Water Quality District
Milltown Dam: Fisheries
Due to concerns that the February, 1996 ice jam on the Blackfoot River might cause serious damage to the dam, Montana Power Company officials ordered an emergency drawdown of the reservoir. Ice coming down both the Clark Fork and Blackfoot Rivers scoured sediments from the reservoir, causing extreme water quality standards violations downstream. Water samples collected by the Missoula City-County Health Department in February, 1996 found total recoverable copper concentrations as high as 770 parts per billion (ppb) after the drawdown, and 440 parts per billion before it occurred. The Montana Water quality standard for protection of fish and other aquatic life was 18 ppb.
Following the event, Montana Fish, Wildlife and Parks completed fish surveys that showed a 62 percent loss of catchable-sized rainbow trout and 56 percent fewer brown trout. Juvenile rainbows were reduced by 71 percent and juvenile browns by 86 percent. In some locations, there were too few fish remaining to complete a population estimate.
Similar ice jams and ice floes occurred on the two rivers in 1986 and 1974.
In 1999, Montana Fish, Wildlife and Parks counted more than 35,000 fish congregating below the dam. The dam blocked fish from traveling to the upper Clark Fork and its important spawning tributaries: the Blackfoot River, Rock Creek, and Flint Creek. Unfortunately, rainbows, cutthroats and bull trout congregated at the Milltown Dam in spring – just when runoff washed metals over the dam and violations of water quality standards routinely occurred. Among the fish blocked were the bull trout, a threatened species, and the west slope cutthroat trout, which is under consideration for listing under the Endangered Species Act.
In addition, Northern Pike thrived in the artificial habitat created by Milltown reservoir. Pike are an invasive, predatory fish. They like slow, warm water — e.g., pond conditions — and spawn on vegetation at the water’s edge. The problem: pike eat native fish. So even if the dam had a fish ladder, cutthroats, rainbows and bull trout would swim over the dam and into a gauntlet of hungry pike.
Since the dam's powerhouse and spillway removal, fish have resumed upstream and downstream movement in the Clark Fork and Blackfoot Rivers. Dam removal contributed to short-term increased mortality of fish downstream of the dam to the Bitterroot confluence, but dam removal is expected to benefit fisheries over the long term.