Framework for Assessing Dynamism and Persistence of Eddy-Sandbar Complexes in the Grand Canyon

Sandbars along the Colorado River in the Grand Canyon are a fundamental part of the landscape - creating habitat for native plants and animals, providing camping beaches, and supplying sediment needed to protect archaeological resources. The Glen Canyon Dam just upstream of the Grand Canyon reduces the amount of sediment available for sandbars and the flows available to deliver sediment. Current monitoring efforts do not and cannot produce a complete (spatial) sample of all the change in storage throughout the Canyon, so we present a method for characterizing eddy-sandbars in terms of their dynamism and persistence to help classify monitored sandbars and use this to infer changes about non-monitored sandbars. We defined dynamism as the degree to which the eddy-sandbar complex experiences scour and fill during a) baseflow conditions, and b) flood conditions. It was quantified as the total volume change experienced from one survey to the next, normalized by the area over which the change occurs. We defined persistence as the proportion of the sandbar above a baseflow shoreline that remains present over time. This was quantified as the average fill ratio for the eddy post-dam, based upon interpretations of aerial photography. To assess dynamism and persistence, we used sandbar topography data available from over nearly two decades, surveyed repeatedly with a combination of multi-beam and single-beam bathymetry, photogrammetry, LiDaR and total station surveys. Utilization of Geomorphic Change Detection software allowed for iterative calculations of changes in sediment volume. Results show that complex relationships exist between dynamism/persistence and geomorphic, hydrologic and vegetative metrics. By exploring these relationships, we gain insight into the dynamism and persistence of eddy-sandbar complexes through time in response to a highly regulated flow regime and experimental flood releases.