Hydraulic habitat composition and diversity in rural and urban stream reaches of the North Carolina Piedmont (USA)

The differences between urban and rural streams regarding hydrological process, channel morphodynamics and ecosystem functioning have been highlighted by a number of studies in recent decades. The need to understand lotic ecosystem functioning in these environments at scales relevant to individual organisms has led to research focusing on hydraulic composition and structure over small areas of channel bed. In this study we map and analyze the hydraulic biotope composition of two urban and two rural stream reaches in the North Carolina (USA) Piedmont to determine if urban flow regimes and attendant channel modification processes might translate into important differences in hydraulic environment, and if so, what those differences are. Hydraulic biotope assemblages were found to vary only moderately in diversity per unit stream length between sites, but were distinctly different in composition. One important control on the differences between rural and urban streams was found to be the localized incision of urban channels into bedrock and saprolite. Resistant rock outcrop in the beds of urban streams creates rapid and riffle biotopes and long stretches of upstream pool habitat by impoundment. Urban reaches were found to be more homogeneous than rural reaches in hydraulic composition and were dominated by pools. Rural reaches, characterized by copious sandy alluvium in the bed, were dominated by runs or glides. Quantitative differentiation of biotopes based on four hydraulic indices generally yields coherent associations, although these may vary in content. Comparisons between hydraulic and biotic diversities suggest relationships favoring biotic functional group aggregation over species richness‐based indices of diversity. Because the majority of published hydraulic diversity analyses are based on coarse‐bed streams, further study of hydraulic diversity in streams with finer substrate is likely to be beneficial. Copyright © 2008 John Wiley & Sons, Ltd.