Response of the Upper Animas River Downstream from Eureka to Discharge of Mill Tailings

The Animas River watershed study area in the San Juan Mountains of Colorado was the site of extensive mining and milling during the late 19th and early 20th centuries. We focus on the reach of the Animas River between Howardsville and Eureka, which receives water and sediment from the headwaters area of the upper Animas River. Using geologic mapping, stratigraphic and sedimentological studies of flood-plain sediment, geochronology, historical records, oblique and aerial photographs, sediment transport calculations, and geochemical analysis of sediment, we conclude the following. Prior to mining, the river valley near the eventual town and mill site at Eureka was composed of shallow braided, gravel bedded channels, and multi-thread channels with well-defined banks downstream. The multi-thread channels were located within a silty flood plain consisting of willow thickets and intermittent and localized beaver ponds. A radical change in the stream and flood-plain environment started shortly after A.D. 1900 and concluded downstream of Eureka with aggradation of channels and burial of older sediment with sheets of gravel. This was caused by milling, not mining or other anthropogenic activities. Mills in and near Eureka supplied huge quantities of tailings to the river, at rates 50 to 4,700 times greater than the natural rate of production of sediment from hillslopes prior to mining. The tailings were introduced at a location where the valley profile is strongly concave, where the river is inherently prone to braiding. The introduction of sand-sized tailings altered the bed-load sediment transport regime, and the result was aggradation and stream braiding. Flood-plain sediment deposited prior to mining has naturally high zinc concentrations close to 1,000 ppm, but the introduction of tailings resulted in an increase of zinc concentrations to more than 4,000 ppm on average and locally to as much as 10,000 ppm. The presence of tailings in stream sediment also increased the concentration of cadmium, copper, lead, and manganese, above the concentrations in prehistorical sediment. Using vanadium as a lithologic tracer for sediment derived from hillslope erosion, we estimate that the fine fraction of streambed and flood-plain sediment deposited after A.D. 1900 contains, in general, two-thirds tailings and one-third natural sediment. Of the original tailings produced, between 70 and 80 percent has been flushed from the study reach by streamflow, and on the order of 10 percent was mechanically reclaimed and placed in a repository. Of the tailings that remain in the study reach, on the order of 10 percent-exists at the surface as easily identifiable tailings beds, but the majority is in the subsurface dispersed within gravel deposits.