LEGACY MINE TAILINGS AND ALLUVIAL DEPOSITS ALONG THE BELLE FOURCHE RIVER, SOUTH DAKOTA: AN INTEGRATED, MULTI-SCALE INVESTIGATION OF CHANNEL DYNAMICS

The Belle Fourche River is the largest tributary by volume to the Cheyenne River, which in turn is a major tributary to the Missouri River in South Dakota. For nearly a century, the Belle Fourche River received large volumes of raw mine tailings from the Homestake Mine near Lead, South Dakota. In 1977, the influx of mine tailings was virtually eliminated by the closure of a tailings retention pond at the mine. The legacy mine tailings remaining in the Belle Fourche river system are a potential ongoing ecological and human-health risk, because they contain toxic concentrations of heavy metals, including arsenic. Previous research has indicated that significant proportions of these legacy mine tailings were stored in the floodplains and terraces adjacent to the river. In 2008, record flooding widened the main channel of the Belle Fourche River and remobilized some of the legacy mine tailings, and these tailings may continue to contaminate the bottomlands and the Missouri River system for decades to come. To understand the nature and magnitude of the ongoing risk associated with legacy mine tailings, the USGS, in cooperation with the U.S. Army Corps of Engineers, is documenting the dynamics of river channel adjustment at different time scales and spatial scales using a combination of LiDAR, geomorphic mapping, dendrochronology, and field geochemistry. The channel adjustments will be compared to the hydrologic record and mining history. This project will provide a case-study of river channel response to both increased and decreased sediment loads, with a special emphasis on the effects of historical land management and its ongoing influence on the water-quality of receiving streams.