SEDIMENT DYNAMICS AND HYDRODYNAMICS IN DEVELOPING CREVASSE SPLAYS IN THE LOWERMOST MISSISSIPPI RIVER: IMPLICATIONS FOR DELTA RESTORATION

Many deltas across Earth’s surface are in a state of degradation, threatened by rising sea levels, reduced sediment inputs, altered hydrodynamics and other anthropogenic changes. The Mississippi River Delta (MRD) exemplifies these impacts. Many scientists and policy makers agree that one of the best ways to restore this system is to partially divert the flow of the Mississippi River into adjacent wetlands and shallow bays, thereby re-initiating the natural land building processes that originally built the MRD. The West Bay Diversion is located ~7 km above the Head of Passes and is the largest diversion operating in the MRD that was specifically constructed for land creation. As such, it provides an ideal location for studies to investigate hydrodynamics and sediment deposition patterns where a diversion leaves the river and enters a receiving basin. Here we report on studies of hydrodynamics and sediment dynamics at the West Bay Diversion that were conducted in 2009. Our results indicate that patterns of sediment distribution in West Bay closely follow basin wide current flow patterns. In the spring and summer of 2009, approximately 2.1 x 10⁶ metric tons of sediment were deposited in West Bay. This is greater than the 1.5 x 10⁶ metric tons of sediment that entered West Bay through the diversion and suggests that the remaining 0.6 x 10⁶ metric tons came from neighboring distributaries. These results are contrasted to a study of the Brandt’s Splay Crevasse, a natural setting in Cubit’s Gap located directly across the Mississippi River from West Bay. Results from Brandt’s Splay indicate that channels can carry sand in suspension up to 6 km from the Mississippi River, and can contribute to sedimentation rates that range from 1-3 cm over a 100- day period. Results from both diversions show unexpected results, with sediments distributed much further than simple models of sediment dynamics would suggest. This findings point to the need for new models of geomorphic evolution of mouth bar and crevasse splay dynamics to inform global delta restoration strategies.