Restoring Rivers in the Twenty-First Century: Science Challenges in a Management Context

Legal mandates force consideration of at least some level of river restoration in many developed nations (e.g., Clean Water and Endangered Species Act in the United States, or the Water Framework Directive in the European Union), but a lack of specifics in legislation compels decision-makers to ask three persistent management questions: (1) How much river restoration do we need? (2) How do we best achieve cost-effective river restoration? (3) How do we know we have restored enough? Moreover, the broader management context is permeated with tremendous inertia to continue development of rivers for societal and economic gain, continual application of small and fragmented restoration actions, and skepticism that river restoration can succeed in the face of climate change and steady population growth. It is in this context that we identify key science challenges for river restoration in the twenty-first century. We suggest that a fundamental shift toward restoring watershed and river processes (process-based restoration) is needed if scientists are to begin developing the tools needed to provide relevant policy answers. The basic conceptual framework of process-based restoration requires that we understand how habitat is formed and changes, how habitat changes alter biota, and how human actions alter both river habitats and the landscape processes that create river habitats. Restoration actions must then directly address human actions that caused habitat degradation, thereby addressing the root causes of biological impacts. Understanding this framework will allow scientists to better address key science challenges for advancing river restoration, including development of ecosystem models to predict what kinds of and how much restoration is needed, an expanded suite of process-based restoration techniques for large river ecosystems, and comprehensive but cost-effective suites of metrics for monitoring river health.