PRELIMINARY CHARACTERIZATION OF THE LOWERMOST MISSISSIPPI RIVER FLOODPLAIN SEDIMENT AND IMPLICATIONS FOR THE RESTORATION OF THE MISSISSIPPI DELTA

Wetland loss and shoreline retreat have been documented in the present Mississippi Delta, and the consequent deterioration of large tracts of marshland habitat poses a substantial ecological crises, as well as a significant threat to infrastructure due to the reduction of wetlands that buffer small coastal communities and the metropolitan area of New Orleans from storm surge flooding. It has been recognized that one of the primary causes of wetland loss in the Mississippi Delta is the construction of levees that prevent overbank flooding and the distribution of fine sediment and nutrients to the adjoining wetlands, a natural process that is necessary to compensate for land drowning due to natural subsidence. Measurements from the juvenile and developing Wax Lake Delta in coastal Louisiana show that greater than 50% of the delta is comprised of fine sand, and therefore it is critical to distribute fine sediment to mitigate land loss in coastal Louisiana. Thus, of fundamental importance to the design of delta restoration projects is to assess the quantity of sand available in the present Mississippi River channel and floodplain. Here we present a preliminary budget for floodplain material in the lowermost 500 km of the Mississippi River, or that portion of the river between the Old River Control Structure and the Gulf of Mexico. The Mississippi River floodplain and its levees are mapped from a Lidar system, and the data are used to construct a 5-m resolution DEM. Levee elevations and the floodplain width are measured, and these data are combined with previously collected river-channel data to determine how the composite geometry of the river cross section varies in the downvalley direction. Channel migration rates are estimated using previously published data. These data are used to calculate a mean annual floodplain material budget and calibrate a morphodynamic model that routes bed and floodplain material through the Lowermost Mississippi River. A secondary goal of this research is to use information on potential sand deposits in the Mississippi River valley to target future field studies on the Mississippi River floodplain and river-channel bars that will assess the potential for the channel and floodplain to deposit and release sandy sediments.