Trap-and-haul is a mitigation strategy at many hydropower dams lacking upstream fish passage facilities, and protocols are needed to maximize its effectiveness. We used biotelemetry to assess the potential benefits of releasing transported adult Chinook salmon (Oncorhynchus tshawytscha) into a cold-water reservoir versus a relatively warm-water tributary before spawning. Warm river temperatures in the study region have been linked to high prespawn mortality in Chinook salmon, and so reservoirs provides a potential thermal refuge where salmon can select preferred water temperatures to regulate maturation processes and reduce disease risks. Over five years, we released 160 salmon into Foster Reservoir (Oregon, USA) and another 102 into the South Santiam River near historic salmon spawning areas further upstream. In total, 70% of reservoir-released salmon entered an upriver tributary after spending a median of 3-95 d annually in the reservoir. Data recovered from 61 archival temperature loggers indicated that salmon were ~3-6 °C cooler per day in the reservoir than in the river. We estimated that cumulative exposure of reservoir-released fish was reduced by 64 degree days (DD), on average (range = -129 to 392 DD), relative to river-released fish. The results demonstrate that adult trap and transport to reservoirs may reduce thermal exposure for some temperature-sensitive populations, and therefore may be an effective strategy to help reduce prespawn mortality. However, reservoir releases were not risk-free: 14% of all reservoir-released fish fell back downstream past the dam versus 1% of river-released fish; the fallback may have been related to homing behaviors, but natal origins were unknown for all fish. We conclude that reduced transport distance, reduced thermal exposure, and potential survival benefits of releasing salmon into reservoirs should be weighed against fallback risks.