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.