FORT KENT — Homes and businesses were evacuated Wednesday as spring floods along the St. John River spilled into the downtown of this Canadian border town, marking what emergency management officials described as a 100-year flood for the St. John.

At least 3 inches of rain combined with melting snow sent the St. John to more than 3 feet above flood stage, causing water to begin rising on Main Street. Before the river crests, it was expected to spill over a dike along the river, officials said.

"We're evacuating all the main streets, going to all the businesses and telling them to close up for safety," Police Chief Kenneth Michaud said.

Hours earlier, the Aroostook County Emergency Management Agency said residents of about 30 to 40 homes were ordered to leave because of rising floodwaters caused by heavy rain and the melting of the region's heaviest snow accumulations in memory.

Evacuations also took place along the St. John River in Van Buren, downstream from Fort Kent, and in the Penobscot County town of Mattawamkeag due to the Mattawamkeag and Penobscot rivers spilling over their banks, according to the National Weather Service.

The International Bridge over the St. John between Fort Kent and Clair, New Brunswick, was closed amid fears that the raging waters could drag it down. The bridge over the Fish River, which empties into the St. John, also was closed as a precaution.

"If that bridge falls over, it would make like a dam, and the water would wash over the main street," Michaud said.

Local officials have been watching the St. John since last week when rising waters caused concern on the Canadian side. Those waters had been receding until a deluge of 3 inches or more of rain over a 24-hour period, said Joseph Hewitt of the National Weather Service.

There was still a half-foot of snow on the ground following a winter that dumped around 200 inches of snow in the region, and the melting snow exacerbated the situation.

"In response to that, a lot of these rivers took off," Hewitt said.

It was the worst flooding ever seen in the community of 4,200 residents, Hewitt said.

An emergency shelter was set up at the University of Maine at Fort Kent, where 20 to 40 people had gathered by late morning.

In Augusta, where Gov. John Baldacci issued a state of emergency Tuesday night because of flood conditions in Aroostook County, the Maine Emergency Management Agency activated its 24-hour operations center.

Baldacci and MEMA officials flew by airplane to Aroostook County, where he then boarded a helicopter to get an aerial tour of the situation and meet with local officials in Fort Kent.

During a conference call, scientists described the flooding on the St. John as "greater than a 100-year event," said Lynette Miller, MEMA spokeswoman.

"It's certainly a dangerous situation. That's why they're doing evacuations proactively. They've been out there early and often with the warnings," she said.

The problems were worst on the U.S. side of the river, but warnings were being issued to residents in low-lying areas around Fredericton, New Brunswick, about 200 miles away from Fort Kent.

Emergency officials in New Brunswick warned people living along the St. John River to brace for the worst flood in decades, with up to 1,300 homes threatened by the rising water. By Wednesday afternoon, more than 300 people had registered with the Red Cross to confirm they have left their homes or plan to do so.

That number was expected to grow as officials warned that floodwaters would continue to rise until Thursday morning, when the crest in the Fredericton area would likely reach a level just below the record level set in 1973.