Swimming in most lakes in Glacier is not a warm experience

But as you tread water, your feet might feel colder than the rest of you. Why is that?

This example demonstrates something called thermal stratification: different depths in a lake being at different temperatures.

And it makes sense! The sun warms the surface of the lake, while cold, dense water sinks to the bottom.

Take Lake McDonald, the largest lake in the park. A team studying trout in the lake took temperature measurements at different depths each month. The surface got to a balmy 66°F in August, but at depths greater than 100 feet, the water stayed around 38°F year-round

Especially in deep lakes, these layers of temperature can prevent nutrients and oxygen necessary for life from cycling throughout the lake. But twice a year—once around now, and once in the spring—that changes.

In the fall, as the air temperature gets colder and colder, surface water cools too. There's a small window of time that the entire lake—top to bottom—is nearly the same temperature!

When this happens, the lake can "turn over," mixing in oxygen throughout the water column before ice forms and seals it away for the winter. (This also happens in the spring, as the ice melts!)

You can't really see it happen, but in some lakes, you can smell it

Decomposition happening at the bottom of lakes gives off sulfur and decaying matter, which can be brought to the surface—giving off a less-than-pleasant smell.

Luckily that odorous phenomenon isn't too common here in Glacier, but you may have experienced it elsewhere!

You can learn more about temperature stratification and lake turnover from our friends at the USGS here: https://www.usgs.gov/ …/water…/science/temperature-and-water…

This photo of a lake surrounded by yellow larches and snow-covered mountains was taken on the traditional land of the ǔmssk̇ǎaṗiiṗiik̇ǔni, Kootenai, Selis, and Qlispe People in 2019.

(This photo of vibrant larch trees and a lake was taken right next door to us in the @FlatheadNF!)