Roughly 100,000 gallons has been contained with booms in a low-lying area filled with floodwaters near the derailment.
Rail cars carrying crude oil are shown derailed about a mile south of Doon, Iowa, on Friday, June 22, 2018.
Olson says Rock Valley's water towers also will be drained as a precaution.
Crews are working to contain and clean up crude that spilled when an oil train derailed in northwestern Iowa.
Despite the rising water outside, the oil spill turned Rock Valley's focus to the safety of the water inside, ultimately deciding to turn off all locally sourced water.
Cleanup of an oil spill caused by the derailment of almost three dozen oil tankers in northwestern Iowa has begun. Sheriff Stewart Vander Stoep (VAN'-dur stoop) in neighboring Lyon County says the train derailed between 5 and 5:30 a.m. Friday just south of Doon.
Omaha's public water utility - Metropolitan Utilities District - said it was monitoring pumps it uses to pull drinking water from the Missouri River.
Some of the cars soon began leaking crude oil into the Little Rock River floodwaters, prompting an evacuation of nearby houses and sparking fears of drinking water contamination.
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.
One or more of 31 tanker cars on the derailed train is leaking into floodwaters surrounding the tracks near Doon, about 40 miles (65 kilometers) southeast of Sioux Falls, South Dakota. Williams said officials hoped to reach the cars by sometime Saturday afternoon. The nearby 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.
Vander Stoep said four homes near the site were evacuated.
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.
The National Weather Service says the Rock River is expected to crest later Friday about a foot below the 2014 record of almost 23 feet (7 meters), when several Rock Valley homes were damaged by the floodwater.
Then an unrelated fire at an industrial site about a mile south of town took emergency crews away from the derailment. Dozens of damaged cars are seen piled on the broken train tracks near the Rock River.