The edge of the Tweedsmuir Glacier — the ridge running diagonally through the middle of this May 7 photo — is surging toward the Alsek River, with the snowpack in front of it being pushed forward. (Courtesy Chris Larsen)

A glacier in northern British Columbia is surging into the Alsek River, which runs through the Yukon, raising concerns of flooding in Alaska.

Photographs of the Tweedsmuir Glacier show the glacier has moved about 300 metres since October, advancing to the edge of the Alsek.

The fast-flowing river is currently eroding the encroaching ice. But there are concerns the ice could form a dam at the river, creating a lake behind it.

That lake would then grow until the ice dam eventually breaks up, causing sudden flooding downstream toward the Alaska community of Dry Bay.

"It's one of the large glaciers up there, and this is an entirely wholehearted surge," Garry Clarke, a glaciologist at the University of British Columbia, told CBC News on Monday.

"The whole glacier is participating, it's advancing over the whole front."

Clarke, who visited the Tweedsmuir Glacier on Aug. 1, said the small amount of snowfall in the area last winter has left water levels lower on the Alsek, possibly raising the risk of an ice dam forming.

"I was a bit pessimistic about this dam forming. And I was down last week actually … and I was really quite startled," he said. "I think this is a much higher probability of making a dam this time than the last."

Emergency plan

The Alsek River starts in the St. Elias Mountains and flows about 250 kilometres across the Yukon and through the northern tip of B.C. before emptying into the Pacific Ocean near Dry Bay.

Officials from Glacier Bay National Park and Reserve in Alaska said they are preparing an emergency plan while they get updates on the ice's movement.

In the meantime, Clarke advised outdoor enthusiasts to be careful if they plan to go on the Alsek River.

"If people were landing planes or rafting down the Alsek, that there'd be a pretty outrageous surprise at some point," he said. "So it's not a manageable risk in a sense."

Chris Larsen, a professor at the geophysical institute at the University of Alaska in Fairbanks, told CBC News that the Alsek River has so far been effective in preventing an ice dam from forming. He added, however, that the possibility of a blockage still exists.

The Tweedsmuir Glacier last surged towards the Alsek River around 1974, leading to floods as far downstream as Dry Bay.

Larsen, who reported the latest glacial surge late last year, estimated that it might have started the previous spring.