What’s Happening Under the Ice?

River during winter, with frozen banks covered in snow.

Water is Amazing

Winter is a magical time to experience nature in our parks. As you explore, you will find it is unavoidable to stop and wonder about our amazing environment around us. Have you ever wondered how a stream continues to flow all winter?

While you might see a layer of ice covering the stream when its cold outside, you will notice that the streams don’t freeze solid. Part of this reason has to do with the exchange between surface water in streams and groundwater, which is water stored beneath the earth’s surface.

In summer, when groundwater enters a stream, it helps keep streams cool, buffering the water from the impact of heat in the air. In winter, groundwater is warmer than surface water and helps prevent streams from freezing solid. In the Credit River Watershed, groundwater temperatures remain consistent throughout the year, on average four to ten degrees Celsius.

But let’s face it: streams in the winter are still downright cold, even with groundwater doing its part. So how do fish and other creatures living in streams manage when temperatures drop? Under the ice, life moves slowly but the action doesn’t stop altogether.

Aquatic Invertebrates

Aquatic invertebrates are small creatures without a backbone that live on a stream bottom. This group includes insects, snails, worms and crustaceans and each has their own way of adapting to the cold.

A small insect sitting on snow.
A winter snowfly. Photo credit: Chantalle Jacob-Okorn

Snails, worms and some species of crayfish burrow into the mud or gravel to keep from getting too cold. Most aquatic insects, like mayflies and dragonflies, spend the winter in the water as an egg or immature larva or nymph. When the weather warms up in spring, they transform into a land-loving flying adult and find a mate. However, winter stoneflies do the opposite. They wait until winter to emerge from the water and transform from nymph to adult. To cope with the cold, these insects produce an anti-freeze in their body and take refuge under the snow or rocks where the air is warmer.


Like many invertebrates, fish are known to burrow into the streambed during winter, searching out crevices between rocks and gravel to rest until spring. Between the rocks, water can freely move and continue to deliver oxygen to burrowing fish like blacknose dace, one of the most common minnows in the Credit River. Other fish, like brook trout, seek out warmer places within the stream like areas with groundwater upwellings. It’s in these places that streams get a boost in temperature as groundwater bubbles up from the stream bottom during the colder months.

A fish in container seen through a clear wall.
A blacknose dace.

In winter, it’s simply too cold for fish to waste their energy on fighting over territory or even feasting. By slowing their metabolism, they don’t need to eat much. Fish typically lose weight during this time as they wait for warmer temperatures to search out food.

Through CVC’s Integrated Watershed Monitoring Program (IWMP), we study conditions and trends in groundwater, stream, forest and wetland ecosystems across the Credit River Watershed. Explore stories and interactive maps about IWMP on our StoryMap Collection.

Do you have questions about life under the ice? Share them with us on Facebook, Twitter and LinkedIn.

By: Adrienne Ockenden, Watershed Monitoring Specialist

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