The Pahsimeroi Valley lies between the Lemhi and Lost River mountain ranges. Water percolates through the broad, pervious alluvial fan in the upper valley and enters the river through ground water and springs lower in the valley. Therefore, productivity in the river is higher than most streams in the upper Salmon River drainage.
A hatchery on the Pahsimeroi River, owned and funded by Idaho Power Company and operated by the Department, traps and rears summer chinook salmon and also traps A-run steelhead which are reared at fish hatcheries in the Hagerman Valley. The hatchery was constructed and is operated as mitigation for lost anadromous production from the Hells Canyon Dam complex. The adult steelhead returning to the Pahsimeroi Hatchery contribute significantly to the steelhead fishery in the upper Salmon River.Resident fish species include rainbow trout, brook trout, bull trout, whitefish, and cutthroat trout. Trout densities have declined consistently over the past 10 years.