Officials on the scene aren't sure whether floodwater from the swollen Little Rock River caused the cars to leave the tracks.
Gov. Kim Reynolds visited the derailment site Saturday as part of a tour of areas hit by recent flooding.
BNSF railroad spokesman Andy Williams said workers have unloaded oil from 10 of the oil tank cars that didn't leak after Friday's derailment about 15 miles south of the Minnesota border.
The sheriff said the "flood is definitely causing a lot of problems".
They're also building a road parallel to the tracks to try to get to the derailed and partially-submerged oil cars.
Lyon County Sheriff Steward Vander Stoep said between 30 and 40 semitrailers containing clean-up equipment had arrived at the scene near Doon, Iowa, by Friday afternoon. The river rose rapidly Wednesday after 5 to 7 inches (13 to 18 centimetres) of rain fell Wednesday and a further downpour on Thursday.
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.More news: Oppo Find X global launch set for today
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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".
The train was carrying tar sands oil from Canada to Oklahoma for ConocoPhillips.
The derailment of 32 oil tanker cars Friday just south of Doon has caused concern for towns and cities downstream, as far south as Omaha, Nebraska, about 150 miles from the derailment site.
Metropolitan Utilities District said it was monitoring pumps it uses to pull drinking water from the Missouri.
City public information officer Travis Olson says the wells were shut down as soon as Rock Valley officials were told of the derailment and oil spill north of them early Friday morning. The spill has raised concerns about drinking water downstream.
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. It plans to drain and clean its wells and use a rural water system until testing shows its water is safe.