LAUSR

Significant growth in mountain rivers research since 1990 has promoted the concept that canyon‐confined mountain rivers have complex topographic features nested from base‐ to flood‐stages due to canyon structure and abundant large bed elements. Nesting means literally structures inside of structures. Mathematically, nesting means that multiple individual features and repeating patterns exist at different frequency, amplitude, and phasing, and can be added together to obtain the complete structure. Until now, subreach‐scale landform structure, including nesting, has not been quantified sufficiently to understand morphodynamic mechanisms that control and respond to such organization. Geomorphic covariance structure analysis offers a systematic framework for evaluating nested topographic patterns. In this study, a threshold stage in mountain river inundation was hypothesized to exist. Above this stage landform structure is organized to be freely self‐maintaining via flow convergence routing morphodynamics. A 13.2 km segment of the canyon‐confined Yuba River, California, was studied using 2944 cross‐sections. Geomorphic covariance structure analysis was carried out on a meter‐resolution topographic model to test the hypothesis. River width and bed elevation had significantly less variability than previously reported for lower slope, partially confined gravel/cobble river reaches. A critical stage threshold governing flow convergence routing morphodynamics was evident in several metrics. Below this threshold, narrow/high “nozzle” and wide/low “oversized” were the dominant landforms (excluding “normal channel”), while above it wide/high “wide bar” and narrow/low “constricted pool” were dominant. Three‐stage nesting of base‐bankfull‐flood landforms was dictated by canyon confinement, with nozzle–nozzle–nozzle nesting as the top permutation, excluding normal channel.