An oil pipeline running under the Yellowstone River has ruptured and leaked hundreds of barrels of oil into the waterway and caused a 25-mile plume, fouling the riverbank.
The break led to temporary evacuations of hundreds of residents along a 20-mile stretch. Clean-up crews used booms and absorbent material as the plume moved downstream.
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. "The parties responsible will restore the Yellowstone River," said Brian Schweitzer, the Montana governor.
ExxonMobil said the pipe leaked an estimated 750 to 1,000 barrels of oil for about a half-hour before it was shut down. Exxon officials had estimated that up to 42,000 gallons of crude oil escaped.
Duane Winslow, the 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," Mr Winslow said.
In a statement, ExxonMobil said it was sending a team to help with clean0up, and that state and federal authorities had been alerted to the spill. The ExxonMobil Pipeline Company "deeply regrets this release", it added.