College students help build Fargo's flood defences
North Dakota has been declared a federal disaster area by US President Barack Obama because of record spring flooding across the mid-Western state.
Floodwaters from the Red River, which is expected to peak later this week, have closed roads and bridges.
National guardsmen and volunteers are reinforcing flood defences.
The rising waters are also affecting the neighbouring state of Minnesota which, like North Dakota, borders the Red River on its route towards Canada.
A blizzard has brought snow and freezing rain to North Dakota and high winds have knocked out the electricity supply to some towns in the state.
Hundreds of miles of highways and roads have been blocked in North Dakota and the neighbouring states of Montana, Wyoming, South Dakota and Nebraska.
Weather experts say the extreme flatness of the Red River Valley means the overflowing water tends to go wide, move slowly and take weeks to recede because the soil is increasingly saturated.
"You will have an extremely wide river," Pat Slattery of the National Weather Service told AFP news agency.
A waterway that now measures 100 yards wide "might turn into between a mile and a mile-and-a-half", the forecaster added.
Some flooded areas of North Dakota are without power
The authorities in North Dakota have used explosives to destroy an ice jam on the Missouri River in an attempt to ease the build up of water.
The flow of water had been blocked by car-sized blocks of ice joining the Missouri from the Heart River tributary, threatening low-lying areas of the state capital, Bismarck.
Bismarck Mayor John Warford said the authorities were "cautiously optimistic" that the demolition would have a positive effect.
Earlier reports said an ice jam north of Bismarck had burst but the National Weather Service later said the blockage was intact, though leaking.
Mr Warford said the ice jam, 16km (10 miles) from Bismarck, was still a cause for concern.
"The fact that it could break at any time is bad news," AP quoted him as saying.
Volunteers have also been called out in the Bismarck area to help fill in sandbags to bolster the city's flood defences.
Army engineers were preparing to release water from the Missouri River's Garrison Dam, north of Bismarck, once flooding in the capital had eased.
The region last suffered a devastating flood in 1997, which left more than $2bn in damage and tens of thousands of people displaced after 570,000 hectares of land were swamped.