In an eroding, mountainous landscape, the supply, sorting and storage of sediment have a profound effect on the distribution of plant and animal life within any particular watershed. This research focuses on the variables that dictate the geomorphic conditions of a channel and its valley, their effect on the sorting and storage of the supplied sediment, and whether or not this in turn affects the distribution and/or density of rainbow trout (Onchorynchus mykiss) populationsthat rely on the supplied sediment to provide spawning gravel. Combinations of field and computer-derived data are used to describe the geomorphic conditions present in the river valleys of Sespe Creek, California. With these data, patterns of sediment storage and sorting are described. The data suggest a landscape where patterns of valley width and gradient are controlled by a combination of lithologic, tectonic, and hydrologic variables. Multiple controlling variables result in a valley pattern where narrow, high-gradient bedrock reaches are interspersed with wide, low-gradient alluvial reaches. Howard Creek, a subwatershed located in the middle reaches of the Sespe, has high rainbow trout densities in all size classes as well as the most potential sediment storage per unit stream length. On the other hand, Alder Creek, a subwatershed located in the lower reaches of the Sespe, has low rainbow trout densities and low potential sediment storage per unit stream length. These results suggest that valley configuration influences rainbow trout densities by affecting the location and amount of valley sediment storage.