Dangers of Aufeis on Alaskan Rivers

Translated from the German as “ice on top,” aufeis is found along the rivers of Alaska’s arctic regions. It’s also one of the most unusual dangers you may face on an Alaskan river.

Aufeis is a sheet of ice that forms during winter, filling a river valley until the summer months. It’s unpredictable, though: Sometimes, river water flowing underneath a river’s frozen surface will be forced up to the top through cracks, and will form more layers of ice—much the same way a hail stone is formed when a droplet of water is carried up and down into the cold atmosphere.

By spring, there could be as much as 10 to 12 feet of ice that has spread out along the sides of the river, into the shrubs and across the tundra. Here are the hazards it can create if you encounter it while on a raft kayak or canoe:

Dangerous Passage

Even when the spring breakup sends the main layers of ice downriver, aufeis can linger and block your way. So, never proceed into a narrow channel without knowing that the path is clear. If you can’t tell, get out and walk down the ice to scout the way. I keep a climber’s ice screw in my kit, and have used it many times to tie the raft to something while scouting. We’ve had to portage our boats and gear over to other channels when the way has proven to be completely blocked.

Aufeis Can Extend for Miles Across the Tundra

I once did a trip down the Hula Hula River where, at one point, our rafts were forced out of the main channel by a persistent spot of aufeis that always seems to be difficult to get by. In this case, our rafts were forced out of the channel, and before we knew it we were in a different streambed, then taken into a different watershed. In 30 years of rafting I have never before started on one river and wound up on a different one without having to portage. In this case, we reached the coast nine miles west of where we were supposed to be—not a danger, thankfully, but a big hassle.

Beware When a River Goes Under the Ice, Not Around it

This is one of the greatest dangers you can confront on an Arctic river. Most of the time, the river will have cleared a path through the ice, often with walls of ice 10 feet high to either side of you. I know of one trip where a boat entered a channel that hit a dead-end— but the boat was able to stop before catastrophe hit. As the group tried to figure out what they would do, they heard a loud boom—and the ice blocking the channel collapsed, damming the channel up instantly. The water began to back up so much that the group actually floated back up the way they had come, until they were able to proceed down another channel.

Flash Floods

Due to aufeis, large areas of ice can collapse and dam up a river’s flow. When that dam finally releases, a catastrophic flood can occur downstream of the aufeis, causing flash floods in camps. That’s why, if you’re camping at the mouth of streams, you have to keep an eye on the water level, and keep your boat tied off well at all times.