The stream-dwelling larvae of the caddisfly Glossosoma spp. are dominant grazers in lotic food webs and are capable of suppressing stream periphyton. We explored a method for developing a scaling relationship between macroinvertebrate density and local hydraulic variables. As an example of this method, we quantified habitat for larval stone-cased caddisflies, Glossosoma califica and Glossosoma penitum, in 3 coastal mountain streams in northern California over 2 y. We applied dimensional analysis to develop a functional relationship from a power law based on dimensionless local hydraulic and larval density variables that was applicable to areas where Glossosoma are present. Glossosoma densities were negatively correlated with streambed relative roughness and positively correlated with the ratio of inertial to gravitational forces in the stream. The proposed functional relationship described 41% of the variance in the spatial distribution of glossosomatid larvae. This expression could predict how density and constraints on effects of these important grazers would change under variable hydraulic conditions. Variogram analysis of Glossosoma spatial density and relative roughness revealed overlap in the variogram range, the separation distance above which point measurements were statistically independent. The analysis resulted in an average variogram range of 0.39 m for Glossosoma density and 0.26 m for roughness height. Abiotic variables are increasingly available from laser altimetry, so even where field sampling is limited the proposed scaling relationship facilitates prediction of larval biomass over a range of scales in lotic ecosystems.