This glacier causes a major pinchpoint in the Alsek river, called Turnback Canyon. Few people have run the canyon because it is intense and blind. The commercial raft trips hire a helicopter ride past the obstacles, lifting the boats in giant slings. Private boaters with smaller budgets grunt for two days carrying and dragging their equipment across the glacier. My friend Tyler of Fun Hog Press did this, and I believe he writes about it in his book on whitewater classics.
The glacier is about 1 km wide and 70 km long. My friend Nelbert says that there are house-sized chunks of ice dropping into the river at an hourly rate, and apartment building sized chunks falling in daily. He probably helped save some kayakers who were up there just a few weeks ago and wanted to run Turnback. He says that the eddies are full of giant chunks of ice and no sane boater would go there.
But back to the glacier, it is "surging", meaning that it is advancing forward at 100x its normal rate. It is kind of like a star that shines brightly just before it implodes....the glacier is dying. The warming trends have caused increased water to percolate through the earth below the glacier, and the whole thing is slip sliding away. Some time this fall the glacier is expected to dam up the river entirely and form a giant lake. The last time the glacier surged was in the 1970's and it briefly blocked the river and formed a lake in midwinter. If the pattern continues normally the spring snowmelt will break through the ice dam and a flood will spread over the wide riverbottoms below. Next summer, or in one of the summers soon to follow, the entire glacier may break into chunks and melt. Tweedsmuir Glacier may disappear, and Turnback Canyon will go in its wake.
SOURCES
http://fairweather.gps.alaska.edu/chris/images/Tweedsmuir_4aug07/index.html (pictures!)
http://cgip.wetpaint.com/page/Tweedsmuir+Glacier?t=anon (history)
http://www.gi.alaska.edu/ScienceForum/ASF18/1878.html
http://www.uaf.edu/news/a_news/20071025085229.html