The remnants of Irene reached Canada Monday after barreling through the northeastern United States where the storm claimed at least 18 lives and caused estimated economic damage of up to $7 billion.
Millions of people were without power along the East Coast after the huge storm -- now downgraded to a post-tropical cyclone -- passed over the Big Apple and headed for Canada.
The Miami-based National Hurricane Center warned late Sunday of "major river flooding occurring in parts of the northeast," after President Barack Obama cautioned that recovery efforts would last for "weeks or longer."
"I want people to understand that this is not over," Obama said in a short statement in the White House Rose Garden.
"I do want to underscore that the impacts of this storm will be felt for some time... Power may be out for days in some areas."
The governor of Vermont, Peter Shumlin, said his state was in "tough shape" while New York state's Andrew Cuomo warned of "tremendous flooding" in the Catskill Mountain area north of Manhattan.
In New Jersey, Governor Chris Christie said roads and rails were "impassable" in some areas, and much of the state had seen "significant flooding."
Localized flooding occurred in the south and east of Manhattan, with more serious incidents in Brooklyn, where the famed Coney Island amusement park took a battering and outlying beaches were swamped.
There was heavy flooding along the low-lying south shore of Long Island where high tides, rain and ocean surge drove waves right up against expensive beach houses. Floods were also reported far inland after torrential rain.
At least 18 deaths were blamed on the storm, which first slammed into North Carolina on Saturday as a Category One hurricane, before turning north up the coast and weakening.
Initial property damage estimates ranged up to $7 billion.
The youngest victim, an 11-year-old boy, died when a tree crashed through his apartment building in Newport News, Virginia.
The Federal Aviation Administration announced that New York area airports -- John F. Kennedy International Airport, LaGuardia and Newark -- would reopen on Monday. More than 10,000 flights were cancelled across the eastern United States.
The New York Stock Exchange said it was set to reopen as normal Monday morning.
"The good news is the worst is over and we will soon return to restore and return mode," New York Mayor Michael Bloomberg said, announcing that 370,000 people ordered to evacuate could now go back home.
More than a million evacuees in New Jersey were also headed home, Christie said.
City officials however warned that commuting into New York could be a nightmare this week with no firm indication of when public transport would be back on track following an unprecedented shut-down just ahead of the hurricane.
"You're going to have a tough commute in the morning," mass transit chairman Jay Walder said.
Walder said buses could start running soon, but subway trains needed extensive testing of lines and equipment.
Irene also left swaths of territory without power, including one million in New York state, most of them on Long Island, according to Cuomo.
In New Jersey, 650,000 people had lost power supplies, while in the greater Washington area, nearly two million people lost electricity. In Massachusetts, 500,000 customers were without power.
Virginia's Governor Bob McDonnell told MSNBC his state had seen the second biggest power outage in its history.
"It's going to be days, perhaps a week, before all the power's restored. We just ask people to be patient," he said.
About 200,000 households have lost power in Canada, officials said.
Some 65 million people live in the urban corridor from Washington north to Boston. More than 4.5 million customers lost power in that area and well inland.
In Atlantic City, a gambling resort on the New Jersey shore, locals started to put their lives back together, while counting the costs of a weekend shutdown during the high season.
"We were expecting to make good money, but you cannot fight with Mother Nature," said Riaz Rajput as he removed plywood storm screens from the windows of his shop.
On Cape Cod in Massachusetts, waves of up to six feet (1.8 meters) crashed over some coastal roads, but most residents and tourists let out a collective sigh of relief.
"We're having a little bit of a hurricane party," said T.J. Wolnar, who was confident his beachfront home could withstand the high winds.
"It's good the storm isn't as strong as it was going to be."
Hurricanes are rare in the northeastern United States -- the last major hurricane to hit New York was Gloria in 1985.
Meanwhile, the National Hurricane Center reported that a new tropical storm, Jose, had formed and was approaching Bermuda.