Relationships of Stream Responses to Hydrologic Changes

Rural to urban land use change is a ubiquitous and formidable challenge in watershed management. Decades of research have revealed that urbanization frequently results in severe stream degradation, but the complexity and variability of stream responses inhibit prediction and informed decision-making. Associations between gross measures of total imperviousness or human population and stream characteristics provide little meaningful feedback for understanding key processes and creating practical mitigation strategies. In contrast, examining the effectiveness of mitigation strategies relative to fundamental biophysical linkages provides a foundation for improved management of aquatic ecosystems in rapidly changing watersheds. The objective of this paper is to provide a process-oriented view of what is known about the physical response of streams to urbanization and stormwater controls, to identify some critical information gaps, and to suggest useful approaches and analysis tools for filling these gaps. In particular, variable responses to altered flow and sediment regimes across different stream types, riparian conditions, and spatio-temporal scales are considered. Decision-based models of channel instability that account for the relative sensitivity of stream types to changes in flow and sediment regimes can improve our ability to set priorities and tailor mitigation strategies to the response potential of receiving waterbodies.