“Ice happens,” says Rod Wentworth, a fisheries scientist with the Vermont Fish and Wildlife Department. For the animals that live in our area’s streams, the big question is where that ice happens.
Anchor ice is the term for ice that forms at the bottom of a stream or river. It can look like a fuzzy, light green blob oozing over rocks at the bottom of a stream. Or it can be gray and smooth, looking like the ice it is.
Anchor ice usually forms when small disks or spikes of ice develop in or along a stream and are tumbled through the water column by a stream’s turbulence. These tumbling ice bits, called “frazil ice,” have been described as an underwater snowstorm. When frazil ice hits a rock or something else underwater, it may stick.
Frazil ice is especially likely to stick to a rock that is partially out of the water. Imagine a winter night when the air temperature is 10 below. The water in the stream, even if near freezing, is relatively warm. The air-exposed rock becomes cold, much colder than the surrounding water, and the frazil ice sticks, filling in between the cobbles and covering the streambed. At this point, it becomes anchor ice.
Because it forms out of bits of ice, anchor ice can be slushy, even lacy. One trapper described “wading through the soft but slippery anchor ice” on a river bottom while tending his trapline.
Anchor ice is, of course, ice that turns nature’s prime ice rule on its head. Ice floats in water, or at least it is supposed to. Water is most dense at about 39˚F (4˚C). As it gets colder than that and starts to freeze, it expands in volume, and, below 39˚F, cold water floats above warmer water. Normally, ice forms on the surface of rivers, streams, or lakes, where it insulates the water below from the even colder air temperatures above. Snow cover over a mountain brook does the same, Wentworth notes.
For the creatures that live in water and are accustomed to surface ice, anchor ice is bad news. Adaptations for surviving the winter are suddenly ineffective. For example, Wentworth says, brook and brown trout eggs develop in gravelly nests, called redds, over the winter. Normally the bottom of a river or stream is a relatively warm spot, and the eggs get oxygen from water flowing through the gravel. When anchor ice forms, the eggs can suffocate because the ice is blocking the flow of water. They can freeze while encased in the ice. Or, if the anchor ice later becomes more buoyant than the cobble or gravel holding it to the bottom, it can lift the streambed with it as it bobs to the surface and send the eggs crashing downstream.
Other bottom-dwelling creatures, such as caddisflies and stoneflies, experience the same fate. Some fish, such as dace, spend a drowsy winter wedged between rocks. Anchor ice can form around and freeze them.
But anchor ice doesn’t have to actually freeze an animal to ruin its day. Anchor ice can cause problems just by taking up a portion of the water channel, forcing fish into the smaller and smaller unfrozen parts of the stream. The close quarters are stressful, as is a rapidly changing environment that may include ice at night and relatively warm water during the day.
Further, it is possible in cold winters for that otherwise-protective surface ice on small streams to thicken until it reaches the streambed; then the whole stream is frozen solid.
This is why about half of the trout in many area streams usually die each winter, Wentworth says. Our local animals have had millennia to adjust to this, however, so while a particular population may disappear, its kin survive in still-unfrozen water nearby.
A problem, says Wentworth, is that we make the stresses of anchor ice worse by taking water out of streams during the winter. Snowmaking activities and the fluctuations from generating hydroelectric power can periodically drain winter stream levels, he says. Low water levels increase the likelihood that a stream will freeze to the bottom, giving fish and other animals less free-flowing water to escape to, and can even take out all the water from a stream, killing fish, their eggs, or other aquatic life.
The solution, he says, can be found in taking water from snowmaking ponds rather than streams and in limiting how much water is taken out of a stream when flow levels are low. The idea is to avoid turning the winter’s lowest flow day into the level for every winter day.
“Increasing how often these harsh conditions occur is something like being sick more often,” Wentworth says. “You don’t want to make your worst day the everyday norm.”