What is the foam gathering around my dock?

While up north this fall I noticed a large area by my dock that was foaming. What is this?
—Leslie Diamond, via e-mail

Don’t worry, this froth is very likely natural. (The other explanations—the bubbles came from pollution; the bubbles came from your fish, because they threw a foam party—are improbable.) Lake and river water contains organic matter, and when that matter breaks down, it releases compounds like the fatty acids in soap. These compounds lower the surface tension of the water, and make it susceptible to foaming when wind, waves, or currents stir it up. Sometimes the foam piles up against docks, logs, or other floating objects. It’s not harmful, but it may look brown or green, and it probably smells earthy or fishy.