Effects of reservoir operations on aquatic macroinvertebrate community composition and production in the Deadwood River, Idaho

Background/Question/Methods

We evaluated effects of reservoir operations on aquatic invertebrates in the Deadwood River below Deadwood Dam in central Idaho. The montane field sites included four sites in the regulated river and six reference sites located upstream of the dam and in nearby rivers. Macroinvertebrate, stream habitat, and water chemistry data were collected every 4 – 6 weeks from June through October in 2010 and 2011. Macroinvertebrate production was estimated over intervals between sampling dates. River temperature, discharge, and wetted streambed area were modeled for alternative scenarios of reservoir operations using a one-dimensional flow model and a high-resolution digital elevation model of the streambed. Results from the flow model were used to predict effects of reservoir operations on macroinvertebrate production using an empirical mass- and temperature-dependent growth model for macroinvertebrates.

Results/Conclusions

Regulated and reference sites had similar physical habitat and water quality with the exception of periphyton abundance and organic matter, which were elevated at sites closest to the dam.  Macroinvertebrate communities at dam-influenced sites had lower taxonomic richness and lower diversity than in reference sites, while density was similar at all sites. Functional feeding groups shifted in regulated reaches, with a higher proportion of invertebrates classified as gatherers, and fewer scrapers and shredders. The largest effects of reservoir operations on macroinvertebrate production occurred during summer months. Higher reservoir discharge during summer led to a decrease in mean daily temperature in the regulated river. Lower water temperature decreased macroinvertebrate production, but these decreases were partially offset by an increase in the area of wetted substrate available to benthic invertebrates.