MORGANZA, La. – A 10-ton steel floodgate was slowly raised Saturday for the first time in nearly four decades, unleashing a torrent of water from the Mississippi River, away from heavily populated areas downstream.

The water spit out slowly at first, then began gushing like a waterfall as it headed to swamp as much as 3,000 square miles of Cajun countryside known for small farms and fish camps. Some places could wind up under as much as 25 feet of water.

Opening the Morganza spillway diverts water away from Baton Rouge and New Orleans, and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi.

“We’re using every flood control tool we have in the system,” Army Corps of Engineers Maj. Gen. Michael Walsh said Saturday from the dry side of the spillway, before the bay was opened. The podium Walsh was standing at was expected to be under several feet of water today.

The Morganza spillway is part of a system of locks and levees built following the great flood of 1927. When it opened, it was the first time three flood-control systems have been unlocked at the same time along the Mississippi River.

Earlier this month, the corps intentionally blew holes into a levee in Missouri to employ a similar cities-first strategy, and it also opened the Bonnet Carre spillway northwest of New Orleans to send water into the massive Lake Pontchatrain.

Snowmelt and heavy rain have been blamed for inflating the Mississippi, and the rising river levels have shattered records all set 70 years ago.

About 25,000 people and 11,000 structures could be in harm’s way.

In Krotz Springs, La., one of the towns in the Atchafalaya River basin bracing for floodwaters, Monita Reed, 56, recalled the last time the Morganza was opened in 1973.

“We could sit in our yard and hear the water,” she said as workers constructed a makeshift levee of sandbags and soil-filled mesh boxes in hopes of protecting the 240 homes in her subdivision.

Some people living in the threatened stretch of countryside — an area known for a drawling French dialect — have already started heading out. Reed’s family packed her furniture, clothing and pictures in a rental truck and a relative’s trailer.

“I’m just going to move and store my stuff. I’m going to stay here until they tell us to leave,” Reed said. “Hopefully, we won’t see much water and then I can move back in.”

It took about 15 minutes for the one 28-foot gate to be raised. The corps planned to open one or two more gates today in a painstaking process that gives residents and animals a chance to get out of the way.

The water will flow 20 miles south into the Atchafalaya Basin.

From there it will roll on to Morgan City, an oil-and-seafood hub and a community of 12,000, and eventually into the Gulf of Mexico.