Geomorphology, flood-plain tailings, and metal transport
John Lambing, Jim
Smith, David Nimick, Charles Parrett (U.S.
Geological Survey) Michael Ramey (R2 Resource Consultants, Inc.), William
Schafer (Schafer & Associates)
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
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.