Valley width variation influences riffle–pool location and persistence on a rapidly incising gravel-bed river

Decades of research have demonstrated that gravel-bed rivers exhibit multiple spatial scales of landform nonuniformity. Landform features such as valley wall oscillations, alluvial fans, terraces, river meanders, point bars, boulder clusters, and logjams contribute to river corridor heterogeneity. Further, studies have shown that the locations of riffles and pools in gravel-bed rivers are closely associated with the spatial pattern of river corridor morphological variability. These studies imply (but do not directly test) that persistence of riffles and pools through time is also conditioned by landform variability. Mechanistically, it may be hypothesized that overbank flows interact hydraulically with oscillating valley walls by way of a mechanism known as flow convergence routing. In such circumstances, can variation in valley width influence channel morphology, forcing riffle–pool location and persistence? To address this question, research was performed on a valley-confined, wandering, gravel-bed reach of the regulated lower Yuba River, California, USA that is incising from reduced sediment supply and flow regulation after a historic period of mining-related valley sedimentation. This study quantified stream planform change 1984–2006 and elevation change 1999–2006 using eight aerial photo sets and two digital elevation models, and it qualitatively assessed channel change in older aerial photos going back to 1937. Analysis revealed that the river has undergone rapid incision (0.11 m/year) and planform change in response to frequent floods, yet eight riffles persisted in the same locations, which coincided with the locally wide areas of the valley. Longitudinal profiles revealed that deep pools were associated with long valley constrictions. Geomorphic analysis and design need to explicitly account for multiple scales of landform nonuniformity in order to rehabilitate natural fluvial processes.