Officials readied mandatory evacuation orders as rapidly rising floodwaters lapped sandbag dikes built to reinforce defences against what is forecast to be North Dakota's worst floods in recorded history.

The city of Fargo implemented emergency plans for vulnerable populations, opening shelters and evacuating nursing homes, officials said.

In a bid to loosen clogged waters swamping the flat prairie state, authorities blew up ice jams with explosives, and residents trapped across the state were rescued by boat and helicopter.

Persistent low temperatures and heavy falls of wet snow hampered the efforts of thousands of volunteers and legions of state troopers building up improvised dikes — even as flood forecasts were vamped up through the day.

"It's uncharted territory," Fargo Mayor Dennis Walaker said. "If nature has anything else to throw at us, it'd have to be a tornado."

North Dakota's Governor John Hoeven told CNN that hundreds of national guardsmen and thousands of volunteers were "working very hard to put the best protection in place" for vulnerable areas against surging flood waters.

A federal disaster declaration

President Barack Obama issued a federal disaster declaration for 34 counties and two Native American reservations as nearly the entire state remained under a major flood warning.

A heavy blizzard knocked out power and dumped wet snow and freezing rain on Wednesday, making many roads impassable and saturating the already sodden earth.

Low-lying homes across the region were evacuated as rivers and creeks spilled over their banks and several bridges and roads were already closed due to flooding as an unusually heavy snowpack began to melt on top of land that has not yet fully thawed.

"The state has turned into a fishing pond and can't absorb any more snow or rain," said Patrick Slattery of the National Weather Service.

The biggest threat was the Red River, which runs along the North Dakota-Minnesota border and flows northward to Canada. In 1997 massive flooding from the river forced tens of thousands of people from their homes — an event looking to be eclipsed by this weekend.

It was already 20 feet (six meters) above flood levels in Fargo and was forecast to rise several more before cresting on Saturday.

Fargo — a city of about 92 000 — was most at risk as it has not developed the extensive flood protection systems of upriver cities like Grand Forks and Winnipeg, Canada.

Volunteers filled sandbags

Thousands of volunteers, many in the city's Fargodome arena, filled 2.5 million sandbags to build miles of dikes as tall as a four-story building to hold back the rising river, which was at nearly 39 feet (11.9 metres) deep on Thursday.

But just as the exhausting bag-by-bag work seemed close to completion, forecasters raised their predicted crest level by a possible three feet.

"Conditions on the Red River at Fargo have grown increasingly dangerous over the past 24 hours," the weather service said in a bulletin late on Thursday.

"The river is currently approaching record levels and showing no sign of slowing," it said, warning that flows upstream of the city have "produced unprecedented conditions" and "the river is expected to behave in ways never previously observed."

The crest could reach as high as 43 feet (13.1 metres) by Saturday and continue at that level for three to seven days, the weather service predicted.

In 1897 the Red River reached a record 40.1 feet (12.2 metres) in Fargo. The massive 1997 flood sent waters here to 39.57 feet (12 metres) high.

Officials began distributing evacuation plans and building a secondary system of dikes to protect against potential breaches after the cold weather has weakened the integrity of the dikes, which must now be raised by another foot.

But they promised residents they were not giving up.

"We will defend every house," said deputy mayor Tim Mahoney, who noted that the river is already lapping at the sandbag dike in his own backyard.

The state capitol of Bismark was granted some relief on Thursday when the Missouri river levels fell back below flood stage after army specialists used explosives, sand and salt dropped from helicopters to break up a massive ice jam.

But large swaths of the city remain under an evacuation order and several stranded residents had to be rescued.

It will take at least a week for the crest to reach the Canadian border because of the flatness of the land, the weather service predicted.

More snow was forecast to fall on the Red River valley in the coming days and rain could worsen flood conditions next week.

AFP

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