As Irene churned over Canada Monday, residents along the battered U.S. East Coast began surveying damage and fretted over the next danger: treacherous flooding.
Downgraded from a hurricane as it lumbered up the coast Sunday, Irene left millions without power across much of the Eastern Seaboard, was blamed for at least 21 deaths and forced airlines to cancel about 12,000 flights .
It never became the big-city nightmare forecasters and public officials had warned about, but it still had the ability to surprise.
And the danger was far from over for many.
Rivers and creeks turned into raging torrents tumbling with tree limbs and parts of buildings in northern New England and upstate New York.
Authorities warned of possibly disastrous flooding in the days ahead, with Vermont's governor saying his state was facing "a full-blown flooding catastrophe."
Video: Irene only got a little bite out of the Big Apple (on this page)President Barack Obama, speaking Sunday afternoon from the Rose Garden, urged people to stay vigilant.
"This is not over," Obama said. "Many Americans are still at serious risk of power outages and flooding, which could get worse in the coming days as rivers swell past their banks."
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Many of the worst effects arose from rains that fell inland, not the highly anticipated storm surge along the coasts.
Residents of Pennsylvania and New Jersey nervously watched waters rise as hours' worth of rain funneled into rivers and creeks.
Irene was downgraded further to a post-tropical cyclone as it crossed the border with Canada on Sunday night, the danger from rising waters was intensifying for many.
Flooding was under way or forecast along rivers throughout New England after Irene dumped 6 inches to a foot of rain on many of the coastal states, according to a detailed map published by NOAA.
'Very serious'
Hundreds of Vermonters were told to leave their homes after Irene dumped several inches of rain on the landlocked state.
Video posted on Facebook showed a 141-year-old covered bridge in Rockingham swept away by the roiling, muddy Williams River. In another video, an empty car somersaulted down a river in Bennington.
"It's very serious for us at the moment in Vermont," Robert Stirewalt, a spokesman for Vermont Emergency Management Agency, told Reuters. "The top two-thirds of the state are inundated with rapidly rising waters, which we anticipate will be an issue for the next 24 hours."
"It's pretty fierce. I've never seen anything like it," said Michelle Guevin, who spoke from a Brattleboro restaurant after leaving her home in nearby Newfane. She said the fast-moving Rock River was washing out the road to her house.
Green Mountain Power warned that Montpelier, the capital, could be flooded twice: once in the initial storm and again if the utility decides it must release water to save the earthen Marshfield Dam, about 20 miles up the Winooski River to the northeast.
"We don't want to do it. But if the dam were to be compromised, it would be a far greater effect," utility spokeswoman Dorothy Schnure said. Residents of 350 households were asked to leave as a precaution.
Nearly 5 million homes and businesses lost power at some point during the storm. Lights started to come back on for many on Sunday, though it was expected to take days for electricity to be fully restored.
Only about 50,000 power customers in New York City went dark, but people there had something else to worry about: getting to work Monday.
The metropolitan area's transit system, shut down because of weather for the first time in its history, was taking many hours to get back on line.
Limited bus service began Sunday and New York subway service was to be partially restored at 6 a.m. Monday , but riders were warned to expect long lines and long waits.
The commuter rail service to Long Island and New Jersey was being partially restored, but the Metro-North Railroad to Westchester County and Connecticut was suspended because of flooding and mudslides.
Story: 'Gold medal forecast' of Irene's path saved lives, expert says
Airports in New York and around the Northeast were reopening to a backlog of hundreds of thousands of passengers whose flights were canceled over the weekend.
Some of New York's yellow cabs were up to their wheel wells in water, and water rushed over a marina near the New York Mercantile Exchange, where gold and oil are traded. But the New York flooding was not extensive from Irene, whose eye passed over Coney Island and Central Park.
The New York Stock Exchange said it would be open for business on Monday, and the Sept. 11 memorial at the World Trade Center site didn't lose a single tree.
Mayor Michael Bloomberg defended his decision to order 370,000 residents to evacuate their homes in low-lying areas, saying it was impossible to know just how powerful the storm would be.
"We were just unwilling to risk the life of a single New Yorker," he said.
Irene had at one time been a major hurricane, with winds higher than 110 mph as it headed toward the U.S.
It was a tropical storm with 65 mph winds by the time it hit New York. It lost the characteristics of a tropical storm and had slowed to 50 mph by the time it reached Canada.
Damaged roads
Chris Fogarty, director of the Canadian Hurricane Centre, warned of flooding and wind damage in eastern Canada and said the heaviest rainfall was expected in Quebec, where about 250,000 homes were without power.
At least 21 people died in the U.S., most of them when trees crashed through roofs or onto cars. One Vermont woman was swept away and feared drowned in the Deerfield River.
Officials worked to repair hundreds of damaged roads, and power companies picked through uprooted trees and reconnected lines.
One private estimate put damage along the coast at $7 billion, far from any record for a natural disaster.
Video: Jersey shore battered, but inland might have it worse (on this page)Twenty homes on Long Island Sound in Connecticut were destroyed by churning surf.
The torrential rain chased hundreds of people in upstate New York from their homes and closed 137 miles of the state's main highway.
Authorities in and around Easton, Pa., kept a close eye on the rising Delaware River. The National Weather Service forecast the river to crest there at 30 feet, well above normal flood stage.
In the South, authorities still were not sure how much damage had been done but expressed relief that it wasn't worse.
