for almost a century, the U.S. Geological Survey has collected hydrologic data at a network of stream-gaging stations throughout the Coeur d'Alene Lake and River drainage basin. Since 1990, extensive water-quality data have been collected for a comprehensive study of potential eutrophication of Coeur d'Alene Lake and for assessment of the environmental effects of past mining and ore-processing activities in the South Fork Coeur d'Alene River valley. Although the South Fork Coeur d'Alene River provided only about 20 percent of the Coeur d'Alene River's annual discharge to Coeur d'Alene Lake, it contributed as much as 84 percent of the annual cadmium and 83 percent of the annual zinc loads entering the Lake. The South Fork contributed at most 14 percent of the annual lead and 21 percent of the copper loads carried by the Coeur d'Alene River to Coeur d'Alene Lake. Cadmium, copper, and zinc loads more than doubled between the upstream and downstream boundaries of the Bunker Hill (Kellogg, Idaho) Superfund site in water years 1993 and 1994; lead load increased 24 percent and 33 percent, respectively, in water years 1993 and 1994. Zinc was transported primarily in a dissolved or colloidal phase, the major source being the South Fork Coeur d'Alene River valley, downstream from the Elizabeth Park gaging station. Lead was transported primarily as particulate material, the major source being sediments eroded from the main-stem Coeur d'Alene River channel and flood plain. Annual lead and zinc loads at Rose Lake were significantly larger than at Harrison or Cataldo, indicating entrainment of trace elements in the reach between Cataldo and Rose Lake, and subsequent deposition or loss in the reach between Rose Lake and Harrison.
