Fluvially Sourced Sediment Supply and Climate Modulate Morphodynamics and State Transitions in Bistable Aeolian Dunefields

Abstract

Alternative stable state theory in aeolian geomorphology describes the occurrence of bare and vegetated aeolian dunes in response to long-term climatic and ecological drivers. Transitions between states, as well as their co-occurrence (bistability), can arise from perturbations to drivers, disturbance, and associated biophysical feedbacks. Here, we examine the role of upwind fluvial sediment disturbance as a driver of aeolian state transitions. We analyze 24 source-bordering aeolian dunefields along 400 km of the Colorado River in Grand Canyon, USA, where active and inactive aeolian dune states occur downwind of fluvial sediment sources. Four decades of topographic change detection following upstream dam emplacement revealed that active aeolian dunes—which had indicators of recent aeolian transport—experienced four times more surface change than inactive dunes lacking such indicators of recent aeolian transport. Persistent aeolian activity occurred where bare upwind fluvial sandbars were maintained as sediment sources, whereas transitions toward inactivity accompanied the loss of bare sandbars and resulted in 50% less topographic change. Contrary to theory, the transition to aeolian inactivity did not correspond to significant reductions in erosion, despite decreased topographic changes. These results broadly demonstrate the effects of biophysical feedbacks within fluvially sourced aeolian environments. Specifically, they show that reductions in aeolian activity are not synonymous with geomorphic quiescence, as other sedimentary processes may become dominant agents of change within these coupled sedimentary systems that are ubiquitous throughout dryland regions on Earth.