Natural and Anthropogenic Influences on the Physical Habitat in Streams

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.

Impact/Purpose

Anthropogenic alteration of physical habitat structure is increasingly recognized as a major cause of ecological impairment in streams and rivers worldwide.  International researchers and resource management agencies have shown increased interest in the application of  USEPA’s physical river & stream habitat assessment approach to assess status and trends in habitat condition. The Brazilian Federal University of Minas Gerais is convening a conference and multidisciplinary work group focusing on hydroelectric development, addressing the question of how anthropogenic activites alter water quality, habitat heterogeneity, biodiversity, and ecosystem services in a large river basin in Brazil.  The author will give an invited presentation describing the EPA’s National Aquatic Resource Survey (NARS) approach for physical habitat assessment and will serve on an international review panel to evaluate plans for extensive regional monitoring and assessment of the impacts of land use and hydroelectric power development and management on aquatic ecosystems in southeastern Brazil.  NARS quantifies and monitors channel size and slope, substrate size and stability, instream habitat complexity and cover, riparian vegetation cover and structure, anthropogenic disturbance activities, and channel-riparian interaction. Like biological assemblages and water chemistry, physical habitat is strongly controlled by natural geoclimatic factors that can obscure or amplify the influence of human activities. In this presentation, Kaufmann describes the systematic approach to estimate the deviation of observed river and stream physical habitat from that expected in least- disturbed reference conditions. NARS applies this approach to calculate indices of anthropogenic alteration of three aspects of physical habitat condition in the conterminous U.S.: streambed sediment size and stability, riparian vegetation cover, and instream habitat complexity. Although anthropogenic activities negatively influenced all three physical habitat indices in the least-disturbed sites within most ecoregions, natural geoclimatic and geomorphic factors were the dominant influences. For sites over the full range of anthropogenic disturbance, analyses of observed/expected sediment characteristics showed augmented flood flows and basin and riparian agriculture to be the leading predictors of streambed instability and excess fine sediments. Similarly, basin and riparian agriculture and non-agricultural riparian land uses were the leading predictors of reduced riparian vegetation cover complexity in the CONUS and within ecoregions. In turn, these reductions in riparian vegetation cover and complexity, combined with reduced summer low flows, were the leading predictors of instream habitat simplification. The EPA’s experience in ecological assessment will benefit the Brazilian effort to assess the effects of land use and hydroelectric development on streams and rivers.

Citation

Kaufmann, Phil. Natural and Anthropogenic Influences on the Physical Habitat in Streams. 2nd Transdisciplinary Workshop of the IBI-Furnas & UFMG Project: Environmental Governance and Sustainability of Water and Energy, Belo-Horizonte, Minas Gerais, BRAZIL, November 18 - 20, 2024.