Teams of federal and state workers have fanned out along Montana's Yellowstone River to gauge the environmental damage from a ruptured ExxonMobil pipeline that spewed crude into the famous waterway.
An Environmental Protection Agency representative said on Sunday only a small fraction of the tens of thousands of gallons of spilled oil is likely to be recovered.
Agency on-scene coordinator Steve Way said fast flows along the flooding river are spreading the oil, making it harder to capture, but that also could reduce damage to wildlife and cropland along the river.
A 40-km slick of oil had reached as far west as Hysham on Saturday night. An estimated 1000 barrels spilled on Saturday before the flow was stopped.
Yellowstone County disaster coordinator Duane Winslow says dozens more ExxonMobil clean-up workers began to arrive in Montana on Sunday morning.
The break near Billings in south-central Montana fouled the riverbank and forced municipalities on Saturday to close intakes.
The river has no dams on its way to its confluence with the Missouri River just across the Montana border in North Dakota.
Winslow 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," he said.
"The timing couldn't be worse," said Steve Knecht, chief of operations for Montana Disaster and Emergency Services. He said the plume was measured at 40km 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."
Brent Peters, the fire chief for the city of Laurel, about 20km west of Billings, said the rupture in the 30.5-cm diameter pipe occurred late on Friday about 1.6km south of Laurel.
About 140 people in the Laurel area were evacuated early on Saturday on concerns about possible explosions and overpowering fumes. They were allowed to return about 4am after fumes had decreased.
Winslow said hundreds of residents downstream were told to evacuate in the early morning hours.
ExxonMobil said it was sending a team to help with clean-up, and that state and federal authorities had been alerted to the spill. The ExxonMobil Pipeline Company "deeply regrets this release", it said.
Crews were putting out absorbent material along stretches of the river in Billings and near Laurel, but there were no attempts at capturing oil farther out in the river. In some areas oil flowed underneath booms and continued downstream.
The smell of oil permeated the air for kilometres downstream and through the city of Billings.
"Right now, the Yellowstone River is at flood stage," Peters said. "The bank isn't stable enough for anybody to get close."
The cause of the rupture in the pipe carrying crude oil from Belfry, Montana, to the company's refinery in Billings wasn't known. Peters and Malek said speculation involved high water that might have gouged out the riverbed and exposed the pipe, which was possibly hit by debris.
"I haven't seen it this high for at least 15 years," Peters said.
Jeb Montgomery of ExxonMobil said the pipe was buried 1.8m below the riverbed.
Laurel, which has about 6500 residents, is known for a huge Fourth of July fireworks display put on by the fire department. Peters said the town can swell to as many as 50,000 people for the event.
He said the fire department plans to hold the event on Monday