Regional Risk Analysis of Channel Instability

In many watersheds, land use changes and hydraulic modifications have directly resulted in accelerated geomorphic activity and excessive sedimentation. Channel adjustment by erosion can be expected to occur if specific stream power exceeds a threshold of critical stream power. Excess stream power in stable meandering channels may result in a variety of responses that potentially range from extreme widening and braiding to extreme incision. Such extensive changes in channel pattern and morphology usually result in significant degradation of water quality and ecological integrity. An energy-based index grounded in geomorphic threshold theory was developed to improve the prediction of channel response to land use changes and hydraulic modifications. A very large and geographically diverse data set consisting of stable meandering rivers, braided rivers, and severely incised rivers was used in conjunction with logistic regression analysis to develop a probabilistic approach to predicting thresholds of channel instability in meandering channels. Simple variables such as slope, median bed material size, 2-year discharge, and drainage area are used as independent variables to estimate the risk associated with varying levels of excess specific stream power relative to sedimentary characteristics. Results indicate that simple indices of specific stream power derived from data that are readily available for most areas can provide accurate predication of thresholds of channel instability at the regional scale. This approach facilitates rapid identification of channels that are at highest risk of severe morphologic change due to an imposed increase in stream power and most sensitive to changes in the controlling variables. The approach provides tools for improved management of stream ecosystems through regional risk assessment, land use planning, and prioritization of stream and watershed management efforts.