"The steady flow of water through the Bonnet Carre Spillway will turn into a torrent today as the Army Corps of Engineers opens the structure for the first time since 1997 in an effort to divert water from a rapidly rising Mississippi River

The corps will open 70 of the spillway flood-control structure's 350 bays, diverting 67,000 cubic feet of water per second from the Mississippi River.

The move is designed to relieve strain on Mississippi River levees, improve navigation for ships and barges that find it difficult to maneuver in the current, and keep the volume of water flowing past New Orleans at 1.25 million cubic feet of water per second or less.

When all the bays are opened, the spillway can divert 250,000 cfs, or nearly 1.9 million gallons of water per second, into Lake Pontchartrain.

Gov. Bobby Jindal is scheduled to give remarks on the opening, which is scheduled for noon.

Corps officials had predicted that an opening wouldn't be needed this year, but recent rains in the Arkansas, Ohio and the Upper Mississippi River Valley have heightened flood worries.

The opening of the spillway in St. Charles Parish will lower river stages in New Orleans while causing a rise in Lake Pontchartrain. Corps officials said they don't expect to open the entire spillway, and that the opening will probably last between two and four weeks. The opening will take place over a period of several days.

Opening the spillway shunts Mississippi River water into Lake Pontchartrain, allowing it to bypass New Orleans.

After more than two weeks of high water, the 8,000-acre spillway already is looking more like a shallow lake as water rushes through gaps between the timber "needles" that keep most of the flood at bay. On Thursday, more than 6,000 cubic feet of water per second were passing through the structure. Water begins to move through the spillway when the river rises to 12 1/2 feet in New Orleans.

Effect on fisheries

The last spillway opening was protested by Carlton Dufrechou, executive director of the Lake Pontchartrain Basin Foundation, who said he didn't think the water levels were high enough to warrant the opening. The flood of fresh water drives out saltwater species of marine life and contains nutrients that can create algae blooms that can kill fish and crabs.

"The lake is going to take a hit, but it will recover," Dufrechou said Thursday. "Public safety comes first."

Mark Schexnayder, a biologist with Louisiana State University, said an opening isn't catastrophic and that studies show that the nutrients have a long-term positive effect on the lake, at the cost of a temporary disruption of fishing grounds.

Historically, oyster production has risen dramatically in the years after an opening, he said.

"The fresh water helps the cypress swamp, which is our first line of defense in the lake," he said. "These openings mimic what used to happen every year before we cut the river off from the lake."

Fisheries activist Cliff Glockner of Lacombe said the verdict is still out on the effect on fishers' catch.

"It's just too early to tell," he said. "If we get a strong east wind, the fresh water will pile up inside the lake and we're going to have problems."

The spillway structure was built in response to the devastating flood of 1927 that killed more than 500 people, prompting the corps to abandon its previous levees-only strategy and add spillways.

One major headache for local officials is keeping thrill-seekers from trying to swim or operate boats in the rushing water.

"We definitely need people to stay out of there," Sheriff Greg Champagne said. "We have airboats that can go in there after them if we need to, but it's a very dangerous situation."

The 8,000-acre spillway has been closed to recreation for the past week as the water rose.

Triggers for action

Under current policy, the Bonnet Carre Spillway is the first to be opened when high water threatens the structural integrity of the levees. The corps can open as many of the 350 bays in the structure as it chooses. The water then travels between guide levees about six miles to the lake.

The opening is triggered when the Mississippi River passes the Carrollton gauge at a rate of more than 1.25 million cubic feet of water per second, if the level is rising or if there is a threat of overtopping or weakening of levees.

During higher-water events, the corps has the option of opening the Morganza Floodway and the West Atchafalaya Floodway.

The Morganza Floodway was opened in 1973, but only because the Old River Control Structure at the mouth of the Atchafalaya River near Simmesport was damaged by flood water and officials wanted to relieve water pressure on the structure.

The Bonnet Carre spillway structure has been opened eight times since it was completed in 1931, the last time in 1997. That year, 298 of the bays were opened. The last full opening was in 1983." (Matt Scallan, The Times-Picayune, April 11, 2008)