The Breton Sound Basin is a 1700 km² estuary on the Louisiana coast that receives fluvial inputs through a freshwater diversion structure at Caernarvon. This diversion is located on the east bank of the Mississippi river, southeast of New Orleans. The Caernarvon structure was built in 1991 as a gravity-fed, five-box culvert with vertical lift gates and a maximum flow of 226 m³ sec^-1. Freshwater discharge averages 21 m³ sec^-1; current minimum and maximum flows are generally 14 and 114 m³ sec^-1, respectively, with summer flow rates near the minimum and winter flow rates 50-80% of the maximum (Lane et al. 1999).

The estuary extends for approximately 80 km to the Gulf of Mexico. The upper portion of the estuary is extensively marsh with a couple small lakes and few convoluted channels. The lower estuary is primarily open water in Breton Sound.