A surging Tweedsmuir Glacier could reach and potentially dam the Alsek River.

Doug Makkonen, a Trans North Air helicopter pilot based in Haines Junction, said Thursday there was no evidence of the spectacular surge when he stopped flying into the area last fall.

But by the time he started hauling supplies in this past spring for the upcoming river kayaking and rafting season, the glacier had already started its march toward the river just south of the Yukon-B.C. border in the St. Elais Mountains.

'It must have been moving during the winter,' Makkonen said in an interview with the Star. 'It was pretty dramatic when I first got down there.'

Using altimeter readings from his Bell 206 chopper, Makkonen estimates the highest point on the face of the surging glacier is about 60 metres.

He also estimates it's advanced up to a couple of kilometres so far, and is now just 200 metres or so from the Alsek.

Should it cross the river, Makkonen doesn't think it would flood the river basin all the way back to Haines Junction.

'But there will be a lake very quickly because of the volume of water,' he said.

Historical evidence indicates that in the mid- to late-1800s, when the Lowell Glacier surged across the Alsek, a huge lake was formed that partially flooded what is now the village of Haines Junction.

And in 1973, a surging Tweedsmuir Glacier pushed into the Alsek River but did not block it completely.

Jeff Bond, a surficial geologist with the Yukon Geological Survey, said this morning the marching Tweedsmuir will likely reach the Alsek again, though he doesn't think it will block it.

When the Lowell Glacier dammed the river back in the 1800s, the Yukon was coming out of a mini-ice age and glaciers in the St. Elias range were much more robust than they are today, Bond explained.

As the Tweedsmuir surges forward, he said, it is weakening by extending itself, growing thinner and cracking, causing large crevasses.

'It will likely reach the Alsek River but it is much more fragmented so the river will likely break it up and wash it away.'

Bond agrees there is very little chance it will back up the Alsek to the extent the Lowell Glacier did more than 100 years ago.

If the Tweedsmuir does enter the Alsek, there may be additional impacts from large blocks of ice being dislodged and washed downriver, through what is known to whitewater enthusiasts as Turn Back Canyon,

He also concurs with Bob Daffe of Tatshenshini Expediting that there is potential for the glacier to deposit large boulders into the river.

Daffe, who was at Turn Back Canyon last month to wed longtime partner Theresa Landman, said it was amazing to witness the surging Tweedsmuir.

'I looked across the river and the glacier was right there, where it had never been before,' said Daffe, who's been kayaking and rafting the Alsek for 15 years.

'It was kind of like thunder when it cracked.'

Landman, he pointed out, is the first woman to kayak Turn Back Canyon when she conquered the turbulent stretch 11 years ago.

But if the glacier does reach the river and the canyon, located about 20 kilometres south of the Yukon-B.C. border, it could very well alter things, he said.

'It is carrying these massive rocks,' Daffe said. 'So just imagine dropping a house-sized rock into the Yukon River. It would do something.'

Chris Larsen, a professor with the Univerity of Alaska Fairbanks' Geophysical Institute, said Thursday that by a stroke of luck, he took altimeter survey measurements of the Tweedsmuir in 2006, as part of a general research project of glaciers in southeast Alaska, northwest B.C. and a tiny corner of the Yukon.

He said with the 2006 survey work, he had a baseline of information to work with prior to the Tweedsmuir surge.

One measurement taken last month showed the glacier has grown in thickness at one point to 120 metres from 40 metres the year before, he pointed out.

Like the local geologist, however, Larsen believes if the glacier does enter the Alsek River, the river could easily wash it away as quickly as it advances.

Larsen said glaciers in that area typically surge every 20 to 25 years.

Witnessing this migration, the professor emphasized, is refreshing in these days of seeing so many retreating and receding glaciers.

Both Makkonen and Daffe are somewhat surprised there hasn't been more scientific interest in the surging Tweedsmuir.

It was amazing to watch over the last few months, said Makkonen.

He said initially, it was the northern corner of the glacier that began to move but the surge now involves the entire front.

The massive flow of ice would sometime push itself 100 metres into the air, then slowly flatten out, again and again, as it moved toward the Alsek.

To have documented the surge with a time-lapse camera of some sort would have been amazing, he said.

'All the way down the valley, it travelled like a caterpillar-type thing,' Makkonen said.