Sedimentary Characteristics of an Estuarine Marsh System and the Roles of Storm Overwash, Micro-tidal Currents, and Organic Production: Big Branch Marsh, Louisiana
The goal of this thesis is to document sediment composition in Big Branch Marsh National Wildlife Refuge, located on the northern shore of Lake Pontchartrain in St. Tammany Parish, Louisiana, to discern the relative influences of fluvial, tidal, and storm overwash transport dynamics of a micro-tidal estuarine marsh system. Collected sediment samples from tidal channel beds, pond bottoms, and marsh surfaces were analyzed for organic content and particle-size distribution. Parameters of sub-environment groups were compared to find differences representing changes in relative transport energy and depositional processes. Whereas some results adhered to the model of fining patterns with increasing distance into the estuarine marsh environment, there were some anomalies that suggested that such a simple correlation cannot be used to predict the sediment distribution for the study area. Anomalies are explained by the combination of two different sediment supply mechanisms: (i) storm overwash and (ii) daily micro-tidal currents. Relatively coarse-grained sediments are uniformly deposited in all subenvironments during overwash events. Subsequently, micro-tidal currents introduce relatively fine-grained sediments to the immediate Bayou Lacombe corridor and proximal marsh sub-environments, but affects diminish toward the distal areas of the system. Thus, coarse sediments deposited by storm overwash in the interior marsh are not effectively "masked" by micro-tidal processes. The combination of these two depositional processes with vertical accretion of organic matter is the mechanism by which micro-tidal marshes are sustained. Results should inform coastal restoration efforts, notably activities to introduce additional sediment into marsh environments to offset relative sea-level rise and shoreline erosion.
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