Rick Kozin, a spokesman for the Polk County emergency operations centre, said hundreds of volunteers were helping to fill more sand bags in an effort to provide a barrier against floodwater.
"There's still a lot of water in the river. The situation still remains serious," he said.
Serious flooding has hit the entire US Midwest, including parts of South Dakota, Minnesota, Wisconsin, Nebraska, Illinois, Indiana, Kansas, Oklahoma and Arkansas.
Houses and cars were swept along by rushing floodwaters in the Iowan city of Cedar Rapids, where the Cedar River breached its banks.
The floodwaters there are now beginning to recede but authorities have called on residents to limit their water use to drinking.
"The city continues to be very dangerous but we are doing everything in our power to get people access to their homes as soon as possible in a safe manner," Jeff Beauregard, Cedar Rapids' mayor, said.
Extreme weather has left at least 16 people dead and thousands homeless in Iowa in the last three weeks, Bret Voorhees, spokesman for the Iowa Homeland Security and Emergency Management, said.
The disaster began when a major tornado struck on May 25. It was followed by heavy rains and on Wednesday another tornado tore across western Iowa.
The flooding, which has swept across America's corn belt, is likely to put further pressure on already high food prices.
Up to 20 percent of Iowa's crops have been lost due to the storms and flooding, according to initial estimates.