GRAIN SIZE OF CREVASSE-SPLAY DEPOSITS IN THE MISSISSIPPI DELTA: IMPLICATIONS FOR COASTAL RESTORATION

Crevasse splays can be used as natural analogs of river diversions and their sediment trapping efficiency has important implications for coastal restoration planning. To quantify sediment trapping efficiency of Attakapas Splay, a recent crevasse splay of Bayou Lafourche (a former Mississippi River course), the grain-size distribution of the splay deposits was investigated. The sediment texture of the splay deposits was studied by a number of cross sections and qualitatively described in the field following the United States Department of Agriculture (USDA) texture classification. Sediment samples were taken to transfer qualitative field descriptions into quantitative data. A total of 43 samples were analyzed. A Horiba LA-300 was used to measure grains less than 106 μm in diameter, a CAM-sizer for grains greater than 106 μm; three trials were conducted for each sample. The average grain-size distribution of the three measurements was used to calculate the content of clay (<2 μm), silt (2-63 μm) and sand (>63 μm) for each sample. The grain-size analysis reveals that the splay deposits are dominantly silty. Considering that the Mississippi River sediment load is silt-dominated, the Attakapas Splay likely trapped most of the sediment load brought to the splay. This makes it substantially different from the Wax Lake Delta that is sand-dominated and has a ~30% sediment trapping efficiency, largely due to its location on the open coast and its exposure to waves and currents that export much of the finer fraction offshore. This striking difference suggests that the Mississippi River sediment load can be more effectively used in wetland creation than in the case of the Wax Lake Delta, a commonly used analog for river diversions. Thus, we conclude that river diversions preferentially be planned in vegetated settings farther inland.