The storm that had been Hurricane Irene crossed into Canada overnight but wasn't yet through with the US, where flood waters threatened Vermont towns and New Yorkers feared a commuting nightmare as their transit system, shut down ahead of the storm, was slowly restored.

The storm left millions without power across much of the Eastern Seaboard, left more than 20 dead and forced airlines to cancel about 9,000 flights. It never became the big-city nightmare forecasters and public officials had warned about, but it still had the ability to surprise.

Many of the worst effects arose from rain 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 funnelled into rivers and creeks. Normally narrow ribbons of water turned into raging torrents in Vermont and upstate New York late Sunday, tumbling with tree limbs, cars and parts of bridges.

"This is not over," President Barack Obama said from the Rose Garden.

Hundreds of Vermonters were told to leave their homes after Irene dumped several inches of rain on the landlocked state. A 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.

Green Mountain Power decided against flooding Montpelier, the capital, to save the earthen Marshfield Dam, about 20 miles up the Winooski River to the northeast. Water levels had stabilised on Monday morning but engineers were continuing to monitor the situation, said spokeswoman Dorothy Schnure.

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 on Sunday and New York subway service was partially restored at 6 a.m. (1000 GMT) on Monday. Riders were warned to expect long lines and long waits.

Commuter rail service to Long Island and New Jersey was being partially restored, but train service to northern suburbs was suspended because of flooding and mudslides.

Airports in New York and around the Northeast were reopening to a backlog of hundreds of thousands of passengers whose flights were cancelled 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 September 11 memorial at the World Trade Centre 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 (177 kph) as it headed toward the U.S. It was a tropical storm with 65 mph (105 kph) winds by the time it hit New York. It lost the characteristics of a tropical storm and had slowed to 50 mph (80 kph) by the time it reached Canada.

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 US, most of them when trees crashed through roofs or onto cars.

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.

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.