WASHINGTON (Reuters) - A US official said he feared serious flooding could lead to disasters after the expected recession of several US states this week.
A frosty wave, accompanied by cold winds from the North Pole and more than 20 people in the United States, began to recede on Friday, easing down on dozens of states where temperatures have fallen to record highs.
Forecasters expect spring temperatures this weekend.
The "unprecedented polar air mass" hit the northern plains, the Great Lakes region and the midwest regions of the United States, leading to very low temperatures ranging from 34 to 51 degrees Celsius below zero.
The ice covered a river in central Chicago, half the size of the Great Lakes, and the waters of Niagara Falls were frozen.
Twenty-one people died from low temperatures after the snowstorm hit the center of the country this week, with record temperatures dropping Wednesday and Thursday, US media reported.
The National Meteorological Center said temperatures "continued to decline" Friday.
"In some parts of the Midwest, the next swing of temperatures will be towards warmth in a short period of time," meteorologist Jeff Masters told AFP.
In northwest Minnesota, temperatures dropped to minus 53 degrees Celsius, to 51 in Wisconsin and 58 below zero in Illinois.
But temperatures will be milder by Monday, as they are expected to reach 10 degrees Celsius.
Local officials said they were concerned that the melting of snow in the Niagara Falls area could raise the water level.
"It is important to remember that rapid climate change and melting of ice could lead to dangerous flooding," James James, regional director of the Federal Emergency Management Agency, said in a statement.
The authorities fear the threat of flooding and its impact on the infrastructure under pressure, especially with the onset of melting of ice.
More drinking water pipelines could be exposed in Detroit and Chicago to crash.
Some aspects of life are gradually beginning to return, especially in the transport sector.
More than 1,500 flights a day were canceled at Chicago airports this week, leaving thousands of passengers stranded in the city, which is a regional hub for flights in the United States.
"New flights have already been booked for most of our customers," American Airlines spokesman Russ Finstein told AFP.
Delta Airlines spokesman Derek Castaneda said the company "started working in normal form today (Friday)."
A number of schools opened in Wisconsin, while the inability to operate buses in some other schools hindered the resumption of school.
The authorities warned citizens to stay for more than a few minutes on the streets, and asked the displaced and the elderly to go to the heating centers available in the area.
The authorities have opened more than 270 "heating centers" in federal buildings, social centers, libraries and even police stations for those in need of a cozy place.
A 18-year-old student named 18-year-old Gerald Belles died on January 30, the University of Iowa said. Local media reported that Bills was found unconscious outside a university building as temperatures were at their lowest in Iowa.
A 38-year-old woman was found frozen in Wisconsin on Wednesday evening in an uninhabited house, while a man was found frozen to death in the backyard of his home on Thursday afternoon, local media reported.
