USACE builds underwater sill to halt saltwater intrusion on Mississippi River

The USACE New Orleans District will construct an underwater sill across the bed of the Mississippi River channel to prevent further upriver progression of salt water from the Gulf of Mexico.

“Once the river volume begins to fall in low water season, we regularly monitor the progression of the saltwater wedge on the Mississippi river,” said David Ramirez, chief of the New Orleans District’s Lower Mississippi River Management Branch. 

“This week, the projected location of wedge’s toe in combination with current National Weather Service forecasts we’ve reached our triggers for constructing the barrier sill.”

The Mississippi River’s volume of water has fallen to a level that allows salt water 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. 

According to USACE, in times of extreme low volume water flow, unimpeded salt water can travel upriver and threaten municipal drinking water and industrial water supplies.

To arrest the upriver movement of the salt water and reduce the risk to freshwater intakes, USACE will construct an underwater barrier sill near Myrtle Grove, LA. 

The sill will be creating using sediment dredged from an area designated for this purpose. 

Also, the sill will take approximately two weeks to complete but will demonstrate benefits in advance of completion.