1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): VINCENT, Kirk R., NRC Associate with USGS, 3215 Marine St., Boulder CO 80303, kvincent@usgs.gov; ELLIOTT, John G., USGS, Denver Federal Center, Box 25046, Lakewood, CO 80225
Keywords: Channel change, floodplain, mining, riparian
The watershed of the upper Animas River in the San Juan Mountains of Colorado was the site of extensive mining and ore milling during the late 19th and early 20th centuries. The valley bottom (gradient = 0.01) below the ore-mill town of Eureka experienced considerable changes. We used geologic mapping, stratigraphic and sedimentological studies of flood-plain sediments, geochronology, historical records, and geochemical analysis of sediments to examine the history and causes of these changes. Prior to mining, the river valley below the eventual site of Eureka was composed of small and relatively stable, multi-thread, gravel-bedded channels with cohesive banks. These were located within a silty flood plain consisting of dense willow thickets and possibly intermittent and localized beaver ponds. Starting around 1900 AD, mills in and near Eureka supplied (coarse sand to clay size) tailings to the river at rates 50 to 4,700 times greater than the pre-mining production of sediment from hillslopes. A radical change in the stream and flood-plain environment started around the turn of the century and concluded with channel aggradation and burial of flood-plain silt-beds by sheets of gravel. The aggradation ended a short time after ore milling ceased. The environment now consists of shallow and mobile, multi-thread, gravel-bedded channels with non-cohesive banks within a sandy-gravel flood plain devoid of vegetation. Flood-plain sediments in the Animas River valley have naturally high metal concentrations, but ore milling resulted in an increase of metal concentrations by as much as an order of magnitude. Using vanadium as a lithologic tracer for sediment derived from natural erosion of the watershed, we estimate that the fine fraction of historical stream bed and flood-plain deposits contain, in general, 2/3 tailings and 1/3 natural sediments. Channel and flood-plain changes were caused by ore milling, not mining or other activities. Because ore milling has ceased, the substrate is now stable enough for vegetation recovery. However, vegetation has not become reestablished during the 60 years since ore milling ceased. This may be because of metal poisoning or an unfortunate sequence of soil-moisture drought and/or substrate scour events.
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