Response of invertebrate drift to dam-release restoration pulse flows from Lewiston Dam on the Trinity River, CA

The widespread construction of dams to regulate rivers has dramatically altered aquatic ecosystems, but these impoundments also provide a unique opportunity to support freshwater conservation goals by implementing functional flow regimes designed to mitigate dam-related impacts on fisheries. Drifting invertebrates are an important food source for stream-dwelling juvenile salmonids such that drift feeding can be an energetically profitable foraging strategy, yet the effect of streamflow alterations on invertebrate drift dynamics is largely undetermined. Drift net samples were collected on four days before and four days during the ascending limb (14-42 m3/s) of restoration pulse flows in April 2020 at four sites located along 48 river kilometers of the Trinity River downstream of Lewiston Dam in northern California. Results provide evidence of an inconsistent response by drifting invertebrate biomass concentration (mg/m3) across pulse flows with increases observed during the first pulse, but little effects or potential decreases in subsequent pulses. Drift response varied among sites, where the effect of pulse flows was greater at sites closer to the dam and that underwent longer durations without disturbance in the preceding months. Weighted mean length (mm) of drifting invertebrates was lower at higher flows, a trend that may be temporally driven by differing aquatic or terrestrial taxonomic groups dominating during base or pulse flows. These findings suggest that dam-release restoration flows can temporarily increase prey availability in the drift for juvenile salmonids, but the exploration of alternative restoration flow actions with meaningful long-term benefits to salmonid populations may be a more beneficial solution