North Dakota dug in Monday for a lengthy fight against the erosion of its levees, as flood levels were forecast to remain near record highs and an approaching snow storm threatened to complicate the work of emergency teams.

One floodwall protecting a Fargo school was undermined Sunday, swamping the campus in what the mayor called a "wakeup call" to those who thought the worst was over when the river crested.

"Those things will continue to happen, I guarantee it," said Fargo Mayor Dennis Walaker. "People have to continue to be vigilant."

The mighty Red River, which divides North Dakota and Minnesota and runs north to Canada, was flowing at three to four times its normal strength, eating away at on the city's 48 miles (77 kilometers) of protective dikes and levees.

A blizzard bringing strong winds forecast to hit the region Monday afternoon could whip up waves as high as two feet which will batter and possibly wash over the city's defenses, the national weather service predicted.

Federal officials warned that as many as 30,000 people could be left homeless in the northern plains if the river breaks through levees which are the only defenses for North Dakota's largest city, Fargo, as well as Moorhead lying on the opposite bank in Minnesota.

But local officials said a series of contingency dikes should protect most neighborhoods and noted that only five homes have been lost so far within the city limits.

"We've had an awful lot of pressure to vacate the city and we refuse to do that," Walaker said.

With the river expected to remain more than 20 feet about flood level for the next week, thousands of volunteers worked to fill another 500,000 sandbags to help reinforce levees.

An army of foot patrols walked the lines of the city's defenses to try to catch and plug minor leaks before they undermined the entire system.

Police said Monday a woman identified as Vicki Bates, 49, had been arrested for driving while drunk on a dike. No damage to the levee was reported.

Officials used helicopters to lower 11 one-ton bags of sand into the breach at Oak Grove Lutheran School in an operation which saved the complex from significant water damage.

"A lot of people want to do the dance on the dike but we've got a long way to go," said Greg Gust, a meteorologist with the National Weather Service who predicted that the river could remain at flood stage for a month.

"Every day the water's up against there they're going to be working to reinforce those dikes because they don't want them to fail," he told AFP.

"It's (a question of) how many fingers I need to plug those holes."

Thousands of people have already fled their homes as this flat prairie state remained blanketed with snow and floods that were nine miles (14.5 kilometers) across in some points.

Water levels in some riverside homes had reached the second floor while small dikes kept others dry in the middle of a deep, muddy lake.

Regular boat patrols checked in on those who refused to leave their homes so they could monitor their pumps and repair leaks in their dikes.

The Red River reached a high of 40.8 feet (12.4 meters) early Saturday in Fargo -- breaking the previous record of 40.1 feet (12.2 meters) in 1897 -- and only inches below the top of the city's highest levees.

By Sunday evening it had fallen to 39.83 feet (12.1 meters).

US President Barack Obama has pledged to do whatever is needed for the stricken region and the US administration stockpiled food, blankets and shelters in case of mass evacuations.

Fargo officials have been trying to raise the 800 million dollars needed to build permanent flood protections since the area narrowly escaped a massive 1997 flood that destroyed Grand Forks.

"I'm sure we're going to have millions of dollars of damage just from putting up those (temporary) dikes," said Deputy Mayor Tim Mahoney, who urged federal officials to provide the funding immediately.

"Go out there it's a war zone ... you wouldn't want to do this every year."

Both Grand Forks, North Dakota and Winnipeg, across the border in Canada, have flood diversion systems which should protect them from the forecasted crests.