An Environmental Protection Agency spokeswoman Sonya Pennock said an unspecified amount of oil could be seen some 40 miles downriver during a fly-over Sunday, and there were other reports of oil as far as 100 miles away near the town of Hysham.
But an Exxon Mobil Corp. executive said shoreline damage appeared to be limited to the Yellowstone between Laurel and Billings, which includes about 20 miles of river.
Exxon Mobil Pipeline Co. President Gary Pruessing said company observers flying over the river had seen "very little soiling" beyond Billings. He said the oil appeared to be evaporating and dissipating into the river as the flooded Yellowstone carries it downstream.
A representative of the Montana Disaster and Emergency Services Division said the company's claim was reasonable but had not been independently verified.
State officials on Saturday had reported a 25-mile-long slick headed downstream toward the Yellowstone's confluence with the Missouri River, just across the Montana border in North Dakota. An estimated 1,000 barrels, or 42,000 gallons, spilled Saturday before the flow from the damaged pipeline was stopped.
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"My guess is that as fast as that water is moving, it's probably dissipating pretty quick," said DES public assistance officer Tim Thennis.
Exxon Mobil also revealed Sunday that the 12-inch pipeline had been temporarily shut down in May because of concerns over the rising waters on the Yellowstone. Pruessing said the company decided to restart the line a day later after examining its safety record and deciding the risk of failure was low.
The company and government officials have speculated that high waters in recent weeks may have scoured the river bottom and exposed the pipeline to debris that could have damaged the pipe. The state has received record rainfall in the last month and also has a huge snowpack in the mountains that is melting, which has resulted in widespread flooding.
"We are very curious about what may have happened at the bottom of the river. We don't have that yet," Pruessing said.
An Environmental Protection Agency representative said only a small fraction of the spilled oil is likely to be recovered.
Agency on-scene coordinator Steve Way said fast flows along the flooding river were spreading the oil over a large area, making it harder to capture. But Way said that also could reduce damage to wildlife and cropland along the river.
Crews were putting absorbent material along short 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.
Up to 100 emergency response workers from Exxon Mobil and its contractors were due on the scene by late Sunday.
But property owners along the river were growing frustrated with the response, particularly in agricultural areas where crops and pastures for grazing were at risk. The Yellowstone River is also popular among fishermen, though areas further upriver from the spill are more heavily trafficked.
Billings-area goat rancher Alexis Bonogofsky said the flooding Yellowstone brought the oil into her summer pastures — pollution she's not sure what to do with. Bonogofsky said she had been unable to get answers through either government authorities or Exxon Mobil.
"My place is covered with oil," she said. "I would like a list that says 'this is what's in crude oil.' ... I called a million times yesterday and got no response."
The 20-year-old pipeline was last inspected in 2009 using a robotic device that travels through the line looking for corrosion, dents or other problems, Pruessing said.
Pipeline control room workers first became aware of a problem with the line when pressure readings dropped early Saturday morning. Pruessing said workers began shutting down the line within six minutes, although it was unclear how long that process took.