Ever wonder what happens to stream life during the winter? How do fish survive? Is ice beneficial? Does it protect fish from predators, or trap them?
When water temperatures lower during the winter, the metabolic rates of aquatic life slow down. Therefore, fish and invertebrates don't need to eat much food. On the other hand, it takes longer for them to transform food into energy when it is cold, so they must conserve whatever energy they have. That is why aquatic animals of all kind burrow into the stream bottom, get underneath stones or woody debris in the river, look for groundwater springs where water temperature is a little warmer, or try to stay in pool areas where the water velocity is slower and they don't need to expend energy fighting the current. Studies seem to indicate that bigger fish survive winter the best, and only half the young fish going into their first winter will make it through the season.
Some young fish will work themselves down several inches into the river's substrate, if there are cobbles and small stones free from packed sediment. It must be something like "going back to the womb," for that is exactly where the female trout and salmon lay their eggs, thrashing their tails back and forth repeatedly to first create a depression as a nest and then to cover their redd.
A winter with little or no snow can be disastrous, because the streambed is likely to freeze. Snow, especially when it is 2-3 feet deep, acts as an insulator. Without it, the cobbles and rock on the river bottom act like radiator fins for heat loss. When enough heat is lost so that some water is slightly below freezing, there will be something like a snow storm in the water.
The storm is called "frazil ice" and is really small ice needles and discs forming, floating, and becoming "sticky." As the frazil discs come into contact with other discs and substrate rocks, it sticks and begins to thicken. At that point, usually at night when there is no solar radiation, the thickening ice turns into "anchor ice," meaning the river is freezing from the bottom up. Anchor ice can penetrate the streambed and kill invertebrates and eggs.
When anchor ice freezes in riffle areas where the river is shallowest, it forms ice dams. The dams can block the water of side channels and tributaries, stranding fish. Also, the river can become a series of twisted and sinuous caverns of water flowing through long blocks of ice. Fish can lose their way, and sometimes become trapped in the ice.
During a thaw or during spring ice break up, havoc ensues. The ice breaks, but so does the streambed, which has actually been lifted. The ice gouges and scrapes, taking gravel and cobbles with it and crushing fish and invertebrates.
Sometimes destruction by ice doesn't occur because an air gap gets created between the surface ice and the water surface, but you need a good number of boulders or large stones in the river for this to happen. (The boulders keep the surface ice from collapsing into the water from its own weight.) If the river has significant anchor ice on the streambed, the water level rises. If the river surface freezes during this rise, and if a blanket of snow then acts as enough of an insulator to melt some of the anchor ice on the bottom, the water level goes back down and an air gap is created. This air gap becomes yet another insulator. If you can hear water running beneath the ice cover of a river, that is a definite indicator that there is an air gap.