Accretion and Vegetation Community Change in the Wax Lake Delta Following the Historic 2011 Mississippi River Flood

During the 2011 Mississippi River flood, discharge to the lower river exceeded that of the 1927 and 1937 floods and the lower river remained above flood stage for nearly 2 months. A combination of WorldView-2 and Land Satellite 5 Thematic Mapper (Landsat 5 TM) imagery was used to assess the impact of this flood event on the Wax Lake Delta, one of few areas where the river is building new land. Vegetation community change was mapped from 2010 to 2011 and related to elevation change using plant species elevation distributions calculated from light detection and ranging (LIDAR) data. Changes in the land area in the delta were also assessed by regressing land area against water level for a series of pre- and postflood Landsat 5 TM images. The results indicate a net growth of 6.5 km(2) at mean water level and 4.90 km(2) at mean sea level. Areal gains were greatest at high water levels, indicating substantial vertical accretion across the subaerial delta. At least 8.7 km(2), or 31.8%, of the area studied converted to a higher-elevation species. The most change occurred at low elevations with conversion from fully submerged aquatic vegetation to Potamogeton nodosus and Nelumbo lutea. Conversion to lower-elevation species occurred across 3.4 km(2), or 12.8% of the study area, while 55.5% remained unchanged. The results highlight the importance of infrequent, large flood events in the maintenance of river deltas and provide a reference for estimating the impact of proposed large-scale river diversions on the Mississippi River Delta.