Page 816 - The Toxicology of Fishes

796 The Toxicology of Fishes

584
500
Clark Fork River 794 773 558

400
Mean Density of All Trout per Hectare 300 Creek km Deer Lodge
Rock

Butte
30

200

100 Clark Fork/Silver Bow
Reference

0
7/3 4/5 3/5 3 4 2/3 4 3 3 4 4 2d 4 2u 1/2 2 4/6 4/6 3/6
Warm
Blackfoot Flint Creek Deer Springs Butte
River Lodge Ponds
Silverbow
Creek

FIGURE 19.11 Mean densities of all species of trout combined in Silverbow Creek and the Clark Fork River, compared
to densities in carefully matched reference reaches in other Montana streams. Reference segments were identified in the
Big Hole, Ruby, and Beaverhead rivers and in the Rock, Flint, and Bisson creeks in Montana. (Adapted from Hillman,
T.W. and Chapman, T.W., in Aquatics Resources Injury Assessment Report, Upper Clark Fork River Basin, Lipton, J. et
al., Eds., Report to the State of Montana Natural Resource Damage Program, Helena, MT, 1995; Hillman, T.W. et al., in
Aquatics Resources Injury Assessment Report, Upper Clark Fork River Basin, Lipton, J. et al., Eds., Report to the State of
Montana Natural Resource Damage Program, Helena, MT, 1995.)

Missouri rivers support brown, rainbow, brook, and cutthroat trout. In contrast, Silverbow Creek supports
no fish. The Upper Clark Fork River supports almost entirely brown trout, with rainbow trout first appearing
in abundance 170 km from the mining and smelting activities, below the confluence of Rock Creek. Typical
native species—bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarki
lewisi)—have been virtually eliminated from the main-stem upper river (Malouf, 1974).
Comparisons of trout abundance could be misleading if large differences in geology, geomorphology,
channel conditions, and habitat exist between streams; this was one source of contention in the early
discussions of mining impacts. Hillman et al. (1995) conducted studies that compared trout populations
in the Clark Fork and Silverbow Creek with reference areas that were matched on several variables.
Silver Bow Creek and the Clark Fork River were first classified in terms of ecoregion, geology, geomor-
phology, and habitat conditions. This resulted in 19 discrete survey segments, 4 in Silver Bow Creek and
15 in the Clark Fork River. References were then selected based on similar ecoregion, geology, land type
association, valley bottom type, stream state type, elevation, valley grade, stream sinuosity, stream grade,
dominant substrate, riparian vegetation type, and channel type. Habitat surveys in both Silver Bow and
Clark Fork and the reference segments were also included in the comparisons. Habitat measurements
included channel width; wetted perimeter width; riffle, run, and pool widths; pool rating; bank angle;
average and thalweg depths; substrate; bank cover; vegetation overhang; canopy cover; bank alteration;
woody debris; sun arc; and bank undercuts. In addition, basic water-quality characteristics were deter-
mined (dissolved oxygen, dissolved organic carbon, temperature, nitrate/nitrogen, conductivity, hardness,
and alkalinity). Reference segments were identified within and outside the Clark Fork watershed.