SEDIMENT DYNAMICS AT THE WEST BAY MISSISSIPPI RIVER DIVERSION

Attempts to restore the Mississippi River Delta often hinge on plans to partially divert the flow of the Mississippi River, allowing needed sediments and freshwater to enter the coastal wetlands and bays to initiate deltaic land-building processes. Despite studies from the Wax Lake Outlet of the Atchafalaya River deltas, and the Caernarvon and Davis Pond freshwater diversions, relatively little is known on the actual depositional processes that lead to land creation at river diversions. Here we report findings from the West Bay Mississippi River Diversion on the modern birdsfoot delta, the largest operable diversion in Louisiana that was constructed specifically for coastal restoration. The diversion consists of a cut in the Mississippi River levee that was constructed in 2003 with the hope of creating 10,000 acres of land over a 20-year project life cycle. These data include CHIRP sub-bottom profiles and a suite of sediment cores that were x-radiographed, and examined for sediment grain size, organic content and naturally abundant radioisotopes (⁷Be and ¹³⁷Cs). Results from this study show that West Bay experiences high rates of subsidence and a range of sedimentary environments, characterized by zones of erosion and deposition. Over the past 5 decades, deposition rates have ranged from 0.7 – 1.5 cm/yr, and appear to be driven by subsidence and flow down the Grand Pass distributary along the western edge of West Bay. More recent depositional rates range from 1.5 to 4.5 cm/yr and sediment dispersal patterns in the bay appear to be linked to flow from both the diversion and Grand Pass, winds, tides and coastal set up, and potentially influenced by subsidence. Overall, these findings suggest that for this location, new land will not be formed on subdecadal time-scales, though it may be formed on multidecadal to centennial time scales, which is consistent with historical studies.