Stream Foam

Walk along a woodland stream and the occasional sight of a glob of foamy stuff comes into view. So where does this sudsy-looking stuff come from? 

Stream foam is most common during periods of heavy rain and snow melt. Surface water mixes with topsoil rich in organic matter like leaves and twigs. The organic matter contains naturally-occurring chemicals called “surfactants”: materials that reduce surface tension in water. The surfactants become mixed with the water runoff much like the flavor of tea leaves become infused into the water in your teacup. Surfactants act to make water easier to spread or “more wet”. Soap is the most common surfactant we use, acting to ease cleaning operations; it allows water to “stretch”…and to form bubbles.

As the stream’s water flows over branches and rocks it becomes oxygenated, a major source of atmospheric oxygen mixing into our global waters feeding fish. Bubbles are formed and, due to the decreased surface tension from the surfactants, these bubbles become persistent and are somewhat stabilized. The bubbles congregate in areas of the stream called eddies—where the shape of rocks, branches, and other stream structors cause a circular flow of water to persist in a given spot.  

There are, however, some materials like detergents and yes, soaps, that can also cause foam, but these foamy areas will usually be found near houses or drainages areas from parking lots. So if you’re out in the woods and are so inclined, scoop up some stream foam and give it a smell test! It may have an earthy, fresh-cut grass or fishy scent. 

There’s another naturally-occurring foam caused by something different. Found along shorelines and beaches, “foam-lines” are caused by the seawater mixing with decaying matter from certain microscopic creatures called algae. Some of this algae can be harmful and the release of air from the collapsing bubbles from these algae can cause eye irritation and breathing difficulties to those with asthma and respiratory issues. Some sea birds can become affected by the damage to waterproofing oils on their feathers, making flight more difficult and compromising survival.  

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