Long-Distance Delivery of Fine Sediment to Wetlands from a Bay-Head Delta: A Possible Analog for Large River-Sediment Diversions and Coastal Wetland Restoration.

Large river-sediment diversions are being proposed as primary tools for wetland restoration and land building in the Mississippi River Deltaic Plain. To date, much emphasis in the study of diversion performance has focused on proximal sand-rich deposition. To better understand far-field dispersal and deposition of fine sediments, which account for the majority of sediment supply, sediment cores from Fourleague Bay, Louisiana and adjacent marshes are being analyzed for ⁷Be, a naturally occurring radioisotope, as well as mineral and water content. The bay, which receives about 2% of total sediment discharge from the Atchafalaya River, is 20 km long and extends southeastward from the river’s outlets. During 2015, time-series cores have been collected from ten sites, five located in the bay and five from the marshes, and have been analyzed for ⁷Be using gamma spectrometry. All sites fall within a distance of 9 km to 25 km from the outlet of the Atchafalaya River. Preliminary results show that cores sampled during the summer season, collected July 2015, contain higher activity of ⁷Be in the marsh cores, with no detectable ⁷Be contained in bay cores. The highest activity of ⁷Be, 3.2 ± 0.79 dpm/g, was found in the top two centimeters of the marsh core closest to the river mouth, site FLM-1. No activity was detectable beyond a depth of 4-6 cm. Surficial activity declined in samples further from the river mouth, with the lowest detectable value being 1.8 ± 0.72 dpm/g in the top two centimeters of site FLM-5. Results show that the range of influence for fine sediment delivery to wetlands is > 10 km, suggesting that the area that may be nourished sediment from a large diversion extends far beyond the footprint of proximal sandy deltaic deposits.