Punctuated Sand Transport within the Lowermost Mississippi River

Quantitative estimates of sand transport within tidally-influenced reaches of large, continental scale river systems have been difficult to acquire, owing to the rigorous physical and dynamic conditions that exist in such settings. Recent advances in observational techniques, however, have provided high resolution data necessary to constrain bed-material flux and to evaluate temporal trends of sand delivery to continental margins. Studies conducted in the lowermost Mississippi River document dune geometries and translation over a range of water discharge conditions using a multibeam bathymetry profiler. Our results indicate that active dunes fields in the lowermost river cover roughly two-thirds of the channel bed and are constrained to straight-reach segments between bends. During low and moderate water discharges, dune geometries are not depth limited and range from 0.5-2 m and 20-40 m for height and wavelength, respectively. Bedform flux, determined by measuring dune translation ranges from 1-5 x 10^-5 m² s^-1. Low and moderate flow conditions persist in the lower river for >95% of an average discharge year. Data collected during high discharge events demonstrate a substantial change for bedform scale. Dune size ranges 5-10 m and 150-200 m in height and wavelength, and bedform flux is 5-8 x 10^-4 m² s^-1. These transporting conditions generally occur every 1-4 years, for an average duration of ~5% of a discharge year. This order-of-magnitude increase in bedform sand flux for the lowermost Mississippi River suggests storage and release of sands within channel reaches, and demonstrates the importance of flood events for delivering coarse sediment to the Mississippi River delta.