Ecological Risk Assessment for the Middle Snake River, Idaho

Note: EPA has updated this document, but this version is provided as a courtesy to the public as a matter of public record.

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An ecological risk assessment was completed for the Middle Snake River, Idaho. In this assessment, mathematical simulations and field observations were used to analyze exposure and ecological effects and to estimate risk.

The Middle Snake River which refers to a 100 km stretch (Milner Dam to King Hill) of the 1,667 km long Snake River lies in the Snake River Plain of southern Idaho. The contributing watershed includes 22,326 square km of land below the Milner Dam and adjacent to the study reach. The demands on the water resources have transformed this once free-flowing river segment to one with multiple impoundments, flow diversions, significant alterations to river habitat, loss of native macroinvertebrate species, extirpation of native fish species, expansion of pollution-tolerant organisms, and excessive growth of macrophytes and algae.

The environmental management goals for this assessment are:attainment of water quality standards, establishment of total maximum daily loads for major pollutants, water for hydropower, recreation, and irrigation, recovery of endangered species, and sustained economic well being. The diversity, reproduction, growth, and survival of representative species from three major trophic levels (fish, invertebrates, and plants) were chosen as assessment endpoints in order to complete an ecosystem level analysis.

Simulation of habitat conditions (temperature, water velocity, and water depth) and review of field studies show that most spawning, rearing, and adult habitats available to native fish species and in the Middle Snake River are undesirable. In addition to high water temperatures, our analysis showed that low flows and sedimentation are main stressors affecting these fish species. These same factors are thought to be responsible for the decline of native snail populations. Risks of eutrophication were estimated by changes in the plant biomass. The simulation of macrophyte growth, under existing conditions in the study reach, indicates the river is eutrophic based on aquatic plant biomass exceeding 200 g/m2. The lines of evidence drawn from the model simulation suggest that nutrients, temperature, flow, and water depth are the major factors controlling macrophyte growth.

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