NEW ORLEANS - On July 11, 2023, the U.S. Army Corps of Engineers, New Orleans District began construction of an underwater sill across the bed of the Mississippi River channel to prevent further upriver progression of salt water from the Gulf of Mexico.
USACE awarded a contract to Weeks Marine for construction of the saltwater sill at approximately river mile 64 near Myrtle Grove, La. The initial phase of construction will build the sill to an elevation of -55 feet. At that height, USACE will monitor progression of the saltwater wedge to determine if additional height is needed to meet the saltwater intrusion mitigation requirements.
The Mississippi River’s volume of water has fallen to a level that allows salt water from the Gulf of Mexico to intrude upstream. The intrusion of salt water into the river is a naturally occurring periodic condition because the bottom of the riverbed between Natchez, MS, and the Gulf of Mexico is below sea level. Denser salt water moves upriver along the bottom of the river beneath the less dense fresh water flowing downstream. Under normal conditions, the downstream flow of the river prevents significant upriver progression of the salt water. However, in times of extreme low volume water flow, unimpeded salt water can travel upriver and threaten municipal drinking water and industrial water supplies.
The construction of the sill is a mitigation feature required as part of the effort to deepen the shipping channel from 40 feet to 45 feet and subsequently to 50 feet. The sill is designed to create a large reservoir to collect and hold saltwater for a period of time equal to the increase in duration of the saltwater intrusion caused by the channel deepening.
USACE constructed a similar underwater sill in 1988, 1999, 2012 and 2022 at river mile 64, near Myrtle Grove, La., to arrest the progression of saltwater intrusion during that year’s low water season. The sill will naturally erode when the Mississippi river returns to the flow levels required to push the saltwater wedge back down the river to the Gulf of Mexico.
Release no. 23-017