Officials have called for help to shore up sandbagged levees in Iowa, following heavy rain that has caused serious flooding across the US Midwest.
Thousands of volunteers stacked sandbags by rivers in Cedar Falls and Iowa City on Tuesday, as officials sought to prevent them overflowing.
More thunderstorms are forecast this week, posing the threat of more rain.
Democratic presidential hopeful Barack Obama cancelled a planned trip to Iowa because of the floods.
Several days of severe weather have caused rivers to burst their banks, reservoirs to spill over and towns to be inundated in several states.
Dozens of people were forced from their homes in eastern Illinois on Tuesday when levees failed on the Wabash and Embarras rivers, flooding large areas of farmland.
'Critical point'
In Cedar Falls, the brimming Cedar River had been expected to spill over a levee into the city overnight but officials said the barrier appeared to be holding early on Wednesday.
City spokeswoman Susan Staudt praised the efforts of volunteers who had piled up tens of thousands of sandbags, saying their efforts had "saved this city, but we are still at a critical point", the Associated Press reported.
The city of 35,000 was evacuated on Tuesday, with sandbags stacked in the streets and tarpaulins taped to windows to protect homes from flooding.
In Iowa City, university students piled sandbags along the Iowa River in a bid to protect their campus from flooding.
Residents of the town of Elnora, in Indiana, were also relying on sandbags and concrete barriers to hold back the White River.
Indiana Governor Mitch Daniels said the state was suffering its worst flooding since the 19th Century.
"There's widespread damage, there's a lot of water where it shouldn't be, there are a lot of homes that have been damaged or destroyed," said Homeland Security spokesman John Erikson, quoted by Reuters news agency.
In Minnesota, Governor Tim Pawlenty declared a state of emergency in two counties affected by flooding on Tuesday.
Officials from the Federal Emergency Management Agency (Fema) are due to begin visiting flood-hit areas on Thursday.
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