Stream and river water slowly lose enough heat to the colder air so ice forms in various ways. In small, slow-moving streams, ice often forms a layer on top of the stream just like it forms on top of lakes. The ice covering the stream tends to be thinner than ice found on lakes because of the moving water underneath. And during winter months, stream levels usually drop and the ice may end up covering an air space with the flowing stream below. Because of the air space between ice and flowing water, the ice doesn’t get any thicker.
In larger, faster streams, the first ice to form is along the slow-moving edges. When the river water gets more uniformly colder, frazil ice forms, and it looks like slush in the water. Winter steelhead fishermen sometimes see what looks like blobs of snow floating downstream. This is frazil ice being formed in the moving, super cooled water, probably right before their eyes. This slushlike ice gets thicker and packs together until it collects on the sides, bottom and top surface of the channel. If you are seeing frazil ice forming in your favorite trout stream, it’s time to put the fly rod away for the season.
Newly formed stream ice tends to block off parts of the channel, but water remains free-flowing in some sections all winter long. Since many streams receive their water from lake outflow, like the Kenai River below Kenai Lake or Skilak Lake, much of the water is too warm to immediately form ice. This is why we frequently see open water under the bridge between Kenai Lake and the start of the Kenai River.
Another major source of water for winter streams is groundwater. Groundwater, by definition, has not been exposed to colder air and is considerably warmer than the rest of the water in the stream. So, we often end up with warmer water flowing between and underneath layers of ice. These flowing channel areas and deeper areas of liquid water are the critical wintertime refuge for resident populations of grayling, trout and whitefish.
When exposed sections of stream water gets very close to freezing, such as later in the winter months, another kind of ice may form. Usually at night or early morning when air temperatures are at their coldest, anchor ice can form. This type of ice looks like a clear layer of ice covering the bottom of the stream. The anchor ice layer may temporarily restrict or block off some of the under-the-ice channels that were flowing previously.
The flowing water can’t descend through the sediment and can’t move laterally so it breaks through the ice somewhere and flows on top of the ice cover. This is called river overflow.
Because the water has considerable inertia and gravity moving it along, huge amounts of water can suddenly begin flowing on top of the ice. River overflow can sometimes be measured in depths of feet and can be quite problematic for travelers. One might expect the overflow to immediately freeze, since it is in contact with cold air. However, if there is snow cover on the ice, overflow water can remain liquid for many days, even when the air temperatures are extremely cold.
Some of the most spooky, and also some of the funniest, stories I have ever heard were told by a well-known Iditarod musher when talking about his experiences with river overflow. In some situations he had his entire sled floating and his dogs swimming in overflow when the air temperatures were minus 10.
In many areas of Alaska, wintertime travel routes are on top of frozen rivers. Most of the time the river ice cover is more than thick enough to support the person, dog sled, snowmachine or even a pickup truck. However, because of constantly changing under-ice channels and the dangers of river overflow, traveling on frozen rivers can be very dangerous, too.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.