Coupled in-reservoir and downstream water-temperature responses to the deep drawdown of a large reservoir, and tradeoffs for fisheries management

Karen M Bartelt , Laurel Stratton Garvin, Stewart Rounds

Across the Pacific Northwest and other regions, the construction and operation of large dams has provided numerous societal benefits but has also fundamentally altered present-day river networks by blocking fish passage and altering streamflows and thermal regimes. Increasingly, evolving management directives require dam operations far outside of historical operational regimes to achieve new objectives utilizing existing dam infrastructure. In the Willamette River Basin of northwestern Oregon where the U.S. Army Corps of Engineers operates 13 dams comprising the Willamette Valley System, deep reservoir drawdowns of over 30.5 m (100 ft) below typical operational lake levels have been implemented at multiple sites as a strategy to facilitate downstream passage of juvenile Chinook salmon (Oncorhynchus tshawytscha). Because reservoir thermal conditions, dam operations, and downstream release temperatures are closely coupled, deep drawdowns must be strategically planned to avoid releasing potentially harmful warm water to downstream reaches where adult salmon spawn. To better understand and quantify these thermal tradeoffs and evaluate mitigation strategies, we analyzed water temperatures upstream and downstream from Green Peter Dam, a high-head dam on the Middle Santiam River in the South Santiam River Basin in northwestern Oregon. We compared release temperatures and reservoir conditions from three deep drawdown years (2023-2025) with historical temperature data and simulated three additional drawdown timing scenarios using a two-dimensional hydrodynamic model under consistent hydroclimatic conditions. Deep drawdowns produced cooler mid-summer release temperatures but substantially warmer maximum release temperatures in autumn relative to historical norms. Maximum daily mean release temperatures of 16.9-18.7 °C in autumn exceeded thermal thresholds associated with egg mortality and coincided with the spawning and egg incubation period for spring Chinook salmon. The magnitude of peak release temperatures in autumn depends on several factors such as late summer and autumn meteorological conditions, the degree to which the reservoir fills during spring, summertime dam operations, and the timing of a deep drawdown. To reduce these peak water release temperatures, results showed two critical strategies to be helpful: (1) using dam outlets near the water surface to export excess near-surface heat and minimize its accumulation in late spring and early summer, (2) delaying a deep drawdown until seasonal cooling of the epilimnion releases more of its accumulation of summer-sourced heat to the surrounding environment, thus minimizing the downstream export of that epilimnetic heat as the drawdown pulls the thermocline down to the level of dam outlets. Implementing these strategies effectively depends on real-time monitoring of reservoir thermal structure, which could inform outlet and drawdown timing decisions. This case study illustrate the utility of process-based modeling for characterizing the close coupling between reservoir thermal conditions, dam operations, and downstream release temperatures. This case study also highlights the importance of managing the thermal tradeoffs of different dam operations from a systems and life-cycle perspective, wherein complementary monitoring and modeling strategies are used to rigorously evaluate those tradeoffs to inform highly consequential dam management operations.