Pulsed-Flow Impacts on Stream Fishes: What Do We Know? What Are the Knowledge Gaps and Synthesis Needs?

The societal benefits of hydropower systems (e.g., relatively clean electrical power, water supply, flood control, and recreation) come with a cost to native stream fishes. We reviewed and synthesized the literature on hydropower-related pulsed flows to guide resource managers in addressing significant impacts while avoiding unnecessary curtailment of hydropower operations. Dams may release pulsed flows in response to needs for peaking power, recreational flows, reservoir storage adjustment for flood control, or to mimic natural peaks in the hydrograph. Depending on timing, frequency, duration, and magnitude, pulsed flows can have adverse or beneficial short and long-term effects on resident or migratory stream fishes. Adverse effects include direct impacts to fish populations due to 1. stranding of fishes along the changing channel margins, 2. longitudinal (downstream) displacement of fishes, and 3. reduced spawning and rearing success due to redd/nest dewatering and untimely or obstructed migration. Beneficial effects include: 1. maintenance of habitat for spawning and rearing, and 2. biological cues to trigger spawning, hatching, and migration. There is a clear need to move beyond case studies toward a conceptual framework conducive to generalization to new watersheds, and prediction of the impacts of future pulsed flow regimes. We present the first steps toward a conceptual model of pulsed flow effects, and suggestions for the development of analytical tools to predict the effects of pulsed flows on fishes and to design mitigation strategies. We suggest that the synthesis needed is beyond the scope of individual field projects and requires an analytical framework that incorporates mathematical representations of flow, temperature, habitat structure, fish life stages by season, fish population dynamics (especially growth, reproduction, and mortality), and multiple fish species. Ultimately, the results of field and laboratory studies must be synthesized and integrated into conceptual and mathematical models, describing the physical and biological processes occurring with pulsed flows, which can be used to predict outcomes in other regulated streams experiencing pulsed flows.