Understanding environmental controls on the hydrodynamics of a river delta's interdistributary islands

River deltas are dynamic systems of environmental, ecological, and societal importance. In coastal Louisiana, wetland loss and increased nutrient loadings are altering the eco-geomorphic equilibrium, raising awareness to the environmental concerns associated with climate and anthropogenic change. Over the last 100 years, nearly 5000 km² of wetlands in coastal Louisiana have been submerged due to a variety of environmental and anthropogenic forces [Day Jr. et al., 2007]. Denitrification in coastal Louisiana wetlands has been shown to reduce the export of nitrogen to receiving waters, which limits the risks of hypoxia and related environmental issues [Rivera-Monroy et al., 2010]. Specifically, inundated interdistributary islands are hypothesized to be zones of potential significant nitrogen cycling. The goal of this research is to understand the hydrodynamics of the inundated interdistributary islands at Wax Lake Delta (WLD) in Louisiana, USA. Previous work has shown that a significant portion of the water at WLD is exchanged between the channels and the islands [Buttles et al., 2006; Hiatt et al., 2012]. We employ field measurements to monitor the propagation of fluorescent dye through the island and to estimate the hydraulic residence time (HRT). We compare the results to modeling output using Delft3D. The field and modeling results are supported by ADCP measurements made during the field study. Water depth, temperature, and salinity data were also collected during the tracer study. Both dye propagation patterns and measured velocities point to different hydrodynamic behavior depending on environmental conditions (tides and wind). For example, dispersion of dye is increased during flood tide and is minimized during ebb tide. HRTs are on the order of hours under the measured environmental conditions, whereas HRTs associated with significant dentrification have been shown to be on a monthly timescale [Branhoff, 2012]. Using principal components analysis, we quantify the contribution of wind and tidal forces on the hydrodynamics of the island interior using measured water level, temperature, and wind data. Wind is shown to be a significant driver of delta island hydrodynamics [Geleynse et al., 2013], along with tides, indicating that the often-excluded wind forces should be included in numerical modeling efforts. Branhoff, B. (2012). Thesis, Louisiana State University. Buttles, J., Mohrig, D., Nittrouer, J., McElroy, B., Baitis, E., Allison, M., et al. (2007). AGU, Fall Meeting 2007 Day, Jr., J. W., Boesch, D. F., Clairain, E. J., Kemp, G. P., Laska, S. B., Mitsch, W. J., et al. (2007). Science, 315, 1679-1684. Geleynse, N., Hiatt, M., Sangireddy, H., & Passalacqua, P. (2013) Geophys. Res. Lett., in review. Hiatt, M., P. Passalacqua, J. Shaw, D. Mohrig (2012). EP33B-0874. AGU, Fall Meeting 2012 Rivera-Monroy, V. H., Lenaker, P., Twilley, R. R., Delaune, R. D., Lindau, C. W., Nuttle, W., et al. (2010). J. Sea Res. , 63, 157-172.