Temperature and depth profiles of Chinook salmon and the energetic costs of their long-distance homing migrations

River warming poses an existential threat to many Pacific salmon (Oncorhynchus spp) populations. However, temperature-mediated risks to salmon are often complex and addressing them requires species- and population-specific data collected over large spatial and temporal scales. In this study, we combined radiotelemetry with archival depth and temperature sensors to collect continuous thermal exposure histories of 21 adult spring- and summer-run Chinook salmon (O. tshawytscha) as they migrated hundreds of kilometers upstream in the Columbia River basin. Salmon thermal histories in impounded reaches of the Columbia and Snake rivers were characterized by low daily temperature variation but frequent and extensive vertical movements. Dives were associated with slightly cooler salmon body temperatures (~ 0.01 to 0.02 °C/m), but there was no evidence for use of cool-water thermal refuges deep in reservoirs or at tributary confluences along the migration route. In tributaries, salmon were constrained to relatively shallow water, and they experienced ~ 2–5 °C diel temperature fluctuations. Differences in migration timing and among route-specific thermal regimes resulted in substantial among-individual variation in migration temperature exposure. Bioenergetics models using the collected thermal histories and swim speeds ranging from 1.0 to 1.5 body-lengths/s predicted median energetic costs of ~ 24–40% (spring-run) and ~ 37–60% (summer-run) of initial reserves. Median declines in total mass were ~ 16–24% for spring-run salmon and ~ 19–29% for summer-run salmon. A simulated + 2 °C increase in water temperatures resulted in 4.0% (spring-run) and 6.3% (summer-run) more energy used per fish, on average. The biotelemetry data provided remarkable spatial and temporal resolution on thermal exposure. Nonetheless, substantial information gaps remain for the development of robust bioenergetics and climate effects models for adult Chinook salmon.