DELTA EVOLUTION DURING A SINGLE FLOOD EVENT IN A RIVER DOMINATED WETLAND

In addition to their ecological and economic importance, river deltas worldwide are known to serve as sediment sinks for material exported from continental interiors. An understanding of the regional and local sediment dynamics within a delta is critical to predicting the future fate of a delta, and for managing that delta for human use. This study takes place in the lower Mississippi River delta, a region undergoing rapid subsidence, and where large scale river realignments are being considered as a means of protecting human lives, property and culture against encroachment from the Gulf of Mexico. Existing crevasse splays can serve as natural laboratories to study the dynamics of such realignment schemes.

Here we present field data that was taken to investigate the sediment dynamics of a river dominated wetland in the lower Mississippi River, with a particular focus on the processes pertaining to the formation of a distributary mouth bar within an actively growing, natural crevasse splay. The data set consists of simultaneous hydroacoustic and optical measurements in the receiving basin and channel network of the splay, and sediment samples and short cores taken on and in the vicinity of the distributary mouth bar. The short cores were dated with Beryllium to provide a 90 day depositional record, which was then correlated with upstream river discharge to show how deposition in the splay varies with fluvial input.

With this data set we are able to 1) identify depositional hotspots for sediments of various sizes, and constrain the flow conditions under which the deposition occurs, 2) test the hypothesis that capture rates for fines in semi-enclosed basins such as the one studied will be significantly higher than capture rates for fines in more exposed deltas such as the Wax Lake delta, and 3) work towards developing a "reduced complexity" numerical model that can be calibrated with field data and then used to identify parameters that significantly affect sediment deposition patterns.