The Army Corps of Engineers announced Wednesday that it will build an underwater dam in the Mississippi River to stop saltwater from advancing further upstream. The advancing saltwater is due to the river being unusually low because of a lack of rainfall further north. Here’s an explanation.

Why is the river so low?

The Mississippi drains 41% of the contiguous United States, equivalent to around 1.2 million square miles. That amounts to all or parts of 32 states plus two Canadian provinces, according to the National Park Service. A lack of rainfall further north leads to lower river levels, as has been the case this year.

Why does that matter for Louisiana?

The river bottom in south Louisiana was recently deepened to a minimum of 50 feet below sea level to allow for larger ships, and in New Orleans it is as deep as 200 feet, so there’s no risk of it running dry here. But there are other concerns, mainly related to drinking water supplies and shipping disruptions locally and further north.

When the river is too low, the flow of lighter freshwater is not forceful enough to keep heavier saltwater from the Gulf of Mexico from advancing upstream. Parishes that receive drinking water supplies from the river – including Orleans, Plaquemines and St. Bernard – are not equipped to deal with saltwater, which can cause problems for those on dialysis or low-sodium diets. Orleans officials in the past have said there’s little chance of salinity uptake for the city’s water supply because the river is so deep in those areas and the saltwater will likely not get that high.

Further, Louisiana’s ports are dependent on river traffic, which was severely disrupted last year when the Mississippi fell to record lows, in part caused by shoaling resulting from the low water flow. That has not happened so far this year, and a National Weather Service forecast on Wednesday said two to four inches of rainfall had fallen over the middle Mississippi and lower Ohio Valleys over the past week, adding that “the runoff has provided some temporary relief to the low water conditions.”

What is the Corps doing to address drinking water concerns?

The Corps has a plan that is automatically triggered when the leading edge of the so-called saltwater wedge is forecast to reach river mile 65, around Myrtle Grove, within 10 days, or when it is forecast to reach river mile 80, around Belle Chasse, within 28 days. Both of those triggers were reached this year.

That means the Corps is moving ahead with a plan to build a sill – essentially an underwater dam that is 1,500 feet wide and 45 feet high – on the bottom of the river to block the saltwater. Because the saltwater is heavier and moves beneath the freshwater, the sill blocks it while allowing freshwater to flow past. The sill will be eroded naturally when heavier river flows return later in the year.

What is used to build the sill?

The Corps dredges sediment from the river itself to build the sill. It is expected to take about two weeks to build.

Is it expensive?

The Corps hires a contractor for the project. Last year, the dredging cost was estimated at $10 million.

Is this unusual?

This will be the fifth time a sill is built, in addition to 2022, 1988, 1999 and 2012. The Corps says conservative estimates indicate that a sill would need to be constructed about once every five years on average.