Proceedings of the 2020 Mississippi Water Resources Conference

Year: 2020 Authors: Moody A., Shiller A.M.

The Bonnet Carré Spillway, a structure that prevents flooding in New Orleans due increased water levels in the Mississippi River, was opened for a total of 122 days in 2019 from February to July. This resulted in a massive release of freshwater into Lake Pontchartrain, which was then funneled into the Mississippi Sound, causing significant chemical and ecological impacts there. Fortuitously, this event occur in the midst of our ongoing sampling campaign aimed at elucidating nutrient distributions and groundwater contributions in the Sound. The impacts from the influx of this freshwater event were mainly focused on the western half of the Sound, closest to outflow from Lake Pontchartrain. Water quality indicators from our time series as well as snapshots from before, during, and after the opening of the Spillway indicate that the western Sound experienced rapid changes in water chemistry. For instance, the average salinity of the Sound decreased from 20 to 5 after the opening of the Spillway. Application of an apparent age model using the ratio of short-lived radium isotopes indicates that the flushing rate of the western Sound during the Spillway opening increased dramatically, changing the physical characteristics of the Sound as well. The high levels of precipitation in the 2019 season led to some elevated nutrient levels across the Sound from increased river discharge.