Quantifying Coastal Fluvial Morphodynamics Over the Last 100 Years on the Lower Rio Grande, USA and Mexico

Downstream changes in fluvial channel morphology are commonly observed in association with the backwater zone, where rivers transition from quasi-uniform flow with normal-flow depth to gradually varying flow. This transition is linked to changes in channel morphology and mobility and resulting fluvial stratigraphy. However, the majority of systems studied to date are perennial rivers with relatively consistent flow conditions. Here we investigate the evolution of a large river with significant flood-to-baseflow variability as it transverses and builds a large delta. We provide the first comprehensive study of the morphology and morphodynamics of the lower Rio Grande, a major continental drainage system that enters the western Gulf of Mexico. We quantify the morphology of the current Rio Grande channel and document spatial trends in channel geometry and kinematics using lidar, historical surveys, and hydrographic analysis. The modern Rio Grande channel morphology does not significantly vary toward the coast. Rather, the channel width, levee, and bed slopes remain nearly constant for similar to 200 river km. We find historical migration rates between 10 and 100 m/yr with no significant reduction toward the coast in contrast to previously studied systems. We propose that this invariant channel geometry and sustained high migration rates are signatures of the channel not requiring adjustment within the lower coastal reach to accommodate baseflow conditions, and the channel remains continuously adjusted solely to peak flow conditions.