NEW ORLEANS — The volume of water in the Mississippi River will again require the U.S. Army Corps of Engineers to build a sill to impede salt water movement from the Gulf of Mexico toward freshwater intakes along the river.

Its the third straight year the Corps has had to build a sill. It also constructed sills in 1988, 1999 and 2012.

Typically, the Mississippi's flow is such that salt water is kept at bay. The flow has slowed enough this year that the front of the saltwater "wedge" is expected to reach 80 miles above Head of Passes within the next month, the Corps said.

The sill will be built near Myrtle Grove, using sediment dredged from nearby. 

According to the Corps, salt water intrusion occurs naturally because the bottom of the riverbed between the Gulf of Mexico and Natchez, Mississippi, is below sea level. Salt water is more dense than fresh water, and in times of extremely low flow, salt water can travel farther upriver and threaten drinking water and industrial water supplies.

When the flow increases, the river will erode the sill.