Physical Habitat in Conterminous US Streams and Rivers, Part 1: Geoclimatic Controls and Anthropogenic Alteration

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 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, however, physical habitat is strongly controlled by natural geoclimatic factors that can obscure or amplify the influence of human activities. We used several approaches to estimate the deviation of observed river and stream physical habitat metric values from those in least-disturbed reference conditions. We then compared these indicators of physical habitat condition in least-disturbed reference sites with those in disturbed sites, which we interpret as a response of these physical habitat metrics to anthropogenic influences. We applied these approaches to evaluate four integrated aspects of physical habitat condition in the conterminous 48 U.S. states: riparian anthropogenic disturbance, streambed excess fine sediment, riparian vegetation cover, and instream habitat complexity. 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 anthropogenic disturbance, streambed excess fine sediment and riparian vegetation cover, versus 15% 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 40%. These physical habitat condition determinations help to explain deviations in biological conditions from reference conditions and inform management actions for rehabilitating impaired waters and mitigating further ecological degradation.

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.

Citation

Kaufmann, Phil, R. Hughes, Steve Paulsen, Dave Peck, C. Seeliger, M. Weber, AND R. Mitchell. Physical Habitat in Conterminous US Streams and Rivers, Part 1: Geoclimatic Controls and Anthropogenic Alteration. Elsevier Science Ltd, New York, NY, 141:109046, (2022). [DOI: 10.1016/j.ecolind.2022.109046]

DOI: Physical Habitat in Conterminous US Streams and Rivers, Part 1: Geoclimatic Controls and Anthropogenic Alteration