April’s Storms Were Relentless. But the Rainwater Is Finally Going Away.

At the start of April a deadly torrent of rains caused flooding across the Midwest, as days of severe storms ravaged much of the central part of the United States. A month later, the Mississippi River in New Orleans was finally cresting this week, as much of the water from those storms is ending its long journey south.

The river has been gradually rising for weeks and reached 16.7 feet on Thursday. This is just below the flood stage of 17 feet, and far from a record, but it’s the highest water level in New Orleans since 2020, and comes amid a four-year drought in the Mississippi River Basin.

“It looks noticeably different than it was just a year ago,” said Robert Florence, a co-owner of NOLA Historic Tours, who led a tour by the river a week ago. “When the water is higher, it accentuates the sinkholes, cross currents, whirlpools and eddies. It feels more alive and powerful.”

The Mississippi watershed is the third largest in the world.

The Mississippi River is complex and huge. Its main stem flows 2,350 miles from the headwaters at Lake Itasca in Minnesota to the Gulf of Mexico, touching 10 states and spreading out into many more with its tributaries.

The water that comes down the river and arrives in New Orleans is a result of rain and snowmelt that has occurred in states as far away as Pennsylvania, Minnesota, Montana, Colorado and Tennessee.

In the case of the water pushing into New Orleans right now, it started out as falling rain four weeks ago, mainly over the upper Mississippi River and Ohio River Valleys. Between April 2 and 6, it turned roads into rivers and flooded communities across Arkansas, Missouri, Illinois, Indiana, Kentucky and Tennessee. Some locations recorded over 15 inches of rainfall.

The rain was part of a sprawling cross-country storm system that also generated thunderstorms and tornadoes, although most of the damage came from the relentless rainfall, fueled by moisture from the Gulf.

Small rivers in the upper watershed rose quickly, some cresting at historic levels, and have already come back down. The flood cycle is longer on the channeled and leveed Mississippi, which is fed by numerous tributaries.

The rain in early April turned into runoff and flowed for days, into the upper portion of the lower Mississippi River Basin and its tributaries, including the Arkansas, White and Red Rivers, and especially the Ohio. It can take weeks for the discharge from the Mississippi tributaries to peak, and for that water to eventually travel down the river.

“This is the situation we’re seeing now, where heavy rains fell in Kentucky in early April and it will be roughly four weeks before that peak discharge reaches New Orleans,” said Kory Konsoer, associate director of the Center for River Studies at Louisiana State University.

As all of that water traveled south, the river swelled along with it, causing additional flooding in some areas, and overtaking roads and farmland that was recently planted. About 250 miles north of New Orleans, Vicksburg, Miss., saw the river rise to near major flood stage earlier this week, then hold there for several days.

The river has been closely watched for weeks.

It was well-known that the river would peak in New Orleans this week. The National Weather Service provides forecasts, and the U.S. Army Corps of Engineers monitors these closely as it manages the river’s plumbing system of levees, floodways and reservoirs built to prevent major flood disasters.

Occasionally, the Army Corps will open spillways and floodways that are designed to divert floodwaters during extreme weather events. It hasn’t opened any since the storms in April, but it did come close.

This week, the agency ran tests on the Bonnet Carre Spillway, 30 miles northwest of New Orleans, to ensure it was ready, if needed, to prevent flooding. The spillway is opened when the river is flowing into the Gulf at a rate of 1.25 million cubic feet per second. But Matt Roe, a spokesman for the Corps in the New Orleans district, said the flow was expected to be just shy of that level.

“This whole event, it has been very close,” Mr. Roe said. “In the earlier forecast, we were predicted to go beyond that trigger point, but the conditions appear to be coming out a little lower.”

The spillway was built in 1931 to remove pressure on the levees protecting New Orleans by diverting up to 250,000 cubic feet of fresh water per second into Lake Pontchartrain’s brackish waters. Since then, it has been used 15 times, five of them between 2016 and 2020.

“It’s not the most water that has moved through in the last decade, but it’s still a lot of water,” Mr. Roe said.