Blackfoot River Resource Investigations

Mark A. Kerr
Montana Water Quality Bureau

     A number of resource investigations were initiated in 1988 in the Blackfoot River drainage to address public perceptions of a declining fishery. Investigations included

                  ·        Monitoring of ambient water quality during summer low flow now and spring runoff conditions
                  ·        Chronic toxicity surveys using the cladoceran Ceriodaphnia
                  ·        A benthic macroinvertebrate survey
                  ·        Analyses of sediment and biota for heavy metal contamination
                  ·        Fishery surveys

    Although some studies are ongoing, inforn1ation to date suggests that the fishery has been impaired by a number of causes and sources. In the headwaters area, heavy metals from historic mining have created acutely toxic conditions. Aquatic life criteria for cadmium, copper, lead, and zinc are frequently exceeded. Cadmium has accumulated in fish tissue in concentrations, which likely impair reproduction and survival. Sediment analyses indicate that contamination has penetrated approximately 25 km downstream since the Mike Horse line tailings impoundment failure in 1975.

    The fishery in the middle river (from approximately Poorman Creek to the Clearwater River) is generally poor. Heavy metals il1 sediment are near backgrow1d concentrations. However, cadmium and zinc contamination of the biota is still detectable. Water quality is generally good, although increasing nutrient and sediment concentrations during runoff indicate greater nonpoint source impacts. Benthic macroinvertebrate communities suggest possible organic enrichment and/ or siltation effects. Siltation, a lack of spawning tributaries, and other environmental factors interact to depress fish populations in most of this reach.

    In the lower river, fishing pressure appears to be limiting the larger size fish in all otherwise good fishery. Water quality analyses and benthic macroinvertebrate indices indicate that water quality is good to excellent.

    The various studies indicate that the fishery is moderately to severely depressed in much of the Blackfoot River. Heavy metals, siltation, degraded tributary streams, and over harvesting may be the most significant limiting factors. Drought conditions during the last few years have exacerbated these detrimental influences.