The Remedial Investigation/Feasibility Study conducted by the U.S. Environmental Protection Agency within the Spokane River Basin of northern Idaho and eastern Washington included extensive data-collection activities to determine the nature and extent of trace-element contaminationwithin the basin. The U.S. Geological Survey designed and operated a streamflow and water quality
monitoring network in the basin during the 1999 water year (October 1, 1998, through September 30, 1999) in support of this Remedial Investigation/Feasibility Study. The objective for
the network was to quantify the absolute and relative magnitude of hydrologic, trace-element, and nutrient loads transported by numerous stream reaches within the Spokane River Basin. Of the 29 water-quality stations in the network, 19 were in the Coeur d?Alene River Basin, 2 were in the St. Joe River Basin, and the remaining 8 were on the Spokane River downstream from Coeur d'Alene Lake. All stations were sampled for whole-water recoverable and dissolved concentrations of cadmium, lead, and zinc. Nitrogen and phosphorus concentrations were sampled at nine stations to determine loads of nutrients into and out of Coeur d'Alene Lake and transported down the Spokane River into the Columbia River. Mean daily discharge during the 1999 water year was about 120 percent of the long-term average. Trace-element loads to the Columbia River were calculated for the basin's terminal station, Spokane River at Long Lake. For whole-water recoverable cadmium, 2,110 pounds, 92 percent of which was dissolved, was delivered to the Columbia River. The Columbia River received 25,000 pounds of whole-water recoverable lead, 29 percent of which was dissolved, from the Spokane River Basin. The largest trace-element load delivered to the Columbia River by the Spokane River was 764,000 pounds of whole-water recoverable zinc, 76 percent of which was dissolved. The primary source of trace-element loads in the Spokane River Basin was the Coeur d'Alene River Basin. The South Fork Coeur d'Alene River was the largest source of dissolved and wholewater recoverable loads of cadmium and zinc. In contrast, the main stem of the Coeur d'Alene River was the largest source of dissolved and wholewater recoverable loads of lead. Within the South Fork, substantial increases in dissolved loads of cadmium, lead, and zinc were detected in excess of those measured by the monitoring network stations upstream from the terminal station, South Fork Coeur d'Alene River near Pinehurst. Much of the added load was attributed to inflow of traceelement-contaminated ground water. Similarly, increases in whole-water recoverable loads of cadmium, lead, and zinc were detected in the South Fork in excess of measured loads; these were attributed largely to erosion and transport of sediment-associated trace elements during increased stream discharge events. Coeur d'Alene Lake received nearly all its trace-element loads from the Coeur d'Alene River. The lake retained the majority of the dissolved and whole-water recoverable loads of lead input to it, but retained almost none of its dissolved and whole-water recoverable loads of zinc. About one-half of the dissolved and whole-water recoverable loads of cadmium was retained in the lake. Within the Spokane River Basin, the largest loads of total nitrogen, 13,000,000 pounds, and total phosphorus, 677,000 pounds, were measuredat Spokane River at Long Lake, the station closest to the Columbia River. At Coeur d'Alene Lake, total nitrogen loads input to the lake from the Coeur d'Alene and St. Joe Rivers totaled 1,890,000 pounds; the lake discharged 2,430,000 pounds. The lake received 253,000 pounds of total phosphorus and discharged 187,000 pounds; thus, 66,000 pounds was retained by the lake.