Almost half the spill - an estimated 100,000 gallons (378,530 liters) - had been contained with booms near the derailment site and an additional boom placed approximately 5 miles (8.05 kilometers) downstream, Williams said.
About 100,000 gallons had been hemmed off using booms out of the estimated 230,000 gallons spilled, BNSF said in a statement on Saturday.
The Associated Press Tank cars carrying crude oil are shown derailed about a mile south of Doon, Iowa, on Friday.
Olson says Rock Valley's water towers also will be drained as a precaution.
Crews are working to clean up a BNSF oil train derailment in Iowa that dumped crude into floodwaters, while officials seek to get a handle on the extent of the spill and its cause.
Rock Valley, just southwest of the derailment, shut off its water wells within hours of the spill. The train derailed early Friday near Doon, north of Sioux City.
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Omaha's public water utility - Metropolitan Utilities District - is 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.
A major part of that work includes building a temporary road parallel to the tracks to allow in cranes that can remove the derailed and partially submerged oil cars. The river rose rapidly after 13 to 18 cm of rain fell in the days prior to the derailment.
Williams says he doesn't yet know how much oil has leaked and how many cars are leaking.
The service has issued flood warnings for several other rivers and creeks in the area, including the Big and Little Sioux rivers, the Floyd River and the Ocheyedan River near Spencer. Cleanup crews were dispatched to the site. That empties into the Missouri River at Sioux City.
The Rock River had already carried some oil to Rock Valley by midmorning, said Ken Hessenius with the Iowa Natural Resources Department. He says it's unclear how much oil has leaked.
The derailment also caused concern downstream, including as far south as Omaha, about 150 miles from the derailment site. Reynolds met with the Rock Valley Mayor, Kevin Van Otterloo and examined the areas of the flooded homes as a result of the over-flowing Rock River.