Two barges carrying a total of about 3,800 pounds of fertilizer broke loose Thursday and floated down the swollen Arkansas River in Oklahoma, spreading alarm downstream as they hit a dam and sank. On Facebook, Webbers Falls, Oklahoma, warned the town's 600 residents: "If the dam breaks, it will be catastrophic!! Leave now!!"

The barges had been floating out of control, on and off again, since Wednesday night near the town of 600, which was under a mandatory evacuation order due to flooding concerns. Aerial footage showed the moment of impact shortly before noon Thursday. The Army Corps of Engineers was checking the dam for damage.

Emergency management officials said they are treating the event like a 500-year flood, CBS Oklahoma City affiliate KWTV reported.

Webbers Falls, Oklahoma, warned the town's 600 residents: "If the dam breaks, it will be catastrophic!! Leave now!!" KOTV/KWTV

The Interstate 40 bridge and another span over the river were closed as a precaution. Over Memorial Day weekend in 2002, a barge struck the Interstate 40 bridge pier at Webbers Falls, causing part of the bridge to collapse into the Arkansas River. Fourteen people died after their vehicles plunged into the water.