Abstract
Fluctuating reservoir water levels create varial zones that tributaries flow through before reaching the reservoir pool during low water levels. Aquatic habitat in eight tributaries of six northwestern reservoirs was distinctly different between varial and un-impacted reference zones. Bull trout migration speeds varied greatly between the two tributaries monitored using radio telemetry (Trail Creek and the Middle Fork Boise River) as well as within Trail Creek. Documentations of predator species were concentrated in the downstream end of the varial zone of Trail Creek where a shallow delta formed annually. Thirty three to 50% of the annual tag loss (mortality or expulsion) occurred in the varial zone. Raising reservoir water levels between the end of irrigation season and the start of bull trout downstream migration could limit the impacts of predators when bull trout are most vulnerable in the shallow deltas after expending the majority of their energy reserves during spawning.