Eleven-thousand of the city's 40,888 residents had already been evacuated, the Dakota Farmer reported Friday.
Authorities said the flooding in Minot likely will be far worse than had been predicted, with the Souris River topping a 130-year-old record by more than 8 feet.
"This'll be historic," National Weather Service meteorologist Harlyn Wetzel in Bismarck, N.D., told The Wall Street Journal.
"This is a Noah flood -- as in big, large, historic," U.S. Army Corps of Engineers district engineering Chief Michael Bart told The New York Times.
National Guard helicopters patrolled the area around Minot Friday under drizzly skies.
"It's very obvious now that the levees have been breached in most places in Minot," Gov. Jack Dalrymple told the Fargo Forum. "The farther west you go, the deeper the water is. Obviously, the surge is from there. It really is remarkable because we all know it's going to be several feet higher as well."
In addition to Minot, Burlington also was underwater. Sandbagging efforts were still under way in Sawyer and Velva.
The river was expected to rise far higher and faster than first forecast, cresting as high as 1,565 feet above sea level, washing 2 to 3 more feet of river water over the city. Waters had already spread across the downtown area, threatening to split the city in two, the Times said.
Nobody could have anticipated a flood of this magnitude, with water levels almost six times what the area's 40-year-old, flood-protection systems were designed to take, Bart said.
The river, swollen by recent downpours, spilled out of its regular channel and cut down a valley to Lake Darling, the last flood-control point north of Minot, Wetzel said.
The corps was releasing about 22,000 cubic feet of water a second from the lake toward Minot early Friday. Flows could rise to 28,000 to 29,000 cubic feet by Saturday, Wetzel said.
In Nebraska, Missouri River flooding still threatened the Cooper nuclear plant but the failure of a levee in northwest Missouri dropped water levels a foot, temporarily easing the threat, the Lincoln Journal-Star reported.