Tracking the Trajectory of Alluvial Channel Adjustment Reveals Along‐River Shifts in Sediment Mobility

Abstract Alluvial rivers adjust their geometry to convey water and sediment downstream. While multi‐river compilations reveal robust downstream scaling relationships, how these patterns manifest along a single river remains poorly constrained. We quantify the coevolution of bankfull width, depth, channel slope, and grain size along 166 km of California's South Fork Eel River using topobathymetric LiDAR, automated bankfull width measurements, and field‐measured grain size. Slope and width exhibit systematic downstream scaling, whereas bankfull depths are more variable and grain size displays only weak fining. Differing downstream adjustment rates produce an order‐of‐magnitude decline in bankfull Shields stress, revealing strong along‐river variability in sediment mobility obscured in multi‐river data sets. Downstream channel widening is the dominant contribution to these spatial patterns in erodibility, reflecting 90% of the decline in reach‐scale erosion thresholds. These observations offer novel insights into river adjustment pathways and can improve how river channel dynamics are represented in landscape evolution models.