United States troops and planes are being sent to the state of North Dakota to help residents flee flooding from the Red River.
The National Weather Service is predicting the river will reach 12.8 metres and forecasts indicate it will remain that high into next week.
Explosives are being used to loosen riverbanks to try to ease the flow of the rising water.
Thousands of people have been evacuated in North Dakota, where flooding has been described as the worst in more than a century.
Hardest hit is the city of Fargo, where the Red River has started to breach its levees, and hundreds of kilometres of sandbag walls.
Officials in neighbouring Moorhead want 2,660 homes there to be evacuated.
Nancy Ward, acting head of the Federal Emergency Management Agency, has been dispatched to Fargo.
On Friday the river swelled to previously unseen heights - surpassing the record of 12.2m set in 1897 in Fargo.
Amid fears that Moorhead and Fargo could be deluged, President Barack Obama sent Ms Ward to oversee the towns' preparations.
Mr Obama issued a federal disaster declaration for parts of Minnesota and the entire state of North Dakota earlier this week.
Authorities in Fargo evacuated about 150 homes in the early hours of Friday, along with hospitals, clinics and a county jail.
A spokesperson for the US Army Corps of Engineers said a small amount of water was leaking through a crack in one of the dykes in Fargo, adding that a contingency dyke had been built behind it.
Some 800 guards are being brought in to monitor the dykes, in addition to the 900 already in place, the mayor of Fargo, Dennis Walaker, said.
Experts say the huge rise in the river's levels was caused by an unusually cold winter, followed by a very quick thaw and heavy rain.
The Red River also flows north into icy conditions, making it less able to cope with rising levels than most rivers in the US, which flow south and widen out as they reach warmer climates.
More snow is forecast to fall on the Red River valley in the coming days, and rain next week could worsen flood conditions.