Understanding controls on island inundation in a river delta using information theory

Delta inter-channel islands are land masses bounded by channels, resulting from bifurcations and junctions that characterize delta networks. Reflecting the morphology of the channels and the relationship among neighboring channels, delta islands are among the best descriptors of a deltaic network. The islands also play a fundamental role in the propagation of environmental fluxes through the delta and support important ecological processes such as denitrification. Often flooded, delta islands remain inundated for long periods of time suggesting that hydraulic residence time in islands may be longer than in channels, thus allowing for significant denitrification to occur. Which forcings among wind, tides, and discharge contribute to island inundation is a key question.

In this work, we apply information theory to identify environmental controls on island inundation in Wax Lake Delta, a naturally prograding delta in coastal Louisiana, USA. We analyze the delta as a “hydrologic web” consisting of connected channels and inter-channel islands to use a network approach and gain insights across spatial and temporal scales. Continuous water level data were collected by sensors placed on five inter-channel islands and a levee for a duration of three months, spanning spring floods and summer flows. Information theory statistics, such as transfer entropy and mutual information, were calculated for all sensors to characterize controls on island inundation in terms of strength, direction, and scale. While high flows (discharge above median) certainly exert significant control on island inundation, wind and tides can cause significant water level set-up and draw-down and control island inundation particularly in the case of low flows (discharge below median). A spatial gradient of inundation response to forcings is observed both at network-scale and at single island-scale. The results quantify the important role played by tides and wind on island inundation in a river-dominated delta. Implications for the transport of sediment and nutrients across the delta network are discussed.