WIGGLES IN WIDTH: INSIGHTS INTO ALLUVIAL CHANNEL DYNAMICS FROM VARIABILITY IN HIGH-RESOLUTION DOWNSTREAM HYDRAULIC GEOMETRY (Invited Presentation)

The shape of a river reflects a balance of climate, hydrology, and sediment transport. Stable channels maintain a robust scaling between bankfull discharge and width, such that in aggregate, Q ~ aW0.5. Remarkably, this discharge-width scaling is consistent across 13 orders of magnitude from laboratory channels to the largest rivers in the world and represents a morphologic signature that is anchored to the threshold for motion. Others have highlighted signals of regional variations due to climate, vegetation, lithology, and land use around this scaling. However, individual rivers are often reduced to one or a few data points, relegating these factors to a fitting coefficient (a). In this contribution, we quantify the degree to which high-resolution variations in bankfull channel width deviate from this robust discharge-width scaling. We propose that these site-specific deviations can provide insights into alluvial channel dynamics and stability at the reach scale.

To constrain site-specific variability in channel width, we developed an automated cross section extraction algorithm where bankfull channel widths are identified from high-resolution topography. We extract bankfull channel width at 1-10 m intervals along the entire length of coarse-grained rivers with variable hydroclimates, drainage areas, and slopes - including the SF Eel River, CA, the Logan River, UT, the Gasconade River, MO, and two rivers in the Colorado Front Range. Topographically extracted channel widths largely follow discharge-width scaling, but the degree of width variability at each site rivals and in some instances exceeds the global variability for a given discharge. The observed variability indicates that while the global data are useful for discerning a governing trend, there is considerably more information on the structure and function of the river corridor encoded within hydraulic geometry. We suggest that the degree of width variability exhibited in each test case encodes information related to variations in particle entrainment thresholds, which themselves may vary based on local slope or hydroclimatic variability. We propose that high-resolution downstream hydraulic geometry may also be used to assess the relative resilience of individual watersheds to shifting climate or discrete perturbations.