program study areas. The only exceptions were two sites downstream from large wastewater-treatment facilities, where median concentrations of total nitrogen exceeded the national median. Maximum concentrations of total phosphorus in samples from six sites exceeded the 0.1 milligram per liter threshold recommended for limiting nuisance aquatic growth. Concentra­ tions of arsenic, cadmium, copper, lead, mercury, and zinc were largest in samples from sites downstream from historical mining and ore-processing areas in the upper Clark Fork in Montana and the South Fork Coeur d’Alene River in Idaho. Con­ centrations of dissolved lead in all 32 samples from the South Fork Coeur d’Alene River exceeded the Idaho chronic criterion for the protection of aquatic life at the median hardness level measured during the study. Concentrations of dissolved zinc in all samples collected at this site exceeded both the chronic and acute criteria at all hardness levels measured. When all data from all NROK sites were com­ bined, median concentrations of dissolved arsenic, dissolved and total recoverable copper, total recov­ erable lead, and total recoverable zinc in the NROK study area appeared to be similar to or slightly smaller than median concentrations at sites in other NAWQA Program study areas in the Western United States affected by historical mining activi-

u.s. geological survey open file report/open-file report

Summary of surface-water-quality data collected for the Northern Rockies Intermontane Basins National Water-Quality Assessment Program in the Clark Fork-Pend Oreille and Spokane River basins, Montana, Idaho, and Washington, water years 1999-2001