LAKES OF LAYERS

Why and when do lakes “turn over,” going through the process of mixing several layers of water? Understanding turnover is another tool for the successful angler.



Gale Doudy



Anglers love to talk, even aside from the occasional bragging about the big ones.

They also love to ask questions, and one of the more common questions some angling guides hear is about the phenomenon called “lake turnover,” the seasonal mixing where the top and bottom layers of water switch places.

“I’ve been asked about this more often recently and thought I’d do more research about it,” said Gunnison River guide Gale Doudy, who recently spoke before a packed house at Western Anglers Fly Shop.

He included an article he had written about lake turnover, a topic of interest to cold-water anglers as the spring fishing season approaches.

Most simply, lake turnover is the seasonal movement of water in a lake.

It’s a minor event, or even not occurring, in shallow, warm-water lakes, while larger, deep lakes, such as those on Grand Mesa, can experience major changes as waters of different temperatures mix.

“Not all lakes and reservoirs stratify,” said Lori Martin, aquatics biologist with Colorado Parks and Wildlife. “This depends on the climate of the area and depths of the lakes and reservoirs.

“In milder climates, stratification may not occur, (and) waters may be well mixed year-round.”

Doudy said understanding lake turnover begins with a basic knowledge of how water temperature affects lake stratification.

“The science of limnology tells us water is heaviest (of greatest density) when it is 39.2 degrees (Fahrenheit),” Doudy wrote.  “Colder or warmer than 39.2 degrees, the water is lighter (less dense) and rises to the surface, displacing the heavier water and causing it to sink to the bottom.”

Lakes and reservoirs generally have three layers of water: the epilimnion (lightest, warmest) is on the top; the metalimnion (sometimes called the thermocline); and the hpyolimnion, the heaviest, coldest layer at the bottom.

The mid-temperature thermocline helps prevent the mixing of the other two layers, until those times when the entire lake reaches 39 degrees.

When a lake’s ice cover melts in spring, the surface layer, which never gets below 32, begins to warm and eventually reaches 39 degrees, matching the temperature of the lower layers.

That equilibrium in temperature allows the first big wind to mix the entire lake. This brings the deeper water, carrying a winter’s worth of decayed plant material, to the surface.

Which is why your favorite lake suddenly seems to have so much moss and algae in it each spring and fall.

With the heat of summer, the lake restratifies, and Martin said the depth and thickness of the layers depends on the physical structures of the lake or reservoir.

Doudy said the layers hold varying amounts of oxygen, with the least oxygen being found in the deeper, coldest layer.

“Depending on the size of the lake, the lower level contains insufficient oxygen and plant life,” he wrote.

In the fall, as the surface layer cools below 39 degrees, it becomes more dense and sinks, carrying with it much-needed oxygen to the depths and forcing the hypolimnion to the surface.

As the weather turns colder, the surface water again becomes more dense and once again sinks.

This seasonal mixing of layers, “an act of natural life,” Doudy called it, is what allows many lakes and reservoir to continue to be productive fisheries year after year.


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