Lateral geomorphic connectivity in a fluvial landscape system: Unraveling the role of confinement, biogeomorphic interactions, and glacial legacies

Abstract

The rates at which sediment from the landscape is recruited to channel networks reflect lateral geomorphic connectivity and have significant implications for biophysical river systems. By strongly influencing sediment supply regime, such inputs control the dynamics and characteristics of the channel as well as the associated habitat and biota. The main objective of this research was to disentangle the relative importance of geomorphic and biological factors which define spatial patterns in lateral connectivity by promoting or restricting bank erosion, avulsions, and slope failures. Focusing on a forested, formerly glaciated mountain basin in the interior Pacific Northwest, we combined extensive field surveys with remote sensing data (LiDAR, aerial imagery) to obtain metrics representing this complex suite of landscape characteristics. The interpretation of evidence yielded by correlations between these variables, a regime model, and qualitative observations, suggest that, at a multi-decadal timescale between major landscape disturbances, lateral connectivity patterns in Sullivan Creek may be, to a large extent, governed by a combination of biotic factors and glacial legacies. In particular, forest canopy metrics were key predictors of hillslope stability as well as bank and valley floor resistance, suggesting a pivotal influence of root reinforcement. On the other hand, instream large wood abundance was closely related to channel splitting, which reflects its role in promoting channel avulsions. More extensive bank erosion in reaches dominated by drift deposits appeared to indicate glacial legacies. Furthermore, a strong association between bank erosion and colluvial inputs suggested a close coupling within the channel-hillslope system. Lateral confinement – defined as the proportion of the channel length abutting against bedrock, terraces, fans and roads – was negatively associated with channel braiding; the relationships linking confinement with bank erosion and the magnitude of colluvial inputs (negative and positive association, respectively) were relatively weak. Taken together, findings emerging from this research suggest that better understanding of lateral geomorphic connectivity patterns in mountain river basins similar to our study site requires that both channel processes and broader, biophysical landscape system be considered.