Habitat effects on depth and velocity frequency distributions: Implications for modeling hydraulic variation and fish habitat suitability in streams

Describing the velocity and depth attributes of stream channels is a basic goal of theoretical and applied hydrology and is also essential for modeling biological processes in streams. We applied frequency distributions (gamma probability functions fit to point velocity and depth data) to evaluate their ability to describe variation in hydraulic conditions at the channel unit scale among contrasting habitat types (pools, glides, riffles, and runs) at different flows in a small trout stream. Velocity and depth distributions differed systematically between habitat types, with linear regression explaining 65 and 72%, respectively, of variation in gamma distribution parameters related to skewness and kurtosis; however, distribution parameters were not significantly related to discharge. Relative depth explained 68–79% of the variation in slopes of at-a-station hydraulic geometry relationships between different habitat types. Differentiation of habitat types in a velocity–depth phase space was reduced at high flows, and differences in hydraulic geometry exponents were consistent with flow convergence at high discharge. Modeling variance in velocity and depth using locally derived gamma distributions, in conjunction with simple hydraulic geometry, provided accurate estimates of reach average habitat suitability for trout. Frequency distributions derived from a set of New Zealand streams provided much poorer estimates of habitat suitability. Frequency distributions are useful as an heuristic tool for understanding and modeling drivers of spatial variation in hydraulics, and provide a simple method to model hydraulic conditions in streams. However, general transferability of frequency distributions between streams would be improved by validating and refining existing relationships between distribution parameters and easily measured stream characteristics, like habitat type, channel size, gradient, and substrate caliber.