Following several days of modest rain there was water once again flowing down the hollows into Bull Creek. These are ephemeral streams, flowing only after a rain, then holding pockets of water for weeks until they gradually dry out. Foam appearing in pockets of turbulence always raises the fear of pollution. So what else could cause foam?
Actually foam like this is a natural phenomenon. Like the suds in a bath tub, it is caused by a surfactant that lowers the surface tension of water. Soap is a surfactant made from oil and lye (sodium hydroxide). This creates a molecule that is water soluble on one end and not on the other, creating a thin layer of lipids on the surface. Think of the solution that kids blow bubbles with.
In nature there are natural surfactants called DOC (dissolved organic carbons). These are formed by the decomposition of algae, aquatic plants, dead leaves and animals that produce oil in the form of simple fats. As they break down over time they produce fatty acids that are lighter than water and won't mix but lay on top in a thin layer. In quiet waters this can appear to be a thin layer of oil. Turbulent water flow mixes air in and viola!...bubbles.
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| Foam at the falls on Bull Creek |
There
is frequently a collection of bubbles in an eddy below a little water
fall at our swimming hole. Occasionally this collection will be quite
large and can collect little particles on top. Some biologists use
this as a means of looking for exuvia (molted insect skin), aquatic
fungi, moss spores, etc.Of course, foam can also come from pollution so if in doubt, it pays to check. More details are here.
Good Read:
Insect brains- bigger than you think.