Citation:

Kaufmann, P., R. Hughes, S. Paulsen, D. Peck, C. Seeliger, T. Kincaid, AND R. Mitchell. Physical Habitat in the Conterminous U.S. Streams and Rivers, Part 2: A Quantitative Assessment of Habitat Condition. ECOLOGICAL INDICATORS. Elsevier Science Ltd, New York, NY, 141:109047, (2022). https://doi.org/10.1016/j.ecolind.2022.109047

Impact/Purpose:

Streams and Rivers are an integral component of our Nation’s aquatic resources, and provide abundant benefits to humans and their environment. Physical habitat quality is essential for the maintenance of biological integrity and fishability in these waters. The USEPA’s National Rivers and Streams Assessment (NRSA), part of the National Aquatic Resource Surveys (NARS), includes the nation’s first statistically valid and policy-relevant assessment of physical habitat condition in these waters across the entire conterminous United States. In this manuscript the authors describe five characteristics of the NRSA approach that enabled a conterminous U.S. assessment of stream/river physical habitat condition. 1) Nationally uniform field methods permitted calculation of a comprehensive set of quantitative metrics rather than qualitative judgements of habitat quality. 2) A focus on four indicators from that set of metrics that were known from previous studies to be important to biota and subject to anthropogenic alteration. 3) Development of a methodology for modeling the influence of geoclimatic factors to estimate expected values of the key indicators for every site in the data set. 4) Comparison of modelled expected conditions with observed values to classify Observed/Expected (O/E) ratios into 3 standard condition classes. 5) Use of probability sample-weighting to infer habitat condition in the targeted population of streams and rivers nationally and ecoregionally. Poor conditions existed in 22-23% of the 1.2 million km (~1,200,000 km) of streams and rivers in the conterminous U.S. for riparian human disturbance, streambed excess fine sediment and riparian vegetation structural complexity, versus 15% for instream habitat cover. Based on the same four indicators, the percentage of stream length in poor condition within 9 separate U.S. ecoregions ranged from 40%. The associations of those key indicators and their O/E values with anthropogenic pressures and biological assemblages in NRSA and other studies demonstrate their biological relevance. The NRSA physical habitat condition determinations presented by the authors help to explain deviations in biological conditions from reference conditions and inform management actions to rehabilitate impaired waters and mitigate further ecological degradation.

Description:

Rigorous assessments of the ecological condition of water resources and the effect of human activities on those waters require quantitative physical, chemical, and biological data.  The U.S. Environmental Protection Agency’s river and stream surveys quantify river and stream bed particle size and stability, instream habitat complexity and cover, riparian vegetation cover and structure, and anthropogenic disturbance activities.  Physical habitat is strongly controlled by natural geoclimatic factors that co-vary with human activities.  We expressed the anthropogenic alteration of physical habitat as O/E ratios of observed habitat metric values divided by values expected under least-disturbed reference conditions, where site-specific expected values vary given their geoclimatic and geomorphic context.  We set criteria for good, fair, and poor condition based on the distribution of O/E values in regional least-disturbed reference sites.  Poor conditions existed in 22-24% of the 1.2 million km of streams and rivers in the conterminous U.S. for riparian human disturbance, streambed sediment and riparian vegetation cover, versus 14% for instream habitat complexity.  Based on the same four indicators, the percentage of stream length in poor condition within 9 separate U.S. ecoregions ranged from 4% to 42%.  Associations of our physical habitat indices with anthropogenic pressures demonstrate the scope of anthropogenic habitat alteration; habitat condition was negatively related to the level of anthropogenic disturbance nationally and in nearly all ecoregions.  Relative risk estimates showed that streams and rivers with poor sediment, riparian cover complexity, or instream habitat cover conditions were 1.4 to 2.6 times as likely to also have fish or macroinvertebrate assemblages in poor condition.  Our physical habitat condition indicators help explain deviations in biological conditions from those observed among least-disturbed sites and inform management actions for rehabilitating impaired waters and mitigating further ecological degradation.

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