Relative sediment supply and excess shear stress drives the evolution of restored side channels in a regulated river

Side channel restoration—including creation, rehabilitation and enhancement—is a common strategy to mitigate habitat degradation in regulated rivers. While short-term ecological benefits are well documented, the longer-term geomorphic evolution of restored side channels remains less understood. In natural systems, side channels typically occur at dynamic bifurcations influenced by slope and sediment supply, whereas restoration efforts in regulated rivers often prioritize static design targets (e.g., a fixed inundation area at a given flow). We monitored two restored side channels along a regulated river in California over a five-year period to investigate how geomorphic and habitat conditions evolve post-restoration. Our objectives were to (1) document geomorphic and habitat changes and (2) assess how excess shear stress and relative sediment supply influence channel evolution. We tracked changes in sediment and large wood budgets, bed profiles, grain size distributions, bar formation, inundation patterns and tracer rock displacement and interpreted these in the context of reach-scale and geomorphic-scale shear stress. Results show that the steeper, upstream site experienced greater erosion and a loss of low-flow inundation area due to higher flow energy and excess shear stress, while the downstream site remained relatively stable and gained inundated habitat. Importantly, reach-scale excess shear stress served as an effective proxy for relative sediment supply, explaining observed differences in geomorphic response between sites. Both channels lost more large wood than they recruited, highlighting the need for integrated sediment and wood management. Normalized rates of geomorphic change declined over time, suggesting that the primary morphological adjustments occurred shortly after construction. These findings underscore the importance of reach-scale context in designing and evaluating side channel restoration and demonstrate how multi-scalar monitoring—particularly incorporating reach-scale excess shear stress—can improve understanding of post-restoration dynamics.