About 100,000 gallons has been contained with booms in a low-lying area filled with floodwaters near the derailment.
ConocoPhillips spokesman Daren Beaudo says each tanker can hold more than 25,000 gallons.
Rail cars carrying crude oil are shown derailed about a mile south of Doon, Iowa, on Friday, June 22, 2018.
The amount of oil spilled also wasn't known by Saturday afternoon, Williams said, but he added that officials hoped to have a better idea once they're able to reach the derailed oil tankers. The city of Rock Valley, Iowa, has already shut down its water wells and plans to drain and clean them until testing indicates the water is safe to drink.
The sheriff said the "flood is definitely causing a lot of problems".
Omaha's public water utility - Metropolitan Utilities District - said it was monitoring pumps it uses to pull drinking water from the Missouri River. About 31 cars derailed after the tracks reportedly collapsed due to saturation from flood waters from adjacent Little Rock River.
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.
Some officials have speculated that floodwaters eroded soil beneath the train track.
Williams says he doesn't yet know how much oil has leaked and how many cars are leaking. It joins the Rock River a few hundreds yard west, which courses south into the Big Sioux River. Cleanup crews have been dispatched.
Doon is about 40 miles (65 kilometres) southeast of Sioux Falls, South Dakota, where several rain-swollen rivers overflowed Thursday.
Evacuations began in Rocky Valley early Friday morning as water poured into homes and basements.
The Rock River had already carried some oil to Rock Valley by midmorning, said Ken Hessenius with the Iowa Natural Resources Department. On the one hand, proponents are likely to argue that pipelines are much safer than trains; on the other, opponents will say that either form of transportation is a recipe for this kind of disaster.
The derailment also caused concern downstream, including as far south as Omaha, about 150 miles from the derailment site. A BNSF spokesman didn't immediately return a call from The Associated Press. 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.