A boat carrying state prison employees collided with a barge on the swollen Mississippi River upstream from Baton Rouge on Sunday, injuring 18 people.

The crewboat, with 20 people aboard, was taking workers who had just ended their shifts across the river, said Angie Norwood, spokeswoman for the Louisiana State Penitentiary in Angola. No one went into the water, she said.

The employees were first taken to the prison hospital, and 12 were transferred to other hospitals. The two most seriously injured were flown for treatment, Norwood said.

Normally, the maximum-security prison uses a ferry to take workers and their vehicles back and forth from the west side of the river to the prison on the east side.

But crewboats — typically used to move workers to offshore petroleum platforms — had been used recently because of the high water, Norwood said.

At Baton Rouge, about 40 miles downstream from Angola, the Mississippi is expected to crest April 21 at 42 feet. Flood stage at that point is 35 feet, according to the National Weather Service.

The Coast Guard, which was investigating, could not immediately say whether high water played a role.

Persistent heavy rains to the north have swollen the busy waterway, leading authorities to open a major spillway for the first time since 1997 in hopes of avoiding problems downriver.

River pilots had voiced worries about the high water, saying it's harder to anchor and navigate in the quick currents.

The Coast Guard had recently limited the number of barges that tugboats could push in the lower Mississippi.

Last month, a freighter ran aground near New Orleans and caused 60 nearby barges to break loose from their moorings. A stretch of river was also closed near Vicksburg, Miss., when a barge sank after hitting a bridge.

The penitentiary, which consists of prison camps on an old plantation, has about 1,500 employees.