On a hike, you see the impact of ice on the land at nearly every step.
Any resident of Buffalo, New York knows what ice does to the roads. In winter, snow will fall, then melt on a warm day, then freeze again at night. The meltwater seeps into miniscule cracks in pavement, and when this water freezes, it expands, widening the crack. Wash, rinse, repeat, and soon enough you have a huge hole in the road, a pothole.
Ice does the same in the Arctic, but on a vast scale. Like everything else up here, it all comes back to light.
While every place on Earth receives the same number of hours of sunlight and darkness throughout the year, the Arctic and Antarctic get their sunlight at shallower angles compared to everywhere else. As a result, the light that hits the ground is spread out over a larger area, providing less energy to each small spot. This is what causes the Arctic and Antarctic to be colder overall than any other part of the world, which allows for the formation of permafrost.
Permafrost was something I learned about in elementary school, when we learned about rainforests, temperate forests, plains, and other biomes. When we learned about tundra, we learned it had permafrost, and... well, that's about it. I don't even think I had a clear sense of what permafrost was until I arrived in Alaska. I certainly didn't realize how critical it was to the ecology of this area.
In short, permafrost is frozen soil. I believe the official definition states that the soil has to be frozen for two years straight to qualify, but I'm not clear on the practical difference between one year of being frozen versus two or more.
The important point about the soil being frozen, really, is that it's frozen with water ice suspended between the soil particles. So permafrost isn't just cold dirt, it's more like an underground sheet of solid ice, with soil in it.
Now, when winter comes in the Arctic, it snows. Not much, really, but when it snows, the snow sticks around. Temperatures never climb high enough to melt any snow or ice until spring. When spring does arrive, and the sun shines again, water flows. Water will collect in ravines and gulches, running down mountains into creeks and rivers, flooding them for several weeks. Water will also seep into the soil as deep as it can go until it hits... permafrost.
In most parts of the world, water will seep underground for tens of feet, perhaps hundreds, through soil and even certain types of rock, before hitting bedrock and forming a water table deep underground. In the Arctic, water never gets the chance to go deeper than a few feet before running into the ubiquitous permafrost. And because permafrost is made up of so much water ice, water can't seep into it. Liquid water simply has nowhere to go. So it doesn't go anywhere.
In spring and summer, plenty of water is available at the surface in fields, sitting calm and still until it evaporates, and hosting swarms of mosquito larvae in the meantime. More water still has made its way underground, resting on the permafrost layer, in the valleys and the hills. And there, it remains until next winter.
In winter, the cycle continues. As new snow is falling, last year's snow is freezing into place underground. Vast amounts of water, pooled in greater amounts here and there, stuck on the permafrost all summer, begin to freeze, to expand. Pushing down into the existing permafrost isn't an option, so this winter's freeze expands upward, shifting some land straight up into small hills called pingos. On the sides of hills, ice can push soil and rocks outward, causing landslides and rockslides, or less dramatically, mounds of dirt and rock pushed out over the grass and lichen.
On the North Slope, this cycle of freeze and thaw can result in dramatic landscape features known as ice polygons, but I haven't seen these in person yet.
Another result of the freeze-thaw is an utterly unique natural disaster, one that takes decades to fully unfold. It's known as a frozen debris lobe, and it's essentially a landslide that travels somewhere in the order of several feet per year. I'm not clear on the mechanics behind it, but it has to do with frozen soil, the freeze-thaw cycle, and gravity. One of these is heading for the Dalton Highway some ways north of Coldfoot, and will make it impassable within the next few decades. Rumor has it the plan is to reroute the Dalton to avoid this slo-mo debris flow for many more years to come, but not forever. This frozen landslide will still hit the new route, but only after a hundred more years or so.
So ice is really everywhere up here, and it's easy to see when you know what to look for. The Koyukuk River valley itself is evidence of ancient glaciers, rivers of ice thousands of feet thick passing between the mountains of the Brooks Range thousands of years ago.
Thanks for reading.