USACE Building an Underwater Sill in La. to Limit Saltwater Intrusion in Mississippi River

USACE is building an underwater sill in the Mississippi River near Myrtle Grove, La., to curb saltwater intrusion caused by the deepening of the shipping channel. This preventative measure will safeguard municipal and industrial water supplies from Gulf saltwater. Similar sills have been installed in previous years to combat saltwater movement in periods of low river flow.

October 2, 2025 - Southeast Edition
USACE New Orleans District

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The New Orleans District office of the U.S. Army Corps of Engineers has started constructing an underwater sill across the bed of the Mississippi River.

The New Orleans District office of the U.S. Army Corps of Engineers (USACE) has started constructing an underwater sill across the bed of the Mississippi River channel to arrest, or stabilize, further upriver progression of saltwater from the Gulf of America.

The federal agency recently awarded a contract to Weeks Marine in Cranford, N.J., for the building of the saltwater sill in the vicinity of River Mile 64 near Myrtle Grove, La.

The initial phase of work will install the sill to an elevation of -55 ft. At that height, USACE will monitor progression of the saltwater wedge to determine if additional height is needed to meet saltwater intrusion mitigation requirements.

The Mississippi River's volume of water has fallen to a level that allows saltwater from the Gulf of America to intrude upstream. That progression is a naturally occurring and periodic condition because the bottom of the riverbed between Natchez, Miss., and the Gulf is below sea level.

Denser saltwater moves upriver along the bottom of the river beneath the less dense freshwater flowing downstream, the USACE explained in an Oct. 1 news release.

Under normal conditions, the downstream flow of the river prevents significant upriver progression of saltwater. However, in times of extreme low volume water flow, unimpeded saltwater 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 ft. to 45 ft. and, eventually, to 50 ft. 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.

The USACE's new construction at River Mile 64 near Myrtle Grove in 2025 is not the first time such work has been done in the Mississippi River. In fact, similar underwater sills were installed in 1988, 1999, 2012, 2022, 2023 and 2024 to arrest the progression of saltwater during low water seasons in those years.

In 2023, the agency augmented the underwater sill from a depth of -55 ft. to a depth of -30 ft. when the initial sill was overtopped by salt water moving up the river channel. A 620-ft.-wide navigation lane was kept on the sill at a depth of -55 ft. to ensure deep-draft shipping continued along the nation's busiest inland waterway.

When the Mississippi River returns to the flow levels required to push the saltwater wedge back down toward the Gulf, natural erosion of the sill will occur, according to the USACE.

Proper Engineering Can Keep Saltwater Wedges at Bay

The congressionally authorized enlargement of the Mississippi River's deep-draft channel from 45 ft. to 50 ft. causes an increase in the duration and extent of annual saltwater intrusion.

Since the river is a source for municipal and industrial water supply, the effects of increasing the navigation channel were carefully analyzed in the studies for the deeper channel. The characteristics and impacts of saltwater intrusion, the impacts of the increase in saltwater intrusion and measures for mitigation of these impacts were all taken into consideration.

The bottom profile of the Mississippi River, known as a thalweg, is deeper than the Gulf of America water surface level up to River Mile 350 Above Head of Passes (AHP), located approximately 15 mi. downstream of Natchez, Miss.

Several factors impact the upstream movement of saltwater from the Gulf, the USACE noted, with the dominant one being the volume of flow in the river. However, flow duration, channel slope, wind velocity and direction, tides and water temperature all influence the movement of saltwater as well.

With its density being greater than that of fresh water, saltwater moves upstream in the form of a wedge. A highly stratified wedge is common to deep rivers with high freshwater flows such as the Mississippi. The leading edge, or "toe" of the saltwater wedge is well defined.

However, mixing occurs along the freshwater-saltwater interface and mixed flows eventually reach the surface. When the rate of erosion of the saltwater along its leading edge is equal to the saltwater flow upstream along the channel bottom, the location of the wedge is arrested.

When freshwater flows increase or decrease the saltwater wedge retreats downstream or advances upstream, respectively, but the movement of the wedge is less responsive to increases in flows than to decreases.