Too Much Water?: Coupled Climate Change-Hydropower Reservoir Releases Impact Stream Hydrogeology and Salmonid Populations in the Bear River Valley, California

Salmonids are expected to be particularly vulnerable to global climate change (GCC) because predicted reduced summer base flows and increased winter flood pulse frequency and intensity may disrupt spawning, prohibit upstream movement, and increase temperature-related mortality. California’s Sierra Nevada waterways are further complicated by hydropower and water supply dam operations, with more than 400 reservoirs disrupting the region’s natural flow regime.  The Federal Energy Regulatory Commission (FERC) manages hydropower dam licensing and requires integrated stream assessments for license renewal, though consideration of GCC is currently not required or welcomed by FERC and hydropower licensees.  Rainfall-runoff models have identified that warming trends will shift regional stream flow patterns and volumes, but because modeling operational spills are highly complex, less is known about how GCC will impact salmonid populations in regulated streams.  The Bear River Valley, a regulated reach of the Bear River, was used as a case-study to characterize potential Sierra stream conditions for salmonids, including hydrology, habitat availability, and geomorphology for 2, 4, and 6° temperature increase scenarios. Hydrology was previously modeled using WEAP21, and predicts that average weekly flows will increase in the winter and decrease in the summer (≤109% and 71%, respectively). We combined historical flow data (1975-2008) with WEAP predictions to determine the linked effects of GCC and hydropower operation on regional life history strategies for the two dominant stream fishes, rainbow (Oncorhynchus mykiss) and brown (Salvelinus fontinalis) trout.  Changes to suitable trout habitat were quantified using two-dimensional (2D) hydrodynamic modeling (River 2D) at the predicted flow scenarios.  Preliminary results identified impacts to several critical life history stages for trout species. The increase in winter pulse intensity will increase the number of channel forming flow events (i.e., mobilization of D₅₀ and spawning gravel), likely to scour brown trout nests and increase fry mortality.  The increase in frequency and duration of winter pulses will reduce brown trout recruitment and recolonization, which could potentially benefit rainbow trout by minimizing competition during spring spawning. Climate change-induced reduced summer flows may not significantly impact regulated stream reaches that release base flows sufficient to maintain water temperature, connectivity, and submerge structural refugia.  This study linked climate change models with 2D habitat modeling to quantify climate change impacts on stream hydrogeology and resident biota, and this method could be used as a tool when managing regulated waterways.