As the brisk temperatures of fall start to settle in, you might notice your favorite fishing spot or neighborhood lake smells a little funky, almost like rotten eggs.

It's usually not pollution; it's fall turnover — a natural, seasonal mixing of the lake that brings oxygen-deprived bottom water to the surface.

This week's cold front could set it off where you live.

How a lake 'flips'

While you're enjoying the lake all summer, warm water stays on top, while cold, dense water stays on the bottom. They're separated by something called the thermocline, which prevents those warm and cold layers from mixing.

Deep underwater where the sunshine doesn't reach, bacteria breaks down dead plants and other organic materials, and because there isn't much oxygen, hydrogen sulfide is released, which is responsible for that awful smell.

(MORE: What the Super El Niño means for your fall and winter)

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As fall air cools the surface water, it gets heavier and sinks, the layers mix and flip, and the hydrogen sulfide from the bottom level rises to the top and is released into the air.

The weather connection

Cold fronts, chilly rain and the strong winds we get during fall can help trigger this turnover by cooling the lake surface and stirring up the water.

A great example is the powerful cold front that slid into the Central Plains this week and will cause temperatures to tumble this weekend in the Northeast and South.

In the spring, lakes will turn over again, reversing the process after winter ice melts.

(MORE: 6 steps to composting success this fall)

Why it matters

If that fall turnover happens too fast, oxygen can drop low enough to cause a fish kill, according to researchers with the University of Nebraska-Lincoln.

But FISHBIO, a fisheries consulting firm, says most fish and other marine wildlife usually avoid low oxygen zones by moving to shallower waters.

Is the smell harmful?

Turnover odors are usually more of an annoyance than a human health concern, says FISHBIO. Hydrogen sulfide breaks down quickly and the wind disperses it. But they also advise to not drink water that smells like rotten eggs since it could contain harmful bacteria.

So if your lake smells a little off after the next cold or windy night, it might just be resetting for the winter.