Characterizing Delta-Scale Connectivity Using Entropic Measures

Abstract:

Hydrologic connectivity in river deltas between distributary channels and inter-channel islands brings sediment and nutrients onto islands resulting in distinct vegetation zonation patterns on island interiors. Deltaic islands can also remain inundated for long periods of time, providing the conditions for denitrification to occur. To understand these dynamics and the overall complexity of the deltaic plain, water, sediment, and nutrient fluxes need to be considered at the network scale. Some key questions are what effect does external environmental forcing, such as river discharge, wind, and tides, have on spatial patterns in the delta, and what role do internal dynamics play in deltaic evolution.

We apply the mutual information and transfer entropy metrics to quantify couplings between environmental controls and delta variables on islands and channels in Wax Lake Delta, a naturally prograding 100km² river delta in coastal Louisiana, USA. Mutual information measures the amount of shared information in a coupling, while the transfer entropy measures the information transfer between two variables. With this method, we are able to analyze different dimensional variables across a variety of spatial and temporal scales. Continuous water levels, water temperature, and turbidity data were collected in three channels and on five islands from November 2013 to August 2014. Continuous surface water nitrate concentrations were collected at six locations on one island in the delta during the summer of 2015. The data occur over a suite of discharge, wind, and tidal conditions. While discharge is a predominant force, winds and tides play a significant role in water movement and residence time inside the islands. Nitrate fluctuations, in response to environmental forcing, vary spatially and temporally at the island scale, which will have an effect on nitrate dynamics on island interiors. Turbidity and water levels show variation at the network scale, leading to patterns of inundation across the delta.