Effects of urbanization on channel instability

Channel instability and aquatic ecosystem degradation have been linked to w atershed imperviousness in humid regions of the U.S. In an effort to provid e a more process-based linkage between observed thresholds of aquatic ecosy stem degradation and urbanization, standard single event approaches (U.S. G eological Survey Flood Regression Equations and rational) and continuous hy drologic models (HSPF and CASC2D) were used to examine potential changes in flow regime associated with varying levels of watershed imperviousness. Th e predicted changes in flow parameters were then interpreted in concert wit h risk-based models of channel form and instability. Although low levels of imperviousness (10 to 20 percent) clearly have the potential to destabiliz e streams, changes in discharge, and thus stream power, associated with inc reased impervious area are highly variable and dependent upon watershed-spe cific conditions. In addition to the storage characteristics of the pre-dev elopment watershed, the magnitude of change is sensitive to the connectivit y and conveyance of impervious areas as well as the specific characteristic s of the receiving channels. Different stream types are likely to exhibit v arying degrees and types of instability, depending on entrenchment, relativ e erodibility of bed and banks, riparian condition, mode of sediment transp ort (bedload versus suspended load), and proximity to geomorphic thresholds . Nonetheless, simple risk-based analyses of the potential impacts of land use change on aquatic ecosystems have the potential to redirect and improve the effectiveness of watershed management strategies by facilitating the i dentification of channels that may be most sensitive to changes in stream p ower.