Roughly 100,000 gallons has been contained with booms in a low-lying area filled with floodwaters near the derailment.
Officials say 230,000 gallons (870,619 liters) spilled.
Olson says Rock Valley's water towers also will be drained as a precaution.
Tanker cars from a freight train carrying crude oil are shown after a derailment along the Rock River south of Doon, Iowa, U.S.in this June 22, 2018 handout still image taken from aerial drone video. The spill has raised concerns about drinking water downstream.
(Doon)- Flooding is believed to have been the cause of a train derailment in Lyon county early this (Fri.) morning that has resulted in the evacuation of some residents there.
Leah Vanderbrink, of Doon, Iowa, said, "Around a quarter to five this morning, our neighbor called and said that a train derailed and had derailed into one of our fields".
Metropolitan Utilities District said it was monitoring pumps it uses to pull drinking water from the Missouri.
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Officials at the scene agreed that floodwater from the swollen Little Rock River played a part in causing the cars to leave the tracks, but said they were not yet sure whether the waters compromised the track, physically pushed the cars off it or played a part in some other way.
Some officials have speculated that floodwaters eroded soil beneath the train track. The Little Rock River rose rapidly after heavy rain Wednesday and Thursday.
It's not yet clear how many tankers are leaking or how much oil has escaped.
Williams says he doesn't yet know how much oil has leaked and how many cars are leaking. Cleanup crews were dispatched to the site. It joins the Rock River a few hundreds yard west, which courses south into the Big Sioux River.
Sheriff Stewart Vander Stoep (VAN'-dur stoop) says four homes near the site have been evacuated.
The Rock River had already carried some oil to Rock Valley by midmorning, said Ken Hessenius with the Iowa Natural Resources Department.
Then an unrelated fire at an industrial site about a mile south of town took emergency crews away from the derailment. Floodwater along the Little Rock River is running over a road north of the track.