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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Progress Report on Surficial Deposits and Geomorphology of Major Drainages of the Upper Animas River Watershed, Colorado

By Rob Blair¹

The surficial deposits and geomorphology of the upper Animas River watershed were examined during fall 1997 with the use of color 1:19,000-scale aerial photographs. The goals of the study include: (1) mapping surficial deposits in the upper Animas River, Cement Creek, Cunningham Gulch, Mineral Creek, and the Middle Fork and South Fork of Mineral Creek drainages; (2) assessing the impact of mining activities on the above tributaries; (3) locating potential sample sites representative of pre-mining deposition; and (4) constructing a brief Holocene geomorphic history of the watershed along these drainages.

Surficial geologic strip maps that represent about one kilometer in width and approximately 125 meters above the modern flood plain have been constructed along each tributary. These maps primarily include fluvial, colluvial, and glacial units. Direct human interference of surficial deposits includes gravel mining and channel modification. Indirect human influence includes aggradation of sand and gravel resulting from increased sediment loads related to the erosion of cleared slopes, mine dumps, and tailings piles.

Mining and associated human activity have directly or indirectly modified an estimated 80 to 90 percent of the fluvial surfaces in the Animas valley from Silverton to Animas Forks. Just north of Howardsville, the flood plain has been entirely reworked. Cunningham Creek and Cement Creek occupy narrow valleys; therefore, road construction and mining activity adjacent to the stream can easily impact the whole channel. For example, the collapse of Lake Emma into the Sunnyside Mine in 1978 and its associated flush of sediment-laden waters down Cement Creek coated the entire active flood plain with muddy sediment. Approximately 50 percent of the Mineral Creek and the South Fork of Mineral Creek valley floors have been modified by human activity. Although these tributary channels have been modified, it appears that it takes only a few years or less for them to revert back to a quasi-equilibrium channel form.

The most prominent pre-mining terrace deposits are found in the wider valley floors along the upper Animas River and Mineral Creek. The most distinct terraces are located near the junction of Cunningham Creek and the upper Animas River. Glacial moraine and high-valley-floor deposits indicate that upper Mineral Creek and Middle Fork of Mineral Creek were dammed when an ice stream from South Mineral Creek blocked the upper tributary drainages approximately 16,000 years ago. Local changes in stream gradient can be found in all tributaries associated with opposing debris fans and rare landslides (Cement Creek and South Fork of Mineral Creek.)

¹Fort Lewis College, Department of Geology, Durango, CO 81302 (blair_r@fortlewis.edu)

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