An Ice Place to Be!

This activity deals with relatively small, freshwater streams and brooks, rather than larger rivers or lakes. Some people think that very little activity occurs in a stream or brook in the winter, and that underwater life is simply waiting for spring thaw. Not at all! In fact, there is a fascinating world under the ice, where creatures and organisms are busily occupied with the dynamic struggle for survival.

Not all freshwater streams or brooks freeze, however. Some remain open because of warmer water from underground springs.

Each underwater habitat (stream or brook) consists of its own specific components (such as rocks or vegetation), along with the particular variety of fish and other creatures that live there.

A riffle is a fast-flowing, shallow stream with a rubble- or boulder-covered bottom. It contains a moderate amount of food for fish, and is a great habitat for young salmon, but a poor one for trout or dace. Adult fish would probably not survive if they tried to overwinter in a riffle.

A pool is generally two to several metres deep, has a sandy or silty bottom and not much current. Though food is limited, a pool’s slow flow helps fish to conserve precious energy because they don’t have to swim as hard as in flowing water. Surface ice usually covers pools early in the winter, but they may also become clogged with frazil ice. This kind of ice is thick, water-soaked slush which limits living space for fish and forces them to use up a lot more energy moving around.
A brook is a small tributary, about two metres across and less than a metre deep. Usually it has a rocky bottom with lots of underwater cover where fish and insects can hide. Frazil ice rarely forms in brooks; rather, brooks generally freeze over. In early winter, however, a brook may collect some anchor ice – ice that forms on rocks in the fast-moving water. Anchor ice, and sometimes the rock to which it is attached, often breaks loose and travels downstream, where it can damage fish spawning beds.

Like all creatures, fish need to conserve energy in the winter. Otherwise they can die. They move around as little as possible and find quiet spots out of currents behind or under rocks or, in the case of larger fish, in deep pools.

Creatures living in winter waters may encounter the following dilemmas:

1. A long, cold winter means that more anchor or frazil ice will form, particularly in pools. The danger is less so in riffles and very little in brooks.
2. A dry winter with average temperatures is a significant threat to fish in brooks, as shallow waters may freeze from top to bottom. This risk is moderate in riffles, and the least in pools where the water is deepest.
3. When fire trucks or snow-making machines pump water from brooks and ponds during a dry winter, shallow waters may also freeze solid.
4. Clear-cutting, farming, or developing land right to the edge of any water will endanger the lives of fish and other creatures in any season. The disturbed soil increases erosion, which can suffocate spawning beds and do other damage.

It’s important to understand the dilemmas that fish face when trying to conserve their strength. When a fish stakes out a territory, it must be defended, and that costs energy. Young salmon also use up energy maturing so they can spawn. This growth depletes precious energy reserves needed to survive oncoming winter.

Researchers have discovered many interesting things while exploring life under the ice. For instance, it is generally thought that frogs bury themselves in the mud at the bottom of ponds, yet they have also been found on the bottom of fast-moving streams and brooks. Some literature even suggests that garter snakes may hibernate in water by taking oxygen in through their skins, like turtles and frogs do.