The river has no dams on its way to its confluence with the Missouri River just across the Montana border in North Dakota. It was unclear how far the plume might travel. 

Cleanup crews deployed booms and absorbent material as the plume moved downstream at an estimated 5 to 7 mph. 

"The parties responsible will restore the Yellowstone River," Mont. Gov. Brian Schweitzer said. 

A 600-foot-long black smear of oil coated Jim Swanson's riverfront property just downstream from where the pipe broke. 

"Whosever pipeline it is better be knocking on my door soon and explaining how they're going to clean it up," Swanson said as globules of oil bubbled to the surface. "They say they've got it capped off. I'm not so sure." 

ExxonMobil spokeswoman Pam Malek said the pipe leaked an estimated 750 to 1,000 barrels of oil for about a half-hour before it was shut down. Other Exxon officials had estimated up to 42,000 gallons of crude oil escaped. 

Duane Winslow, Yellowstone County director of disaster and emergency services, said the plume was dissipating as it moved downstream. "We're just kind of waiting for it to move on down while Exxon is trying to figure out how to corral this monster," Winslow said. 

"The timing couldn't be worse," said Steve Knecht, chief of operations for Montana Disaster and Emergency Services, who added that the plume was measured at 25 miles near Pompeys Pillar National Monument. "With the Yellowstone running at flood stage and all the debris, it makes it dang tough to get out there to do anything."