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NEW ORLEANS (AP) — The Mississippi River is falling at New Orleans and crews have begun closing a historic flood control structure that diverts the river's water into a brackish lake, the Army Corps of Engineers said Tuesday.
The process could take about 10 days to two weeks if the weather remains good, Corps spokesman Matt Roe said.
At the river's peak flow, 206 of the Bonnet Carre Spillway's 350 bays were opened, pouring out 213,000 cubic feet (6,000 cubic meters) of water every second — enough to fill the Empire State Building in less than three minutes, or the Superdome in less than 10 minutes.
Each bay is opened and closed with 20 huge timbers called needles.
"We think we can close about 20 bays a day," Roe said. "I think the goal would be 15 to 20 a day, depending on weather and other conditions."
The spillway, a 1.5-mile-long (2.4-kilometer) structure parallel to the river, is opened to relieve stress on levees protecting New Orleans when the Mississippi is flowing at 1.25 million cubic feet (35,400 cubic meters) per second — fast enough to fill the Superdome in less than two minutes.
"Flows are dropping below that trigger point and the river is dropping to where they can safely get out there and drop the pins back in," Roe said.
The water flows across nearly 8,000 acres (3,200 hectares) of land into Lake Pontchartrain.
The Feb. 28 opening was the 13th ever for the dam 28 miles (45 kilometers) above New Orleans. It was the first time the structure has been operated in consecutive years since it was completed in 1931.
New Orleans isn't at risk from floodwaters flowing down from the upper Mississippi River, said Kai Roth, a National Weather Service senior hydrologist.
"The upper Mississippi River is a much smaller river than it is down here. ... By the time it gets here, the river's so much higher and wider and deeper that it can handle the floodwaters really well," he said.
The Mississippi is at major flood stage in Vicksburg and Natchez, Mississippi, and Baton Rouge, Louisiana, but "the levees pretty much contain it," he said.
Roth said that when the river floods this far south, the Ohio River, which merges with the Mississippi at Cairo, Illinois, is generally a greater contributor than the upper Mississippi.
The spillway was opened last year because of high water from heavy rains in areas drained by both the Ohio and upper Mississippi.
"The analogy I've heard is the upper Mississippi is like a four-lane highway and the Ohio is like a seven-lane," said Roth, who works in Slidell, north of New Orleans. "By the time you get down here, it's a 10- or 12-lane highway."
The upper Mississippi is rising, but the Ohio has begun falling, he said.
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