The Influence of Drying on the Aeolian Transport of River-Sourced Sand

Transgression and regression of water levels (stages) have impacted the evolution of aeolian landforms and sedimentary deposits throughout geologic history. We studied this phenomenon over a 5-day period of reduced flow on the Colorado River in Grand Canyon National Park, AZ, USA, in March 2021. These transient low flows exposed river-channel sand deposits to the air, causing progressive desiccation (drying) and thereby making these deposits susceptible to aeolian transport. We measured aeolian threshold friction velocities (u(*t)) for sand saltation and PM10 dust emissions, as well as other characteristics, on a subaerially exposed sandbar and downwind aeolian dunefield during each day of the low river flow. The sandbar transitioned from supply-limited to transport-limited aeolian sediment transport conditions during the regression in river water stage. A possible tipping point between the two transport conditions occurred approximately 48 hr after the drop in river flow. The empirically measured u(*t) decreased as the sandbar sediment dried with increased subaerial exposure time. Theoretical estimates and empirical measurements of u(*t) corresponded closely on the aeolian dunefield and on the sandbar when it was drier during the third and fourth day of the experiment. Eighty-seven percent of the variability in u(*t) was explained by empirical models that provide practical estimates of aeolian transport potential of subaerial river sediment deposits using monitoring data that are commonly available in this and other river systems. The work provides theoretical insight into the response of aeolian processes to sediment supply changes driven by periods of anthropogenic activity, drought, and climate change. Plain Language Summary Along the Colorado River in Grand Canyon National Park, windblown river sand provides important wildlife habitat, sandy areas for camping, and a protective cover for archeological sites but has decreased since construction of the upstream Glen Canyon Dam. One river management tool for increasing windblown sand is to release artificial "floods" from the dam to deposit sand above the level of typical river flows so it can be redistributed by wind. An alternative approach could be to lower river flows and expose sand that is usually underwater, allowing it to dry sufficiently to be moved by wind. Here we examine whether reducing flows to a low level (stage) for 5 days increases windblown sand. We learned that 48 hr after water stage dropped, a formerly wet sandbar had dried sufficiently so that wind speed, rather than dry sand abundance, had become the main factor limiting the amount of windblown sand. After 72 hr, the ability of wind to transport sand from the previously submerged sandbar was equivalent to an adjacent sand dune that was never inundated. These results show that low steady flows combined with wind offer an alternative to flooding for maintaining sandy landscapes in river valleys.