A network-based analysis of river delta surface hydrology : an example from Wax Lake Delta

River deltas are dynamic ecosystems of environmental, ecological, and societal importance. In coastal Louisiana, land loss and increased nutrient loading are altering the eco-geomorphic equilibrium, raising concern for the environmental concerns associated with climate and anthropogenic change. Over the last 100 years, nearly 5000 km2 of wetlands have been submerged due to a variety of environmental and anthropogenic forces. Wetland drowning and costal retreat is predicted to continue, threatening both human and environmental interests. As a result, mitigation efforts in the form of planned river diversions designed to build new land by natural delivering sediment to the once-leveed floodplains have been proposed. Denitrification in coastal wetlands has the potential to limit the risks of hypoxia and related eco-geomorphic issues by reducing the nitrogen export to receiving waters.
The goal of this research project is to understand and quantify to propagation of environmental fluxes through a delta distributary system. Water fluxes in the delta distributary network are not solely propagated within the distributary channels and interdistributary areas, taking the form of inundated island interiors surrounded by subaerial levees, are hydrologically important. At Wax Lake Delta (WLD) in coastal Louisiana, roughly 50% of the flow is exchanged to the island swamps, suggesting that significant portions of nutrients are transported to the island interiors. The hydraulic residence times (HRTs) of the islands are estimated to be 15 – 29 hours and 11 – 18 hours on Mike and Pintail Islands, respectively; both being well below HRT estimates for significant denitrification. Spatial variability in network structure, specifically channel width and frequencies of confluences and bifurcations, influences the transport dynamics within the delta. However, flow partitioning in major distributary channels at WLD is relatively constant with time, supporting the hypothesis that network structure controls flux dynamics.