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U.S. Geological Survey
Scientific Investigations Report 2005-5011

Stream-Sediment Geochemistry in Mining-Impacted Streams: Sediment Mobilized by Floods in the Coeur d'Alene-Spokane River System, Idaho and Washington

By Stephen E. Box, Arthur A. Bookstrom, and Mohammed Ikramuddin


old black-and-white photo taken from a low-flying airplane showing flooded town along riverbank
South Fork of the Coeur d’Alene River at Kellogg, Idaho, near peak streamflow at approximately 10 a.m. P.s.t. on December 23, 1933.

Environmental problems associated with the dispersion of metal-enriched sediment into the Coeur d’Alene-Spokane River system downstream from the Coeur d’Alene Mining District in northern Idaho have been a cause of litigation since 1903, 18 years after the initiation of mining for lead, zinc, and silver. Although direct dumping of waste materials into the river by active mining operations stopped in 1968, metal-enriched sediment continues to be mobilized during times of high runoff and deposited on valley flood plains and in Coeur d’Alene Lake (Horowitz and others, 1993). To gauge the geographic and temporal variations in the metal contents of flood sediment and to provide constraints on the sources and processes responsible for those variations, we collected samples of suspended sediment and overbank deposits during and after four high-flow events in 1995, 1996, and 1997 in the Coeur d’Alene-Spokane River system with estimated recurrence intervals ranging from 2 to 100 years. Suspended sediment enriched in lead, zinc, silver, antimony, arsenic, cadmium, and copper was detected over a distance of more than 130 mi (the downstream extent of sampling) downstream of the mining district. Strong correlations of all these elements in suspended sediment with each other and with iron and manganese are apparent when samples are grouped by reach (tributaries to the South Fork of the Coeur d’Alene River, the South Fork of the Coeur d’Alene River, the main stem of the Coeur d’Alene River, and the Spokane River). Elemental correlations with iron and manganese, along with observations by scanning electron microscopy, indicate that most of the trace metals are associated with Fe and Mn oxyhydroxide compounds. Changes in elemental correlations by reach suggest that the sources of metal-enriched sediment change along the length of the drainage. Metal contents of suspended sediment generally increase through the mining district along the South Fork of the Coeur d’Alene River, decrease below the confluence of the North and South Forks, and then increase again downstream of the gradient flattening below Cataldo. Metal contents of suspended sediment in the Spokane River below Coeur d’Alene Lake were comparable to those of suspended sediment in the main stem of the Coeur d’Alene River above the lake during the 1997 spring runoff, but with somewhat higher Zn contents. Daily suspended-sediment loads were about 100 times larger in the 1996 flood (50-100-year recurrence interval) than in the smaller 1997 floods (2-5-year recurrence intervals). Significant differences in metal ratios and contents are also apparent between the two flood types. The predominant source of suspended sediment in the larger 1996 flood was previously deposited, metal-enriched flood-plain sediment, identified by its Zn/Pb ratio less than 1. Suspended sediment in the smaller 1997 floods had metal ratios distinct from those of the flood-plain deposits and was primarily derived from metal-enriched sediment stored within the stream channel, identified by a Zn/Pb ratio greater than 1. Sediment deposited during overbank flooding on the immediate streambank or natural levee of the river typically consists of sandy material with metal ratios and contents similar to those of the sandy streambed sediment in the adjacent river reach. Samples of overbank deposits in backlevee marshes collected after the 1996 flood have metal ratios similar to those of peak-flow suspended sediment in the same river reach, but generally lower metal contents.

Download the text for Scientific Investigations Report 2005-5011 as a 57-page PDF file (sir2005-5011.pdf; 2.4 MB)

The last 16 pages of this report consist of a set of three tables. These are also available below in their original Excel format (these .xls files also work nicely with assistive technology such as screen readers).

Go to the tables folder and get the three Excel documents (sir2005-5011t1.xls, sir2005-5011t2.xls, and sir2005-5011t3.xls; 168 KB)

Download a compressed package of all three tables as a .zip file (; 56 KB)

For questions about the content of this report, contact Steve Box

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Maintained by: Michael Diggles
Created: December 12, 2005
Last modified: December 12, 2005 (mfd)