Unintended Consequences of Selective Water Withdrawals From Reservoirs Alter Downstream Macroinvertebrate Communities

Regulated rivers downstream of dams often exhibit highly modified thermal regimes in addition to modified hydrologic regimes; downstream river temperatures can be seasonally much warmer or cooler than unregulated streams. Selective water withdrawals can be used to minimize thermal impacts to downstream reaches. However, other water quality parameters, in‐reservoir production, and species composition also vary with depth. In the South Fork McKenzie River, in the Pacific Northwest of North America, an existing large dam was retrofitted to mix and export water from multiple depths, allowing dam operators to adjust the outflow temperatures to be in sync with the upstream thermal regime. We examined benthic macroinvertebrate responses after implementation of selective water withdrawal, comparing it to thermally similar flow‐through conditions, expecting to see shifts in the community over time to resemble upstream, undammed reaches. However, species composition and traits downstream of the dam became more dissimilar to upstream and to flow‐through after selective water withdrawal. These changes included increases in non‐insect taxa and taxa known to feed on plankton and detritus. The most likely explanation for this transition is that selective water withdrawal from multiple depths of a stratified reservoir included epilimnetic releases, which resulted in increased export of plankton and organic material downstream. This strategy favored taxa able to capitalize on resource subsidies from the reservoir. Our findings highlight the interconnected nature of responses to dams and dam operations, including the importance of considering biotic communities in addition to temperature and flow when planning water management strategies. Plain Language Summary Large dams form deep reservoirs that are often stratified in summer, warm at the surface and cold at depth. When water is released from one depth, there can be dramatic shifts in downstream water temperatures and impacts to instream ecological communities. To remedy this, dams are being retrofitted to release water from multiple reservoir depths. We studied stream invertebrates upstream and downstream of a dam where modifications allowed the release of water from multiple depths to match upstream water temperatures, comparing this to flow‐through conditions. We expected to see increasing similarity between the invertebrate communities at the downstream and the upstream reference site. However, when the dam released w