ST. LOUIS -- Roughly 300 gallons of crude oil spilled early Friday into the Mississippi River near St. Louis after a vessel on the rain-swollen waterway slammed into a fleet of docked barges, causing 14 of them to briefly break away, according to the Coast Guard.

None of the oil was found to have washed up on the river's banks as of midmorning Friday, though crews still were assessing the possible environmental fallout of the spill -- the equivalent of seven barrels -- and the precise cause of the accident about 1 a.m. near Alton, Ill., Coast Guard Lt. Colin Fogarty said.

Fogarty credited a worker with quickly shutting off oil being loaded onto one of the barges, blunting the severity of the spill.

The accident forced about an hour-long closure of a four-mile stretch of the river.

"The Mississippi River is incredibly dynamic and now moving so fast with so much sediment that it has to break down the oil," swiftly sending it downstream, he said.

The breakaway happened on a night when heavy rainfall pummeled the region, swelling the river and its currents that in recent weeks have proved especially troublesome to shippers.

"The high water undoubtedly played some factor, even if a small one in this instance," Fogarty said, noting the waterway's strong current makes it difficult for vessels such as the one that hit the barges "to put the brakes on."

"It makes navigation significantly more difficult," he said.

Friday's accident came less than two weeks after the fast-moving Mississippi broke 114 barges loose from their moorings just south of St. Louis; four of them hit the Jefferson Barracks Bridge and 11 of the cargo vessels sank. That forced a shutdown of a 15-mile stretch of the waterway and the six-hour closure of the bridge that inspectors ultimately said escaped significant damage.

Fogarty has said that breakaway could have been caused by various factors related to the elevated current of the rain-swollen river.