Proactive river corridor definition: Recommendations for a process-based width optimization approach illustrated in the context of the coastal Pacific Northwest

We propose a holistic approach to define a river corridor as the minimum space needed to sustain key river functions based on an understanding of the desired functions of that corridor and the processes governing channel and floodplain formation. Giving such space is a fundamental nature-based solution to river management, as it allows the river to use its own energy to maintain flood conveyance and habitat function. The review of existing river corridor concepts shows that these often focus on one or two potential functions of a river corridor and may not be well suited as tools to optimize eco-geomorphic river function. We argue that evaluating the effects of river corridor width on multiple processes can provide an objective means to optimize delineation in areas where development encroaches onto floodplains and channel migration zones. Key processes are linked to channel migration and include floodplain rejuvenation, emergence of a dynamic patch mosaic of riparian habitat that sustains a functioning large wood cycle, and effects of constriction and confinement on channel dynamics and morphology. Quantification of these processes for an example river shows the most rapid gains for habitat and flood protection values up to the threshold for planform-controlled conditions and an asymptotically reducing rate of gain in function above that threshold. For the example river, the threshold width approaching that asymptote is substantially more than its constrained condition but much less than the width of the floodplain and channel migration zone, offering a compromise for managing rivers with developed floodplains.