The sill, or underwater levee, is being built for a third year in a row to head off the potential for New Orleans area drinking water to be threatened.

For the third year in a row, Louisiana is constructing an underwater levee in the Mississippi River to slow an influx of salt water from the Gulf of Mexico.

Salt water is moving upriver from the Gulf of Mexico. Drought in the central United States has weakened the mighty river’s flow to the point that it can no longer push out saltier water from the Gulf. For Plaquemines Parish, which draws directly from the river, this intrusion is a threat to its water source.

Construction has begun on a saltwater sill near Myrtle Grove to help mitigate the effects of saltwater intrusion up the Mississippi River.

As the Mississippi River water level lowers, the possibility of a saltwater intrusion increases, and for the third year in a row, southern Louisiana will be facing one.