Commenting on a recent water quality post, Simon asked a really great question. What causes the foam that we sometimes see in our streams? Is this an indicator of pollution? After tracking our really pure Ravensdale Creek all the way up to Ravensdale Lake, I also saw some indicators of foam in the Creek. How could this be? This is one of the best, least degraded, of any significant stream in our area or other urbanized areas in the PNW. So, I decided to do a bit of research.
The good news - foam is not necessarily an indicator of pollution in our streams and lakes, though it can be. To quote from a Cottonwood Creek TMDL report from Alaska: "foam often is seen accumulating against logs or on the banks of streams, or along the shores of lakes on windy days. The development of foam occurs due to changes in the water surface tension and physical introduction of air. There is a slight tension on the surface of water caused by the chemical attraction among water molecules. This tension is what allows some insects to move along the water surface and what causes water to "bead up" on your car during a rain storm. Certain molecules are called surface active agents or surfactants. Foam is produced as air, introduced in the turbulence of stream riffles .... bubbles to the water surface."
Certainly, there are human sources of surfactant molecules. Household cleaning products such as detergents, shampoos, toothpaste and cosmetics are good examples. But there are also many naturally produced organic surfactants that are released from algae and plants when they die and begin to decompose. These organic surfactants are called DOC's, but we don't need to go there. Human introduced practices that result in even greater plant or algae growth, such as phosphorus in turf fertilizer, serve to "fertilize" such plant growth. But absent these human factors, a certain level of plant growth and decay is natural. For Ravensdale Creek, this is the case and any foam we see is likely quite natural.
So if you see foam, no need to panic. But, we do need to sample the quality of water for confirmation.