Guides here jokingly refer to the Alsek as a Class V Float Trip. There may be rivers with more concentrated rapids than the 140-mile-long Alsek, but looks can be deceiving. The Alsek, which is surrounded by massive glaciers and giant peaks as high as 15,000 feet, drains one of the largest non-polar ice fields in the world. This translates to furiously fast, massively voluminous water that rockets out of the mountains at a much steeper gradient than, say, the Yukon.
The water temperature here averages 33 degrees so if you flip, you freeze—dry suits are mandatory. To amp up the adrenaline factor, the Alsek runs through Kluane National Park, home to the highest concentration of grizzlies in North America. Then there’s Turnback Canyon, the five-mile Class VI stretch so deadly that it must be portaged by helicopter. Twelve-day trips from $3,795; best time to go is June.