I have so often wondered what causes foam in the creeks, but I always forget to look it up. Today, I was going through some old photos and found this one, which I took specifically to remind me to look it up:
Here’s what I found out:
The surface tension of the water is reduced by a surface active agent, also known as a surfactant. This reduced surface tension causes air to be trapped into tiny bubbles at the surface in areas where there is turbulence in the water thus creating foam. The foam starts out white, but will turn brown as particles of sediment get caught in there, too.
The surfactants can be either man-made or natural. Man-made surfactants include chemicals present in many household cleaning products. According to Jeffrey Davis, an aquatic ecologist:
The most widely used synthetic surfactants today are linear alkylbenzenesulfonates (LAS) listed on most products as sodium or ammonium laureth or lauryl sulfate.
If the foam you find in your creek is localized and sweet-smelling, perhaps someone washed their hair or dishes! If the foam has a fishy or more earthy smell, then suspect a natural cause.
The natural surfactant is called DOC (dissolved organic carbon). DOC comes from the decomposition of a wide variety of plant material including algae and aquatic plants, but also the leaves from trees that line the creek. Foam caused by DOC is often widespread, occuring in more than one spot along the creek. You may see more foam after a wind or rain storm as more organic material is washed into the creek and the water is running faster causing more turbulence.
So there you have it! Now you know. You can be such a smarty pants next time you are hiking with friends and they ask about the foam!