Hydrologic and topographic variability modulate channel change in mountain rivers

- Hydrology modulates channel change through the activation of river corridor topography.
- Geomorphic covariance structures show that channel change is associated with distinct topography.
- Particle trapping, flow convergence, and flow steering were important channel change mechanisms.
- Flow steering accounted for 97% of the total channel change at the highest discharge analyzed.
- Process blending occurs in mountain rivers due to both topographic and hydrologic variability.

SummaryThe relationships between flow hydrology, topography, and channel change in mountain rivers is important to understanding landscape evolution, the structure and persistence of aquatic habitat, and also the physiochemical cycling of upstream derived organic and inorganic materials. There is a paucity of detailed studies that analyze the joint roles of hydrology and topography in controlling multiple mechanisms of channel change in mountain rivers. In this study, gravel and cobble channel change in a bedrock river canyon were analyzed in light of a controlled yet natural experiment where 4491 metric tonnes of rounded gravel and cobble was augmented below a sediment-barrier dam in a 1200 m long mountain river reach that had no prior sources of rounded gravel or cobble and still experiences floods above the bankfull discharge. The overall study goal was to investigate how flow hydrology can modulate multiple channel change processes depending on the topographic features engaged by the flow. Channel change was assessed via differencing of high resolution repeat topographic and bathymetric surveys, along with cm-scale aerial photography post injection. Statistical tests used to implicate topographic feature-specific mechanisms of channel change that vary with discharge included analyzing geomorphic covariance structures of flow dependent width, bed elevation, and channel change as well as autocorrelation of flow width spatial series. Stage dependent topographic steering was inferred from associations of erosion and deposition with changes in 2D model derived flow directions at multiple discharges. A variety of mechanisms of channel change were qualitatively and quantitatively confirmed including particle hiding, topographic steering, eddying, and flow convergence. No single mechanism explained the observed patterns of channel change but rather it is thought that process-blending occurs, as modulated by the interactions of flow hydrology with complex topography. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamic spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.