The Bay of St. Louis (St. Louis Bay; French: Baie Saint-Louis) is a shallow-water, partially enclosed estuary of the northeast Gulf of Mexico along the southwestern coast of Mississippi. The estuary receives freshwater input from one blackwater, or swamp-land, the on the west and the Wolf River on the east and some smaller streams ; these are mixed in the bay with salt water from Mississippi Sound and the . The waters are comparatively well mixed, with an average salinity of less than 20.