"Thank God it weakened a little bit," said Virginia Gov. Bob McDonnell, who toured a hard-hit Richmond neighborhood where large, old-growth trees uprooted and crushed houses and automobiles.
In Norfolk, Va., where storm surges got within inches of breaking a record, most of the water had receded by Sunday. There was isolated flooding and downed trees, but nowhere near the damage officials predicted.
"We can't believe a hurricane came through here," city spokeswoman Lori Crouch said.
In North Carolina, where six people were killed, the infrastructure losses included the only road to the seven villages on Hatteras Island.
"Overall, the destruction is not as severe as I was worried it might be, but there is still lots and lots of destruction and people's lives are turned upside down," Gov. Beverly Perdue said in Kill Devil Hills.
Video: Cantore: Irene was a 'wind and rain monster' (on this page)In an early estimate, consulting firm Kinetic Analysis Corp. figured total losses from the storm at $7 billion, with insured losses of $2 billion to $3 billion.
The storm will take a bite out of Labor Day tourist business from the Outer Banks to the Jersey Shore to Cape Cod.
This year has been one of the most extreme for weather in U.S. history, with $35 billion in losses so far from floods, tornadoes and heat waves.
Irene was the first hurricane to make landfall in the continental United States since 2008, and came almost six years to the day after Katrina ravaged New Orleans on Aug. 29, 2005.
Irene had led to the deaths of at least 21 people in eight states as of Sunday evening:
CONNECTICUT:
? In Prospect, one person was killed in a fire that Gov. Dannel P. Malloy said was apparently caused by wires knocked down by the storm.
FLORIDA:
? In Volusia County, 55-year-old Frederick Fernandez died Saturday off New Smyrna Beach after he was tossed off his board by massive waves caused by Irene. The Orlando Sentinel reports the high school teacher had a large cut on his head, apparently from hitting the sea floor.
? In Flagler County, 55-year-old tourist James Palmer of New Jersey died Saturday in rough surf. Family members say they lost sight of him after he waded into the surf in North Florida. He was pulled to shore and his wife attempted CPR, but he was pronounced dead at the hospital.
MARYLAND:
? In Queen Anne's County, Md., 85-year-old Anne Bell was killed when a tree knocked a chimney through the glass roof of the sunroom where she and her son were sitting. They'd gone out into the sunroom after the power went out in her Queenstown home. Bell was struck by debris, causing severe trauma.
NEW JERSEY:
? Celena Sylvestri, 20, of Quinton called her boyfriend and then 911 early Sunday seeking help getting out of her flooded car in Pilesgrove, police said. Her body was found eight hours later in the vehicle, which was about 150 feet off the road, police said.
Video: Jersey shore battered, but inland might have it worse (on this page)
NEW YORK:
? A man in his 50s was electrocuted in Spring Valley when he tried to help a child who had gone into a flooded street with downed wires. The child was in very serious condition at Westchester Medical Center's burn unit, said a spokesman for the Rockland County Emergency Operations Center.
? State police said they recovered the body of a woman who apparently drowned after she fell into Onesquethaw Creek in New Scotland, near Albany.
NORTH CAROLINA:
? In Nash County, a man was killed after a tree limb fell on him outside his home Saturday morning as outer bands from the storm brought near hurricane-force gusts inland.
? Goldsboro police say a 15-year-old girl died Saturday afternoon after the SUV carrying her and family members collided with another SUV at an intersection where Irene had knocked out power to traffic lights. She was one of four family members thrown from the vehicle; the family was returning to northern Virginia from Myrtle Beach, S.C.
? Authorities in Pitt County say a man was found dead in his home after Irene's winds toppled a tree onto his house.
? Another man in Pitt County drove through standing water, went off a road and died after striking a tree on Saturday.
? A mother in Sampson County died Saturday morning when a tree fell on a car carrying her and two family members.
? New Hanover County deputies on Sunday afternoon recovered the body of Melton Robinson Jr., who had been missing since falling or jumping into the Cape Fear River as storms from Irene reached North Carolina on Friday night.
PENNSYLVANIA:
? Michael Scerarko, 44, was killed Sunday when a tree fell on him in his yard. Scerarko, of Stroudsburg, pushed his son out of the way but could not get out of the way himself, police said.
? A 58-year-old Harrisburg man was killed Sunday morning when a tree toppled onto his tent, state police said. The man was one of about 20 people at a party on private property in East Hanover Township, Dauphin County, some of whom who decided to sleep outside.
? A man in a camper was crushed by a tree in northeastern Pennsylvania's Luzerne County, state emergency management officials said. Police found his body shortly before 11 a.m. Sunday.
? A motorist was killed when he lost control of his car on the Pennsylvania Turnpike in Carbon County, skidded over an embankment and hit a tree. State officials attributed the accident to the storm.
VIRGINIA:
? Newport News authorities report that 11-year-old Zahir Robinson was killed when a large tree crashed through his apartment shortly after noon.
? In Brunswick County, a tree fell across a car Saturday afternoon, killing 67-year-old James Blackwell of Brodnax. The driver was taken to the hospital with minor injuries.
? Chesterfield County police say a man died at a Hopewell hospital Saturday after a tree fell on a house that he was in.
? A King William County man, 57-year-old William P. Washington, was killed when a tree fell on him as he was cutting another tree on Saturday night. Washington was trying to get home at the height of the storm when a fallen tree blocked his path on a road, about four miles from his home, and he and another motorist tried to clear the way.
The Associated Press and Reuters contributed to this report.