MYRTLE GROVE, LA — The operational trigger has been met for the U.S. Army Corps of Engineers to construct an underwater sill across the bed of the Mississippi River channel to arrest further upriver progression of salt water from the Gulf of Mexico. The Mississippi River’s volume of water has fallen to a level that allows salt water to intrude upstream. Salt water from the Gulf of Mexico moves upriver in a wedge shape that may stretch up 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, triggering construction of the underwater sill. The underwater barrier sill, built to arrest the upriver movement of the salt water and reduce the risk to freshwater intakes along the river, will be constructed near Myrtle Grove, Louisiana, using sediment dredged from an area designated for this purpose. The sill will take approximately two weeks to complete once the contract has been awarded and the dredge mobilized, but it will demonstrate benefits in advance of completion. The intrusion of salt water into the river is a naturally occurring periodic condition because the bottom of the riverbed between Natchez, Mississippi, 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.