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Lakes

Lakes aren't like a bathtub filled with water that just sits there. They're almost always moving. Lakes "stratify" in the summer. That means a lake forms layers of different temperatures. The sun heats the water on the surface. But, way down deep the water is still cold. The oxygen in the water stays in the warmer layer. Stratified Lake illustration.

In the fall, the warm surface layer cools and sinks. When it sinks, it stirs up all the lake water. The lake becomes cooler. It's called "turning over." Turn overs add oxygen to water at the bottom of the lake.Lake Turnover illustration.

Stream carrying detritusStreams flowing into lakes also add oxygen. Many high mountain lakes depend on streams to get oxygen and fresh cold water. Streams bring in pieces of wood and plants, called - ready for a new word? - "detritus" (d-try-tis). Detritus rots and turns into nutrients for water insects and plants.
Alpine Lake

Fish facts

- Where do you think most fish usually like to be in a lake?

If you guessed along the shore or where a stream flows into it, pat yourself on the back.

- Where do fish go in the winter?

Fish don't hibernate, but they do become less active. They live off the fat reserve they built up during the summer, when meals were easier to come by. In the winter, if there is even a little open water on a lake, that's where the fish gather.

Fish under water

Fisheries biologists work to protect and improve fish habitat.

- They study how using streams may alter fish habitats.
- They measure fish in streams for size, age, and health.
- They count the number and types of fish in a certain area.
- They check on the quality of the fish habitat.

https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5042891.html