DEPOSITIONAL PROCESSES OF THE FORT ST. PHILLIP CREVASSE SPLAY ON THE LOWER MISSISSIPPI RIVER FROM 210PB AND 137CS GEOCHRONOLOGY

In 1973, a natural crevasse opened in the lower Mississippi River bank near Fort St. Philip, due to flooding of the Mississippi River. To better understand land-building dynamics near an evolving crevasse splay, sediment cores were collected in the Fort St. Philip area in 2013. Cores were analyzed by gamma spectrometry of the radioisotopes Pb-210 and Cs-137, and published satellite imagery has been reviewed. Image analysis suggests land loss from the time the crevasse formed until 1998, followed by land gain. Sediment profiles of Pb-210 and Cs-137 are complex, and appear to tell record change from a marine-dominated estuarine system beyond the river to one influenced by both fluvial and marine sedimentation, after the crevasse. This is suggested by core Pb-210 profiles, which show a mid-depth minimum, above and below which activities increase slightly with some scatter. We interpret the minimum to occur at the approximate time of crevasse opening, when the river became connected to the estuary via a new distributary channel. This newly added fluvial source of water and sediment may have reduced Pb-210 activities supplied to the seabed. Subsequent sediment accumulation from a mostly fluvial source then produced the upward-increasing Pb-210 profile, from which we calculate sediment accumulation rates. The transition from losing land to gaining land around 1998 is broadly consistent with our measured patterns of sediment accumulation, which are now likely influenced by both fluvial processes, as well as marine processes such as hurricane resuspension and storm surges. These observations highlight the complexity land building near an engineered river-sediment diversion such as those now planned for the lower Mississippi River.