The Yellowstone River continues to rise near Laurel, Mont., making it difficult for crews to clean up the oil spilled from Exxon Mobil's ruptured pipeline that burst Friday.

The river is flowing four times as fast as it usually does this time of year because of melting snow. The high waters are making it difficult for crews to sop up oil and put oil booms in place.

The National Weather Service predicted the river would peak at Billings Tuesday afternoon.

Investigators are still determining what caused the pipeline to burst upstream from a refinery in Billings, where it delivered 40,000 barrels of oil a day. The initial theory is that powerful high waters brought up debris that damaged the 12-inch pipeline.

"At this time we do not know the root cause. This is a very unusual event. Obviously we had a large amount of oil enter the water very quickly," Exxon Mobil Pipeline Co. president Gary Pruessing said.

Up to 1,000 barrels, or 42,000 gallons, of crude oil oozed into the legendary Yellowstone before the leak was stopped, according to Exxon Mobil estimates.

Company officials say it appears the spill is confined to a 15-mile stretch of river.

The Environmental Protection Agency said in a statement Monday afternoon that officials were still taking air and water samples to determine the impact.