Observed Thresholds of Stream Ecosystem Degradation in Urbanizing Areas: A Process-Based Geomorphic View

Recent studies suggest that channel instability and aquatic ecosystem degradation occur at low levels of watershed imperviousness in humid regions of the U.S. In an effort to provide a more process-based explanation of observed thresholds of aquatic ecosystem degradation in urbanizing areas, U.S. Geological Survey (USGS) gauging data and simple hydrologic techniques were used to examine likely increases in discharge associated with varying levels of watershed imperviousness. The probable increases in these flow parameters for an annual flooding event were compared with analyses of a data set describing 270 stable and unstable channels from diverse geographic regions. Results suggest that a 10 – 20% increase in watershed imperviousness can result in a level of specific stream power that exceeds that corresponding to a high risk of instability for streams poised near a geomorphic threshold. Although risk of channel instability may be associated with different levels of watershed imperviousness, factors such as specific stream power relative to sediment caliber and channel entrenchment provide a more direct assessment of channel sensitivity to urbanization. Linking potential increases in specific stream power with the risk of channel instability establishes a more process-based connection between imperviousness and observed thresholds of aquatic ecosystem degradation.