HIGH RESOLUTION MONITORING OF OUTFLOW FROM A RIVER DIVERSION IN A LOUISIANA ESTUARY

Breton Sound is located on the southeastern side of the progradational delta lobe of the Mississippi River. It formed by subsidence subsequent to the abandonment of the St. Bernard delta complex 4000 ybp. Since 1956 approximately 3,400 acres of marsh in Breton Sound have been lost naturally, as part of the delta cycle. However, this process has been accelerated by oil and gas activities, saltwater intrusion and sediment and nutrient starvation from reduced freshwater inflow. The Caenarvon freshwater river diversion was created in 1991 to pump up to 8000 cfs of Mississippi River water into the headlands of the Breton Sound estuary with the goal of mitigating this wetland loss. The project seeks to accomplish this by controlling salinity and supplementing nutrients and sediments from the Mississippi River. The bays are important to oyster production and as breeding areas for shrimp and food fishes, while the marsh areas produce food for a number of animals including alligators, and a large amount of migratory waterfowl. In addition, the area acts as a sink for the excessive amounts of nutrients in the Mississippi River, helping to reduce the annual hypoxic zone off the coast of Louisiana. In 2002 the Louisiana Department of Natural Resources added levees and other water control structures to better utilize flow of water from the diversion. 6 Sonde discrete water sampling monitors were placed in important hydrologic locations throughout the area to further understanding about the effects of the control structures on water flow through the system. Water levels were measured once each hour for a period of three years starting in 2001, providing data both before and after structure installation. Analysis of the data demonstrates the effectiveness of the structures’ ability to deliver water to places that were previously hydrologically isolated. The dataset itself provides a very high temporal resolution for an extended period of time. This yields more complete information about velocity and amplitude of a pulse of water’s movement through the system than was previously available. Understanding the residence time of the pulse will allow more efficient and effective management of the diversion.