BASELINE DEPOSITION RATES, LEAD-RICH SEDIMENT, COEUR D’ALENE RIVER FLOODPLAIN, IDAHO

In north Idaho, downstream from the Coeur d’Alene (CdA) silver-lead-zinc mining region, lead-rich sediments (containing at least 1000 ppm of lead) cover about 60 km² of the 80-km² floor of the main stem of the CdA River valley. Although tailings have not been discarded directly into tributary streams since 1968, frequent floods continue to transport metal-enriched sediment down-valley and onto the CdA River floodplain.

Between 1991 and 1995, we measured thickness of sediments above the 1980 Mt. St. Helens volcanic-ash layer at 125 sites within the area covered by lead-rich sediments. We calculated a baseline sedimentation rate (cm/10y) for each site by dividing thickness of post-1980 sediment (cm) by the time span (y/10) from 1980 to the sampling date. We collected vertical channel samples of sediments from the 1980 marker layer to the surface at 77 of the measurement sites. We had those samples analyzed for lead (Pb) and other metals contained in tailings from the CdA mining region. We classified sample sites by depositional setting (riverbank, levee, palustrine, littoral, or limnetic), and calculated summary statistics for each setting, and an area-weighted average of the depositional-setting medians.

Means and standard deviations of sedimentation rates and Pb concentrations in Pb-rich sediments of the baseline interval are: 6.9 ± 5.3 cm/10y and 3400 ± 990 ppm for riverbanks, 3.7 ± 3.8 cm/10y at 3600 ± 1600 ppm for levees, 2.5 ± 1.7 cm/10y at 2800 ± 2100 ppm for lateral marshes, 2.4 ± 1.4 cm/10y at 2300 ± 900 ppm for littoral zones of lateral lakes, and 3.8 ± 0.8 cm/10y at 4100 ± 1200 ppm for limnetic zones of lateral lakes. Area-weighted averages of medians for depositional settings are 2.9 cm/10y at 2900 ppm of Pb for the non-riverine 54 km² of the floodplain that is covered by Pb-rich sediments.

Large volumes of Pb-rich sediments, previously deposited along floodwater flow paths, continue to act as secondary sources of Pb-rich sediment. Unless removed or stabilized, secondary sources of Pb-rich sediment in the South Fork basin, plus an estimated 10 Mm³ of Pb-rich sediments in the CdA River channel, and 7 Mm³ along its banks and levees, probably will continue to supply Pb-rich sediment to floodwaters for several centuries.