Page 805 - The Toxicology of Fishes

Mining Impacts on Fish in the Clark Fork River, Montana: A Field Ecotoxicology Case Study 785

TABLE 19.1
Copper Concentrations in Fine-Grained (<64 µm) Sediment and the Metal-Tolerant Caddisfly
Hydropsyche Species from above Warm Springs Ponds, Immediately below the Outfall from the
Pond System (–2.1 km), and Sites Further Downstream in the Clark Fork River
Kilometers from Head Copper Concentration (µg/g dw) Copper Concentration (µg/g dw)
of Clark Fork River in Hydropsyche Species in Sediments
Silverbow above ponds 365 1663
–2.1 (Silverbow at the outfall 31 169
of the Warm Springs Ponds)
11 122 1650
45 114 1200
85 62 748
190 54 277

increased. Within 10 km, metal contamination reappears as banks begin slumping into the river (Table
19.1). Metal loads in the water of the Clark Fork between 1991 and 1995 showed that erosion of historic
mine tailings from highly contaminated banks and floodplains in the approximately 40 km below the
pond anomaly was responsible for most of the continuing contamination downstream (Hornberger et
al., 1995). Anecdotal evidence suggests that the system of ponds itself is a uniquely productive habitat.
This appears to help support a biologically rich habitat just below their outlet. Visibly hardy trout
populations, presumably migrants from streams less contaminated than Silverbow Creek, live and grow
in the ponds and appear to migrate into the river downstream.
Invertebrates are highly abundant in the stream just below the ponds, although the community lacks
metal-sensitive species. It is the only habitat in the upper Clark Fork where amphipods (Hyalella) are
found. Brown trout in this section, and a few kilometers downstream, are more abundant than anywhere
else in the river. From 1973 to 1978, brown trout averaged about 500 trout per mile during spring
sampling in this area (Figure 19.3). During the period from 1979 to 1985, the numbers of brown trout
nearly tripled, averaging about 1300 trout per mile. Brown trout densities of 1600 to 2400 trout per mile
were recorded from 1985 to 1996, but in 1998, only 750 brown trout per mile were found.
In the early 1990s, the first 2 km below the ponds was characterized by extensive willow growth along
a shallow riparian zone. Tailings-laden banks did not occur for about 5 km downstream. Remediation in
the late 1990s (Figure 19.4) removed the willows and associated soils and replaced them with a relatively
broad cobble riparian zone. This removed some metal contamination but also destabilized the bed of the
stream. A paucity of studies since 1998 prevents further determination of whether the long-term effects
of remediation have been to improve or reduce fish abundance in this small area; nevertheless, a peak
of trout abundance remains in this area. The peak confounds correlation of trout densities in the Clark
Fork with proximity to mining activities. In the absence of exposure data showing the pond effect, this
peak has exacerbated contentious discussions regarding the impact of the mining and smelting activities.

Upper Clark Fork River
Floodplain and Bank Contamination
The modern watershed of the river still has widespread slickens through its first 60 km (in the Deer
Lodge Valley). Cut banks containing a meter of contaminated fine sediments occur all along the upper
308 km of the river. These thick banks of fine-grained sediments typically are contaminated with metals
characteristic of mine wastes (Axtmann and Luoma, 1991; Moore et al., 1989). Their lead isotope ratios
match those of the mine, rather than the watershed (R. Bouse, USGS, unpublished data). In the upper
60 km of the Clark Fork River, copper concentrations are 1000 times greater than is typical in the banks
of tributaries. Banks closer to Milltown Reservoir are visually similar to topsoil, but copper concentrations
can be 20 to 25 times higher than is typical of uncontaminated tributaries. Thus, every time the upper
river meanders and undercuts a bank, sediments that contain substantial concentrations of metal are
reintroduced to the river by the slumping bank. The distributed nature of the contamination in the