Ask Sam: What Makes That Weird Foam In Rivers?

May 3, 2019

Looks delicious, am I right?
Credit Pixabay | jochemy

Each Friday on Morning Edition, Outside/In host Sam Evans-Brown answers a question from a listener about some quirk of the world around us. (Do you have a question for Sam? Submit it here!)  

This week, Brendan in Alton asks: "I’m calling from the bank of the Merrymeeting River. Every year around the time that we get some heavy spring rains, we always see this foamy, cream-colored fluff floating down the river. And we were wondering: is that good? Is that bad? Is it edible? It kinda looks like the stuff on the top of a root beer float.”

Let’s just get to the important public service announcement straight away: Don’t eat the foam, Brendan.

If the sight of dirty river foam has you salivating, perhaps you could swing over to Shibley’s or JP China and get something in your stomach... they’re right around the corner.

Now, about that foam.

“It’s not bad,” explains Michele Tremblay, President of the New Hampshire Rivers Council, “unless you’re seeing discharge from a pipe nearby that might be from a dishwasher or a washing machine, this is completely natural.”  

If you’re wondering if your foam is natural or unnatural, try giving it a sniff.

“I have a study site downstream of a car wash that sometimes smells like soap and has bright white, soapy smelling foam,” writes Emily Bernhardt, who teaches biology at Duke University, in an email, “but if your foam is sort of creamy and smells like fish, compost or cut grass—that's a natural foam. It’s not harmful, it’s just some cool chemistry mixed with a bit of turbulence.”

What cool chemistry? Are you ready for a new word? Surfactant. A surfactant is anything that reduces the surface tension of water, and probably the most common example is soap. While this may seem counter-intuitive, it’s actually reduced surface tension that makes stable bubbles possible.

“The foam that accumulates in the streams sometimes is caused by dissolved organic matter,” says Scott Bailey, a geologist up at the Hubbard Brook Experimental Forest. “So you can think of it as kind of like tea. You steep some tea leaves in water you end up with a colored substance that’s got materials that have leached out of those tea leaves, and the same thing happens in our forests.”

You see this in spring (as Brendan rightly observes) because it’s raining and the rivers are high. More rain means more water passing through the ground, more bits of organic matter getting dissolved and swept along as forest tea, and more turbulence mixing that water up and making bubbles.

“And actually we do see this later in the season and I have seen it in the fall as well,” says Tremblay, “So I think we are seeing the result of a lot of rain.”

Okay, that was fun, but can we talk about bubbles now?

Now, can I bring this back to something I pointed out earlier: the fact that the reason that soap and forest tea helps the formation of bubbles is because they *decrease* surface tension?

The reason for this is because of something called the Marangoni effect. You might have seen this demonstrated in middle school science class with a bowl of water sprinkled with pepper and a drop of soap.

The reason that happens is that the soap creates a spot with lower surface tension, and the water flows away from that spot. For complicated physics reasons that I only half understand, this same effect stabilizes bubbles and makes foam possible.

So, I’ve become convinced that the existence of bubbles is a MIRACLE of physics, and we should be thankful everyday.

Oh and also don’t eat the foam. Did we say that yet? Yes, we did. Who cares let’s say it again. Also, don’t drink untreated river water, and don’t eat the foam. Yeesh, Brendan.

Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to, OR call the hotline, 1-844-GO-OTTER.