Authorities in Minot in the state of North Dakota have told roughly 12,000 residents to leave their homes immediately, as the Souris River threatens to flood.
Sirens sounded throughout the city around 1300 local time (1800 GMT), alerting residents of an impending deadline to evacuate.
“All residents must evacuate, Zones one through nine,” an announcement said.
Heavy rainfall has bloated the river, which flows down from Canada.
The Souris River is expected to hit nearly 1,563ft (476m) above sea level this weekend, topping the previous flood record set in 1881.
“We are trying to patch up as many holes [in flood barriers] as we can to give people as much time as we possibly can to get them out safely,” said Minot Fire Department spokesman Dean Lenertz.
As part of a flash flood warning, the National Weather Service said residents “in the mandatory evacuation zones should be moving to higher ground now”.
Heavy rains and melting snow have raised water levels in Canadian reservoirs in the Souris River basin during the past several weeks, forcing unprecedented water releases further south in North Dakota.
On Tuesday afternoon, the Minot Daily News reported that many city residents were moving as many possessions as possible out of their homes and businesses through congested traffic and muddy streets.
“What I see right now is probably the most devastating in terms of the number of people directly impacted and what it will do to damage homes as water begins to overtop the levees and fill in behind,” North Dakota National Guard Maj Gen David Sprynczynatyk told the newspaper.
Minot is North Dakota’s fourth largest city, with a population of roughly 40,000.
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North Dakota braces for flooding