Ecogeomorphology of fluviodeltaic systems : effects of vegetation on delta morphodynamics
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Vegetation exerts a strong control on landscape dynamics that can greatly influence landscape morphology and sediment transport. This dissertation aims to increase our understanding of vegetated delta morphodynamics using physical experiments and fieldwork. The first study presents the results of the first set of delta experiments using alfalfa for vegetation. Plants amplified the depositional nature of experimental deltas such that channels were blocked by vegetation. Discharge fluctuations between floods and base flow increased channel relief and limited vegetation colonization to maintain the channel network. The second study seeks to understand the morphological differences between deltas with relatively high and low discharges and how channel-plant feedbacks change with discharge. High discharge deltas had statistically different channel networks and delta shapes compared to low discharge deltas, and the input discharge controlled how and where plants established on the delta, creating different vegetation patterns and channel-plant interactions. Regardless of discharge, plants enhanced channel bifurcation and smoothed shorelines. The third study examines the timescales controlling vegetated delta channel network development by comparing experiments with varying seeding densities and delta sizes. As deltas grow, their channel volume increases such that sufficiently large deltas can maintain an underfilled channel network with channel memory across several flood cycles. Plants initially enhance channel bifurcation, but if the number of plant patches on the delta does not continue to increase with delta size (i.e. patches merge), then vegetation can block channels and accelerate channel filling. There is therefore a competition between timescales of delta growth and patchiness development that determines if and when a channel network can be maintained. The last study utilizes seasonal observations from the Wax Lake Delta, Louisiana to understand the hydrodynamics of a single delta island. Flow in and around the island is most strongly controlled by seasonal changes in river discharge. However, low river discharge or proximity to the bay can increase the influence of tides such that flow through secondary channels connecting the islands and main channel network can be reversed. I hypothesize that plants increase water storage in the island lakes during rising tide to create favorable conditions for secondary channel flow reversal.