Title
Geomorphology, flood-plain tailings, and metal transport in the upper Clark Fork Valley, Montana
Abstract
The Clark Fork valley of western Montana is adversely affected by metals derived from past mining and smelting activities. To aid remediation planning by the U.S. Environmental Protection Agency, the sources, transport, and deposition of metals were assessed. This required examining the geomorphic history and deposition of flood- plain tailings, determining river migration rates across the flood plain, and tracking the current transport and accumulation of sediment and metals through the 120-mile reach from the Warm Springs Ponds to Milltown Reservoir.
Flood-plain tailings were mapped and the range of metal concentrations determined. Average annual meander-migration rates of the Clark Fork in the Deer Lodge valley were quantified from aerial photographs taken in 1960 and 1989. Estimated sediment and copper loads from individual sources were integrated in mass-balance calculations to estimate input, transport, and deposition. Transport rates were calibrated to water-quality monitoring data and hydrologic conditions of 1985-95.
Under post-1990 source conditions and 1985-95 hydrology, mass-balance calculations indicate that streambank erosion is the largest source of copper input to the river, comprising about 56 percent of the total copper input along the 120-mile reach. Upstream input (Silver Bow and Warm Springs Creeks) accounts for about 5 percent, whereas tributaries and streambed exchange each account for about 8 percent. Flood-plain runoff and ground water together account for about 24 percent of the copper input. Not all of the copper input to the river is transported downstream; instead, about 47 percent is deposited on point bars. When 1985-95 estimates of copper input are adjusted to long-term (1930-95) flow conditions, the percent contributions from individual sources are similar, with bank erosion still providing the largest input of copper (60 percent).
High flows in 1996 and 1997 resulted in larger sediment and copper loads than those measured in 1991-95. Average annual copper yields (tons per river mile) for the expanded 1991-97 period, which had flows similar to long-term hydrology, were 2-3 times larger than yields for 1991-95. In particular, reaches between Deer Lodge and Turah contributed substantially more copper per river mile in 1996-97 than during the low-flow years of 1991-95. These reaches actually contributed as much copper per mile as the reach above Galen. The largest copper yield per mile is contributed from the reach between Galen and Deer Lodge, regardless of flows.
This material is published in U.S. Geological Survey Water-Resources Investigations Report 98-4170.
Start Date
14-4-2000 12:00 AM
End Date
14-4-2000 12:00 AM
Document Type
Poster
Geomorphology, flood-plain tailings, and metal transport in the upper Clark Fork Valley, Montana
The Clark Fork valley of western Montana is adversely affected by metals derived from past mining and smelting activities. To aid remediation planning by the U.S. Environmental Protection Agency, the sources, transport, and deposition of metals were assessed. This required examining the geomorphic history and deposition of flood- plain tailings, determining river migration rates across the flood plain, and tracking the current transport and accumulation of sediment and metals through the 120-mile reach from the Warm Springs Ponds to Milltown Reservoir.
Flood-plain tailings were mapped and the range of metal concentrations determined. Average annual meander-migration rates of the Clark Fork in the Deer Lodge valley were quantified from aerial photographs taken in 1960 and 1989. Estimated sediment and copper loads from individual sources were integrated in mass-balance calculations to estimate input, transport, and deposition. Transport rates were calibrated to water-quality monitoring data and hydrologic conditions of 1985-95.
Under post-1990 source conditions and 1985-95 hydrology, mass-balance calculations indicate that streambank erosion is the largest source of copper input to the river, comprising about 56 percent of the total copper input along the 120-mile reach. Upstream input (Silver Bow and Warm Springs Creeks) accounts for about 5 percent, whereas tributaries and streambed exchange each account for about 8 percent. Flood-plain runoff and ground water together account for about 24 percent of the copper input. Not all of the copper input to the river is transported downstream; instead, about 47 percent is deposited on point bars. When 1985-95 estimates of copper input are adjusted to long-term (1930-95) flow conditions, the percent contributions from individual sources are similar, with bank erosion still providing the largest input of copper (60 percent).
High flows in 1996 and 1997 resulted in larger sediment and copper loads than those measured in 1991-95. Average annual copper yields (tons per river mile) for the expanded 1991-97 period, which had flows similar to long-term hydrology, were 2-3 times larger than yields for 1991-95. In particular, reaches between Deer Lodge and Turah contributed substantially more copper per river mile in 1996-97 than during the low-flow years of 1991-95. These reaches actually contributed as much copper per mile as the reach above Galen. The largest copper yield per mile is contributed from the reach between Galen and Deer Lodge, regardless of flows.
This material is published in U.S. Geological Survey Water-Resources Investigations Report 98-4170.