Managing fish populations requires detailed understanding of the habitat and other environmental conditions that fish need to grow and survive. FISHBIO has evaluated the habitat use of juvenile salmon, steelhead, and rainbow trout in several Central Valley streams and watersheds. As one example, we have conducted snorkel surveys on the Stanislaus River to assess fish density in various habitat types. In addition to recording fish counts, sizes, and school dimensions, we characterized the habitat of select river reaches by documenting water depth, velocity, and substrate type, as well as the presence of overhanging vegetation and woody debris. Our surveys spanned a three-year period that included river flows ranging from 200 to 3,000 cubic feet per second. We documented how the area of suitable salmonid habitat changed with river flow, which water managers can use to develop ecologically sound river flow prescriptions.