Natural and Anthropogenic Influences on the Physical Habitat in Streams
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Effective environmental policy decisions benefit from stream habitat information that is accurate, precise, and relevant. The recurring National Rivers and Streams Assessments (NRSA), carried out by the U.S. EPA and its collaborating partners, require physical habitat information sufficiently comprehensive and precise to facilitate interpreting biotic data, and to address habitat concerns in their own right. NRSA characterizes the major habitat features that may operate as controls or limiting factors on biotic assemblage composition under natural or anthropogenically disturbed circumstances. Within sample reaches, the field approach employs a randomized, systematic design; locating habitat observations on reaches with lengths 40 times their low flow wetted width. Two-person crews typically complete NRSA physical habitat measurements on wadeable streams in 1.5 to 3.5 hours or boatable measurements within a river float of 5 to 8 hours. The resultant field measurements quantify major dimensions of channel morphology and stream habitat, allowing calculation of measures or indices of stream size and gradient, substrate size and stability, habitat complexity and cover, riparian vegetation cover and structure, anthropogenic disturbances, and channel-riparian interaction. We reduce the complexity of the raw field data by calculating metrics to summarize stream reach habitat characteristics. Beyond simple descriptions, the national assessment evaluated habitat indicators that reflect channel responses to basin-riparian disturbances, or elicit biotic responses when altered. In large regions, human land use disturbances typically overlay wide ranges of natural geomorphic factors that control both habitat characteristics and biotic assemblages. We discuss our use of process-based empirical models to estimate the degree to which streams deviate from “natural” or “reference” conditions. We also illustrate the ecological relevance of NRSA habitat indicators through their response to anthropogenic disturbance pressure and their likely influence on instream biota.