Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Geosciences (PhD)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Shaw, John B.

Committee Member

Naithani, Kusum J.

Second Committee Member

Sharman, Glenn R.

Third Committee Member

Covington, Matthew D.

Keywords

coastal dynamics; coastal wetlands; experiment; marsh; mass balance; river delta; sediment deficit; sediment transport

Abstract

Globally, many of the largest river deltas contain vast marsh platforms that are currently threatened due to a combination of anthropogenic alterations to rivers and increasing relative sea level rise. Restoration and management plans for river deltas depend on optimizing riverine sediment accumulation in marsh platforms. However, the accumulation of organic material in marsh platforms is often neglected in predictive models and the interaction of ecogeomorphic processes governing marsh accumulation with the physical processes governing river delta growth is poorly understood.

Herein, I investigate this complex relationship through a combination of field and experimental studies. I show that in coastal Louisiana, organic material accounts for ~25% of all sediment accumulation by mass, which has historically been neglected from predictive sediment budgets for this area. The incorporation of organic material into a sediment budget for coastal Louisiana shows a much smaller sediment deficit than previous estimates that neglected organic sediment accumulation. I show that the organic material supplements sediment accumulation rates in both areas connected to and disconnect from rivers. Similarly, I show that mineral sediment accumulation is important not only in areas connected to the river (the source of mineral sediment), but also in the regions disconnected from the river. This field study shows that these two processes are intertwined and work together to create a dynamic delta top.

I then work to understand how marsh (organic) sediment deposition impacts the formation of a river delta and its distributary channel network by analyzing data from an experimental delta with marsh deposition (treatment) and one without marsh deposition (control). I use kaolinite clay as a marsh proxy in the treatment experiment and deposit the marsh in low-lying regions of the delta top. Marsh deposition in the treatment experiment was based on a simplified model that relates primary productivity to elevation relative to sea level. I show that marsh sedimentation in an experimental setting alters the morphology, mass balance, and channel dynamics of a river delta. Notably, the the delta morphology and channel profile of the treatment experiment is more akin to global river deltas. The treatment experiment had a smaller median slope in the terrestrial marsh window. Further, the treatment channels had a long backwater reach not often present in the control channels. Even the incorporation of a small amount of marsh sediment (~8%) was enough to alter the system completely. Thus, I suggest that the ecogeomorphic processes that govern marsh sedimentation have a first-order control on delta formation, specifically the accumulation of non-riverine delivered, low bulk density, highly compactable sediment on marsh platforms, and should be incorporated in numerical, experimental, and field studies.

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