OKLAHOMA CITY (AP) — Two unmanned barges that broke loose on the swollen Arkansas River in Oklahoma crashed into a dam hours later and sank Thursday, and they didn't cause the structure to fail, which officials feared might happen.

The barges carrying about 3,800 pounds (1,725 kilograms) of phosphate fertilizer between them became unmoored Wednesday night in the Port of Muskogee, which sits at the confluence of the Arkansas and two other rivers and is about 40 miles (65 kilometers) southeast of Tulsa.

At some point, the vessels were caught and secured to trees along the soggy riverbank, but they broke free again Thursday when the trees uprooted, said Tricia Germany, a spokeswoman for the Muskogee County Emergency Management Agency.

The barges eventually crashed into the dam near the town of Webbers Falls shortly before noon and quickly sank.

"They did a direct hit, they kind of turned a little bit," Germany said of the crash.

Webbers Falls officials on Wednesday ordered the town's roughly 600 residents to evacuate due to the threat of flooding from the swollen river, which was at 42 feet (12.8 meters) as of 11:45 a.m. Thursday, according to the National Weather Service. That is 14 feet (4.27 meters) above flood stage. Later Wednesday, they sounded the alarm about the runaway barges on the town's Facebook page, warning they could hit the lock and dam: "If the dam breaks, it will be catastrophic!! Leave now!!"

The lock and dam didn't break, though the Army Corps of Engineers was going to closely inspect them for damage, said Kenna Mitchell, a spokeswoman for the Oklahoma Department of Transportation. Two downstream bridges, including an Interstate 40 bridge, remained closed after the crash until further notice.

"We don't know how long that's going to be. Safety is the priority for this," Mitchell said.

If the warning about the runaway barges seems alarmist, it shouldn't: During Memorial Day weekend in 2002, a manned barge struck one of the I-40 bridge's supports, causing part of the bridge to collapse and killing 14 people whose vehicles plunged into the river.

As for the fertilizer that was on the runaway barges, the amount that got into the river is significant but should dilute quickly because of the waterway's heavy flow, said Jay Wright, environmental programs manager for the Oklahoma Department of Environmental Management.

"It's soluble in water. If it's exposed to the water, it's going to dissolve in the water," he said.

The crash happened during a period of powerful storms in the central United States, including a system that spawned deadly tornadoes overnight. The wet weather has caused heaving flooding, and the threat from swollen waterways persists.

On Wednesday, at least one unoccupied home fell into the Cimarron River near Crescent, Oklahoma, about 35 miles (55 kilometers) north of Oklahoma City, and erosion left several homes hanging over the swollen current.

One death has been attributed to the flooding in Oklahoma. The Oklahoma Highway Patrol said 53-year-old Laura Renee Moorman of Perkins drowned Tuesday after she drove around a road barrier and her vehicle was swept away by water near Perkins, about 45 miles (72 kilometers) northeast of Oklahoma City.

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