The Red River breached a dike on Fargo's north side early Sunday morning, which sent water flowing onto a school campus before it was contained, local officials said.
The flooding did not lead to an evacuation of the surrounding area. However, residents were instructed to plug their sewers and keep an eye on their basements.
The extent of damage at Oak Grove Lutheran School is not yet known.
The school's principal, Morgan Forness, said the Army Corps of Engineers, the National Guard and city officials tried unsuccessfully to contain the water to one campus building after a flood wall panel gave way around 1:30 a.m. local time.
"They made a gallant effort ... but the power of the river is just too much," he told local KFGO radio. "They gave it everything they had, and it just -- we couldn't contain it. It came to the centre of campus, and now, it's inundating all of the buildings."
City officials said the flooding was caused by erosion and was started when water began seeping through the floor of one building and then spread throughout the 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.
The Red River continued its slow retreat on Sunday morning. It crested at 12.44 metres around midnight on Friday and has since dropped to 12.27 metres, which is still more than 6.7 metres above flood stage.
Officials fear that water could still breach the levees and flood parts of the city.
Fargo mayor Dennis Walaker said breaches "will continue to happen. I guarantee it."
The river may yet fluctuate by up to a foot and remain dangerously high for another week.
Residents and volunteers were to continue piling up sandbags on Sunday morning after attending church services.
According to forecasters, the river is receding due to cold temperatures freezing water that would normally flow into the river.
When that water thaws, the flood threat will likely have passed.
In the meantime, National Guard troops, as well as local volunteers, have been inspecting the 56 kilometres of levees around Fargo for breaches.
"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."
Despite the receding water levels, flooding has damaged basements and yards along the river and forced hundreds of residents from their homes.
Emergency crews had to rescue about 150 people from their homes in nearby communities in Minnesota, and about 20 per cent of households in Moorhead, just across the river from Fargo, have been told to evacuate.
The flooding has been caused by a combination of heavier-than-average snowfalls, spring rains and a rapid thaw in Fargo, which has caused the swell in the Red River.
A winter storm forecasted to arrive in the region early this week could still yet cause a break in the levees and massive flooding.
While snow won't affect river levels, winds may cause two-foot waves that could send water over the tops of the dikes, according to Dave Kellenbenz of the U.S. National Weather Service.
"That's something we're going to have to watch closely as we move into next week," Kellenbenz said.
Manitoba prepares
Meanwhile, authorities in Manitoba are working around the clock to prepare for what could be the second-worst flooding in the province's history.
Forecasters believe that the crest of the Red River will cross into Canada from North Dakota on April 5 and arrive in Winnipeg a couple of weeks after that.
With files from The Associated Press and The Canadian Press
