Exploring the Potential of Remote Sensing-based River Temperature Tool for Improving Columbia River Reservoir Management Towards Fish Abundance Outcomes

The thermal condition of riverine ecosystems significantly influences fish survival and migration. Understanding the spatial relationship between water temperature and fish abundance requires a comprehensive spatiotemporal overview of river temperature. In this study, we used multi-decadal spatiotemporal river temperature estimates from the Thermal History of Regulated Rivers (THORR) tool to explore the relationship between water temperature, fish abundance, and migration patterns. We demonstrated the potential of such a tool and the corresponding analyses to inform and improve reservoir management for fish abundance. Our assessment, based on the mass balance concept, considered the influx and efflux of migratory fish during the fall season in the Hanford Reach along the Columbia River to determine fish retention. We found that the proportion of fish leaving the reach increases with rising water temperatures. Although fish appear to travel faster at higher temperatures according to THORR-based analyses, discharge- focused dam operations upstream did not result in downstream cooling, thus failing to improve thermal conditions for fish in the downstream reach. A long-term multi-decadal trend showed a significant increase in fall water temperatures beyond stressful levels over the past decade. These findings underscore the critical need for balanced dam operations that consider both discharge and temperature requirements to ensure optimal conditions for fish survival and migration. The insights provided by the THORR tool not only enhance our understanding of riverine thermal dynamics but also offer a valuable resource for developing sustainable water management practices in regulated rivers where fish passage is critical. By leveraging THORR's capabilities, we can better plan ways to protect aquatic ecosystems, support biodiversity, and promote the resilience of fish populations amidst climate change impacts.