Every year while working along the creek, I notice foam on the creek surface. I’ve always wondered what causes this, and I worry that it is caused by pollution.
Foam in creeks is normally caused when the surface tension of the water is reduced and air is mixed into the water causes bubbles to form. This can be caused by human pollution, but it is normally a natural phenomenon. When organic matter such as leaves and sticks get in the creek and start decaying, they break down and release organic compounds that act as surfactants that reduce the surface tension of the water. These organic compounds include fatty acids and oils that are similar in chemical composition to soaps. When these oils are pushed against downed trees and the movement of the creek, waves and turbulence introduce air into the water, creating the foam.
The foam is either white or tan in color. Usually along Wilson Creek, the foam is brownish in appearance, since tannin is introduced into the water when wood decays. Foam in creeks is more common on fall and spring mornings. When the foam is caused by human pollution and is not a natural phenomenon, it is usually bright white in appearance, has a chemical odor, and normally slimy to the touch.