Floods and cold front passages: Impacts on coastal marshes in a river diversion setting (Wax Lake Delta Area, Louisiana)

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Diversion of the Mississippi River is considered the most effective way of offsetting Louisiana's coastal plain land loss. The Wax Lake delta, a man-made diversion that represents mostly sand-rich distributary mouth bar deposits, is cited as a model for land building from a large river diversion. The fine-grained suspended load is mostly deposited outside of "the delta." Numerical simulations of diversion delta building focus primarily on sand deposition. The fine-grained suspended load is only secondarily considered. Research results reported here suggest that natural processes associated with the synergistic relationship between floods and cold front passages can effectively distribute suspended sediments to maintain and rebuild wetlands outside the sand-rich delta. Measurements (fixed acoustic Doppler current profilers) of current speed, current direction, and water level in a mainland marsh channel, north of Atchafalaya Bay, indicate that as cold fronts approach, local water levels can be elevated by as much as 1 m, and normal discharge into the bay is reversed so that flow is into the coastal plain marshes. LIDAR bank elevations indicate that overbank flow is initiated by these events. Streamside sediment accretion (2-y monitoring period) using feldspar plots indicates up to 6.8 cm. Longer term Cs-137 data reveal an average sediment accretion rate of >1 cm y over 50+ years. Marsh flooding results in denitrification of nitrate in river water plus nutrient uptake resulting in increased plant productivity, considerably greater than for similar coastal plain marshes free of riverine sediment input. Fine-grained suspended sediments are important for rebuilding Louisiana's coastal plain, which is naturally constructed mostly of riverine silt and clay plus organic matter. -1

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Journal of Coastal Research

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