Almost half - 100,000 gallons - of the spilled oil has been contained with booms near the site, but some of the crude oil did make its way into the floodwaters and nearby rivers.
ConocoPhillips spokesman Daren Beaudo says each tanker can hold more than 25,000 gallons.
Tank cars carrying crude oil are shown derailed in Lyon County, Iowa, Friday. In the meantime, the city is getting its water from the nearby Rock Valley Rural Water system, which Olson says is not in danger of being contaminated by the spill.
She issued a disaster proclamation Saturday for Lyon and three other counties in response to rain-fueled flooding and the train derailment, the paper reported. The train derailed early Friday near Doon, north of Sioux City.
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Omaha's public water utility - Metropolitan Utilities District - said it was monitoring pumps it uses to pull drinking water from the Missouri River. Officials on the scene aren't sure whether floodwater from the swollen Little Rock River caused the cars to leave the tracks.
Crews spent Saturday containing the spill and building a temporary road to move equipment to the crash site to make it easier to remove the piled-up train cars and advance the cleanup, the Sioux City Journal reported.
Some officials have speculated that floodwaters eroded soil beneath the train track.
No information was immediately available on how much oil each of the tankers was carrying. Williams had earlier said 33 oil cars had derailed. The Rock River joins the Big Sioux River before merging into the Missouri River at Sioux City.
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Most people have been allowed back into their homes in Rock Valley, Iowa community after they were evacuated because of flooding. He says the oil is being carried downstream into the Rock River a few hundred yards west.
Then an unrelated fire at an industrial site about a mile south of town took emergency crews away from the derailment.
Evacuations have occurred downstream at Rock Valley, where the Rock River is expected to crest later Friday. The Rock River, which is usually about 100 yards wide (about 91 m), is now about half a mile (804 m) wide, according to Iowa Natural Resources Department spokesman Ken Hessenius.
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