Hundreds of people clad in caps and heavy coats gathered at the massive Bonnet Carre Spillway in St. Charles Parish on Sunday morning to watch as it was opened for the first time in four years.

Two red cranes moved along the railway tracks atop the flood-control structure and began plucking out some of its 7,000 wooden timbers, allowing water from the swollen Mississippi River to flow into Lake Pontchartrain more than 5 miles away.

For some, the appeal of the scene was very simple.

Gerald Donaldson, 66, who lives nearby, has seen the spillway opened several times before, but he still came to watch it again Sunday. “It’s a lot of water, and there’s something about water,” he said. “It just kinda grabs you.”

Anne Landry, 35, a fifth-grade teacher at Rudolph Matas Elementary School in Metairie, came to videotape the event for her class, which has been studying the source and tributaries of the Mississippi River and discussed the spillway last week.

Her friend, Jennifer Lojszczyk, 38, a first-grade teacher at Catherine Strehle Elementary School in Avondale, said she was interested in the science behind the spillway and how an infusion of freshwater from the river changes sea life and conditions in brackish bodies of water like Lake Pontchartrain.

But for the most part, Lojszczyk tagged along for what she called “the wow factor.”

It was a view shared by New Orleans Mayor Mitch Landrieu. “It’s an engineering miracle,” he said, emphasizing that the spillway protects homes and lives in his city, 28 miles downriver.

In an opening ceremony that included Landrieu, U.S. Rep. Garret Graves and other officials, Maj. Gen. Michael Wehr, head of the Mississippi River Division of the U.S. Army Corps of Engineers, praised the wisdom of “our forefathers” for creating the spillway more than 80 years ago.

The Corps uses it to ensure that the rising river doesn’t overwhelm the levees in and around New Orleans, said Wehr, who climbed into a crane shortly after 10 a.m. to ceremonially remove the spillway’s first 10 “needles”: 11-foot-long timbers about 8 inches by 12 inches in diameter.

The structure features 350 linear bays of 20 needles each. All told, 400 of the needles were removed Sunday, opening up 20 bays. The Corps will remove more timbers each day as needed to divert water from the rising river. Officials said 30 bays could be opened Monday, depending on the river’s speed and level.

When it’s completely open, the spillway is designed to divert 250,000 cubic feet of water per second away from New Orleans and the lower portion of the river. It’s unclear how many bays will ultimately open this year or how long they will remain open.

In May 2011, the structure’s most recent use, 94 percent of the bays were opened, but the spillway still exceeded its official capacity, handling a maximum of nearly 320,000 cubic feet of water per second over its 42-day use, Corps spokesman Ricky Boyett said. In 2008, river levels dropped acceptably over a 31-day span even though fewer than half of the bays were opened.

The Bonnet Carre’s opening is just part of a statewide response to unusually early flood conditions on the river.

Corps scientists are still monitoring weather and river-level forecasts before deciding to unleash water through the Morganza Spillway, which is located upriver from Baton Rouge and sends water into the Atchafalaya River Basin. A Morganza opening this year would be only its third use since the structure was completed in 1954.

Parts of the Atchafalaya Basin will flood even if the Morganza doesn’t open, so in the areas where water seems imminent, property owners have moved to higher ground and tried to protect what’s left with sandbags.

The Governor’s Office of Homeland Security and Emergency Preparedness reported Friday that it had 842,000 sandbags in stock and had already issued 26,000 of them to Avoyelles Parish, 13,000 to West Feliciana Parish and 13,000 to the Louisiana State Penitentiary at Angola, which is bounded by the Mississippi River on three sides.

The Bonnet Carre Spillway was built after the great river flood of 1927. When activated, it opens up more than a mile of the Mississippi’s east bank and pulls river waters into a 5.7-mile floodway that empties into Lake Pontchartrain and, eventually, the Gulf of Mexico.

Sunday’s opening marked the 11th — and earliest — time the spillway has been used since it was completed in 1931. Heavy December rainfall in the Midwest caused the Mississippi and its tributaries to overflow, triggering severe flooding in Illinois and Missouri. The river is expected to crest at 17 feet in New Orleans by Wednesday.

On Wednesday, when the river reached 15 feet at the Carrollton gauge in New Orleans, the Corps began daily patrols of the levees from Baton Rouge south to Venice. On Friday, officials reported they were tracking some deficient areas in the levees, including broken pavement, animal burrows, erosion, shrinkage cracks and “an area of slides” just south of the Algiers Lock.

At 4 a.m. Sunday, the river stood at 16.24 feet in New Orleans, up from 15.8 feet on Saturday and far above the 7-foot levels seen as recently as Dec. 1, before the Midwestern floodwater reached this far south. By 3 p.m., the river had dropped to 15.95 feet in New Orleans, thanks to a low tide and the spillway’s opening, Corps spokesman Matt Roe said.

Because of the sediment it carries, the diverted river water can be viewed from satellites as it enters Lake Pontchartrain at the end of the Bonnet Carre floodway. “It starts to spread, in a semicircular plume, which moves larger and eastward through the lake,” said coastal scientist John Lopez, of the Lake Pontchartrain Basin Foundation.

Lopez has long advocated for better ways to capture the river sediment from the spillway’s flow so that it can replenish places such as the Labranche Wetlands and the Maurepas Swamp. “A fraction of the available sediment in the floodwater, if discharged directly into the adjacent wetlands, would have a profound beneficial effect,” he wrote in a report.

Modest modifications made to the “guide levees” that edge the floodway between the river and the lake would allow such a discharge into adjacent wetlands, Lopez said.

Lopez closely studies the Pontchartrain Basin and described what happens in the lake when the spillway is opened. In about a week, the river water will reach the Causeway, he said. Within about two weeks, the fairly shallow lake will “be essentially all river water.”

When the spillway opens, the fish that prefer brackish water sense the approach of the river water and migrate out of the lake temporarily, Lopez said. But once the spillway is closed, tidal effects put salty water back into the lake, and six months later, the lake is back to its normal condition, he said.