MYRTLE GROVE — The U.S. Army Corps of Engineers will build an underwater sill across the bed of the Mississippi River to stop saltwater from moving upstream. 

The USACE said on Friday that the river's water level has fallen low enough to allow saltwater from the Gulf to move upriver in a wedge shape that may stretch approximately 20 miles from the bottom to the surface of the river. "Based on current river conditions and forecasts, the toe of the saltwater wedge is expected to reach River Mile 80 Above Head of Passes within the next 28 days, triggering construction of the underwater sill," the USACE said.

According to the Corps, the new underwater sill, which is a tall sediment barrier that stretches the width of the river, will stop the saltwater from continuing upstream. 

Saltwater intrusion occurs naturally because the bottom of the riverbed between Natchez, Miss., and the Gulf is below sea level, according to the USACE. It said that the saltwater moves near the bottom of the river, beneath the less dense freshwater. The sill allows the freshwater near the surface to keep flowing while stopping the salt water underneath it.

The USACE said that saltwater intrusion becomes dangerous when the water level gets too low, allowing saltwater to move farther upstream and possibly get into drinking water and industrial water supplies. 

The Corps said it will build the sill near Myrtle Grove and will take around two weeks to complete once the contract has been awarded. It will maintain the sill until the river pushes the saltwater back downstream, and then the sill will naturally erode.