Members of the National Guard look for breaches in the levee system in Fargo. (Marlin Levison/Star Tribune/AP) The slowly receding Red River breached a dike on the city's north side early Sunday, sending water flowing into buildings at a school campus before it could be contained, city and school officials said.

The extent of the damage at Oak Grove Lutheran School wasn't immediately known. The surrounding neighbourhood was not evacuated, but residents in some areas were told to plug their sewers and monitor basements.

Principal Morgan Forness said city officials, the Army Corps of Engineers, and the National Guard unsuccessfully tried to contain the water to one building after a permanent flood wall panel at the school buckled around 1:30 a.m.

"They made a gallant effort … but the power of the river is just too much," he told KFGO radio. "They gave it everything they had, and it just — we couldn't contain it. It came centre of campus, and now, it's inundating all of the buildings."

The city said the flooding was caused by erosion and began when water came up through the floor of one building and infiltrated the rest of campus.

"I think there's a little bit of divine intervention here — in the sense that we built a secondary dike to help protect the school, and that's gonna probably end up helping to protect the neighbourhood," school president Bruce Messelt told KFGO.

After cresting at about midnight Friday at 12.44 metres, the Red River had dropped to 12.26 metres by early Sunday — still more than 6.7 metres above flood stage. Fargo fears that more water could burst past the levees and submerge parts of the city.

Dangerous water levels expected to last the week

The river may fluctuate up to 30 centimetres and remain at dangerous levels for a week, meaning people will have to endure an agonizing several days before they reach the point they can relax.

The city was to resume sandbagging Sunday morning and was requesting more volunteers. Many were expected to turn out after church services that are a staple of life on Sunday mornings in Fargo, a heavily Lutheran town of more than 90,000.

"I just hope that everybody doesn't let up. We can't let our guard down," said Al Erickson, a 47-year-old banker whose two-storey home is across the street from a golf course that is now a giant water hazard.

"The city as a whole will be OK, but there may be neighbourhoods that still may have some trouble."

Forecasters say the river is retreating because cold temperatures have been freezing water that normally would be flowing into the river. By the time that water thaws, the biggest flooding threat should have passed, Hudson said.

Officials in charge of the flood-response effort deployed high-tech Predator drone aircraft, called up more National Guard troops and brought in hundreds of bags that each hold a tonne of sand and could be dropped by helicopter into breaks in the levees.

Levees around Fargo stretch for 56 kilometres

The National Guard has been dispatching inspection teams to the levees, joining a cadre of volunteers who are being asked to do the same. The task is monumental, with more than 56 kilometres of levees around Fargo.

"I don't think there's an inch of riverfront on the Fargo side that doesn't have some kind of levee," said city engineer Mark Bittner. "We encourage neighbourhoods to get together and have their own dike patrols and assist us."

Bruce Boelter walked the entire length of a roughly kilometre-long stretch of sandbag dike to eyeball the manmade wall separating his subdivision and the Red River. Neighbour Tony Guck joined him halfway.

Each felt a special stake in the dike they helped build.

"If we don't protect this, it's gonna get us. It's basically for our own security," said Guck, 42. "I'm just planning on coming out every six hours and walking it."

Water has forced hundreds of residents in the Fargo area from their homes and submerged basements and yards in an untold number of houses along the river.

Emergency crews in boats had to rescue about 150 people from their homes in neighbouring communities in Minnesota, while about 20 per cent of households in Moorhead have been urged to leave.

Snowy winter, sudden spring melt blamed

The flooding was brought on by heavier-than-average winter snows, spring rains and a rapid thaw of the snowpack that sent the Red River to record-high levels in Fargo, North Dakota's largest city.

A winter storm was predicted to hit North Dakota early next week, although the snow isn't expected to affect the flooding in Fargo. Still, wind from the storm could cause two-metre waves that could send some water over the tops of dikes, said Dave Kellenbenz, a meteorologist with the National Weather Service.

"That's something we're going to have to watch closely as we move into next week," he said.

The variation in flood forecasts was a roller-coaster throughout the week for Fargo, with the projection edging upward twice before being lowered Saturday. Fargo Mayor Dennis Walaker opened a briefing earlier in the day by apologizing for criticizing the weather service.

Greg Gust, a warning co-ordination meteorologist for the weather service, said the predictions are complex. They come from round-the-clock work by hundreds of scientists, engineers and other experts.

Some of those people brave the river for measurements of volume, flow and temperatures. They also use computer models for mathematical and statistical analyses.

But even with improved forecasting methods, the river's record levels and the volatile temperatures don't allow anyone to be certain, and the weather service continued to hedge its prediction Saturday.

The main focus for the Fargo area will be on whether the long line of levees will be able to hold up against the floodwaters — regardless of their level. Engineers say that anytime water is pressed up against a levee for a considerable period of time, there is a risk of catastrophic flooding.

"The saturation usually becomes the enemy of a levee over time," Jud Kneuvean, chief of emergency management for the U.S. Army Corps of Engineers in Kansas City. "It can cause the embankment to be less stable and slide."

Word of the river's possible retreat couldn't come at a better time for 91-year-old Jim Sundahl, whose Moorhead yard has already been swallowed up by floodwaters. He has been waging a furious battle to keep the waters from his home, where he was born.

"I'm happy about it, I'll tell you that," Sundahl said. "But it won't do us any good for four or five days."