Residents in Iowa have described the floods as 'heartbreaking'
At least 24,000 people have been forced from their homes in Iowa by flooding, which has also severely damaged crops in America's main corn state.
Authorities issued mandatory evacuation orders as Cedar Rapids, Des Moines and other Iowa cities were partly deluged.
The disaster was triggered by storms blamed for at least nine deaths in the US Midwest this week.
South Dakota, Minnesota, Wisconsin, Nebraska, Illinois and Indiana have also been affected.
TV crews crowded around Barack Obama as he shovelled sand
Iowa Governor Chet Culver has declared most of the Hawkeye State a disaster area.
In Illinois, local Senator Barack Obama was out helping local people fill sandbags on the banks of the Mississippi river at Quincy.
The authorities are predicting record water levels there in the coming days.
"Since I've been involved in public office we've not seen this kind of devastation," the Democratic presidential hopeful said.
The floods have wrecked the Midwest's corn and soybean crops, helping push already high food prices to record levels this week.
State capital Des Moines was dealing with its first major flooding on Saturday as water poured out of the Des Moines river when a levee gave way.
One of the worst affected areas is Cedar Rapids city, where 438 streets have been submerged and nearly 4,000 homes evacuated.
Officials in the city - which is the state's second largest with a population of about 120,000 - warn its drinking water supply is under threat.
The cost of the damage in Cedar Rapids alone has been estimated at $737m (£380m).
City Mayor Brian Fagan said: "It's a bit overwhelming. This is an endurance competition."
The Cedar river crested on Friday night at nearly 32ft (9.75m), 12ft (3.66m) higher than the previous record, set in 1929.
"This is our version of Katrina," Johnson County Emergency Management spokesman Mike Sullivan said in Iowa City, in a reference to the hurricane that caused devastation in New Orleans in 2005.