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Rivers in Winter |
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Temperatures are frigid and snow is flying-winter is here and in full force! You may wonder where river critters, from flies to fish, go when ice begins to form along the banks of the river. Just as we have ways of adapting to cold weather, such as putting on boots and down jackets, animals have strategies for surviving the cold and ice.
How do plants and animals survive?
The key to winter survival for most aquatic insects and animals is to lower their activity levels to a point where their needs for food and oxygen are greatly reduced and to find a location where they will not freeze. Most inhabitants of a river are cold-blooded; when the water temperature drops, so do their body temperatures. They can live in the coldest water, as long as it doesn't freeze solid. Stream flow at this time of year is at a seasonal low. Parts of the stream will freeze, along the edges and in shallow stretches. So, most aquatic animals move to deeper pools in the river where ice hasn't formed.
Photosynthesis continues to occur in a river during the winter. Although some aquatic plants cease to grow during the cold months, many plant species, such as blue-green algae, are still active and continue to provide a source of oxygen for fish and insects in wintry waters. Most aquatic insects are still in the river, but much less active. Some types of insects spend the winter in the river as nymphs or in a larval case to emerge in the spring as adult flies.
What happens to fish?
Fish "get along just fine in the winter" according to a stream biologist. The cold is a signal for them to leave shallow riffle areas. They over winter in deep holes in the river to avoid being frozen in ice. The metabolism of fish slows down in the winter so they don't need as much food as they do in warmer weather. Their food sources, the insects, are still present. Native fish, such as bass and northern, drop back downstream looking for deep holes in which to over winter. In spring, they migrate back upstream to spawn.
What about frogs and turtles?
You're not likely to find these critters in the winter. That's because they burrow down into the stream bottom and sides, safe from frost and predators like raccoons and weasels. When spring comes with warmer temperatures, these animals emerge from their muddy resting areas to make a living again in the flowing water.
Where does the flowing water come from when everything is frozen?
Most of the water that keeps creeks flowing in the winter comes from groundwater springs. Some water also comes from upstream lakes and wetlands. Groundwater is usually clean, so water quality is generally good over the winter. Problems start in late winter and early spring when snow melt and heavy rains wash over frozen ground.
So, in spite of its icy appearance, rivers continue to support life during this coldest part of the year.
Taken from the UW-Extension Environmental Resources Center publications "River Life in the Winter" #S003 and "Streams and Creeks prepare for winter" #S005. Both are available free, to download visit the ERC publications page.