Stream geomorphology, bank vegetation, and three‐dimensional habitat hydraulics for fish in midwestern agricultural streams

Abstract

Past work on physical habitat in streams has not explicitly considered how differences in channel planform and bank vegetation influence the three‐dimensionality of habitat hydraulics. This study statistically compares frequency distributions of bed elevations, a stage‐independent index of variability in flow depth, and three‐dimensional velocity components for four stream reaches in east central Illinois that have different geomorphological conditions and types of bank vegetation. The analysis shows that bed elevations in a straight channelized reach are significantly less variable than bed elevations in the other three reaches. Distributions of downstream velocities do not differ significantly for two reaches with similar bank vegetation but substantially different channel morphologies, whereas distributions of cross‐stream and vertical velocities are sensitive to differences both in channel planform and bank vegetation. Channel curvature enhances the variance of cross‐stream and vertical velocity distributions through the production of large‐scale helical motion. Conditions that result in net cross‐stream flow, such as abrupt changes in curvature or deflection of the flow laterally, systematically influence the mean of cross‐stream velocity distributions. Corresponding fish studies indicate that the straight, channelized reach has the lowest biotic integrity of the four sites. A detailed comparison of fish population characteristics between this reach and an unmodified reach immediately upstream reveals that the unmodified reach has significantly greater species richness, species diversity, and total biomass than the channelized reach. Thus geomorphological complexity, through its influence on the three‐dimensionality of habitat hydraulics, appears to significantly influence fish community characteristics.