Have you ever been walking by a river and noticed a strange foam built up where the water is hitting the shore or where a branch is dangling out into the waves? I know I have. Until recently, I always just sadly shook my head and assumed the worst – pollution. I’d walk on thinking of my summer home in McCarthy, Alaska and how I’d never see that in the backcountry of Wrangell-St. Elias National Park. Turns out, I was wrong! If you pay close enough attention, you can find this on many rivers and lakes. But thankfully, that foam is not always a sign of pollution. So, what is it?

Decomposing matter in the water – think twigs, insects, leaves, etc. – create a fatty and oily (lipid) substance that floats on the surface, unseen by us. Because this substance doesn’t mix with water, it stays invisible on the surface until wind or intense churning agitates the lipids and creates foam! You’re more likely to see this foam occurring on rivers and lakes that are at a lower elevation and surrounded by foliage.

Curious if the foam you’ve seen is natural or pollution? There’s an easy way to test it! Scoop up some of that foam and water into a water bottle (a wider mouth bottle will be best). Give it a shake. And voila! You’ll either see the foam breakup and dissipate (natural and harmless) or you’ll see the foam grow (likely caused by pollutants, like a detergent or soap). So next time you’re walking by a body of water and see this foam, try performing your own scientific experiment!

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