INFLUENCE OF SUSPENDED LOAD ON THE DUST EMISSION POTENTIAL OF MISSOURI RIVER SAND BARS

The thick Peoria Loess that mantles the southern bluffs along the 59-mile segment of the Missouri National Recreational River is assumed to be derived from wind erosion of expansive late Pleistocene braid bars in glacial melt water streams choked with suspended load. Modern day suspended load values are greatly reduced since the construction of Gavins Point Dam near Yankton, SD. The flood of 2011 regenerated many sand bars in the Missouri River, providing an opportunity to measure their dust emission potential. Suspended load data was collected by using a depth integrated sampler on transects of the Missouri River and its tributaries downstream of the dam. Additionally, PI-SWERL (Portable in situ Wind Erosion Lab) measured the amount of dust generated by bare sand bars. Sand bar and suspended load samples were collected and analyzed for grain size and composition. Each bar was mapped using GIS by the type of surface including fluvially deposited sand and gravel with occasional silt rich zones, eolian sand dunes and wind ripples, and gravel lags. Results indicate that low percentages of silt in the sand bars are consistent with the low suspended load values of the Missouri. Tributaries may only affect suspended load of the Missouri River and the availability of fine sediments for sand bar dust emissions when tributary discharge is high. Rapid eolian and fluvial erosion of the sand bars make bars short-lived dust sources. Even though suspended load concentrations and frequency of bar generation are greatly reduced from late Pleistocene conditions, dust emission measurements from flood-generated bars can provide a first approximation for volumes of dust produced by sand bars.