It is usually caused by naturally occurring dissolved organic compounds in the water that act as "surfactants" and reduce the surface tension of the surface film of water. This allows fine bubbles and froth to form, accumulate on the surface, and be moved into calm areas by wind and water currents.

Learn more about anionic surfactants used in detergents here.

Natural or Man-made?
Most of the compounds that lead to foam are fatty acids that come from decomposing plants and animals and are chemically similar to additives in soap products. People often blame shoreline foam on detergents, but they usually don't create long-lasting foam and tend to quickly lose their sudsing ability. Surfactants are the key active ingredient of detergents and increase the wetting and cleaning power of water.

Industrially polluted effluents that caused great masses of foam were much more common in the past before about 1964 when the detergent industry introduced a new surfactant that greatly reduced wastewater foaming. The Clean Water Act of 1972 led to further changes in the formulations of soap and detergent, wastewater treatment and the elimination of most of the worst offenders in point sources. Local sources can still produce excess foam but would likely be diluted relatively quickly.

Extreme case of shoreline foam. image courtesy of Robert Korth, UWEX/UW

A number of environmental agencies report that natural foam usually has an earthy or fishy smell while detergent foam has a perfumy fragrance.

Environmental consequences
The foam itself, if natural, is simply an interesting part of the ecosystem. However, if it is derived from human activities, there may other pollutants associated with it that may affect human and environmental health. If detergent-based, the phosphorus in the product can lead to eutrophication effects which include excess algal and plant growth, higher suspended solids and reduced oxygen levels. It may also be an indication of excess storm water runoff which can contribute a variety of pollutants, including some toxic compounds. Since foam is most likely to be produced during periods of high runoff, the public should treat it with some degree of caution because it may contain disease causing organisms from sewer overflows and stormwater runoff, in addition to chemical contaminants.

This can't be real. . .    images courtesy of The Independent, A Communtity Newspaper in Dundee, Michigan Unbelievable amounts of foam were generated below a dam on the Raisin River in Dundee, Michigan in March 2003. The fluffy stuff piled about 12 feet high from bank to bank. Although the river is known to generate considerable foam in the spring, this year was exceptionally "impressive." The Michigan Department of Environmental Quality believes that a confluence of factors contributed to a very high volume of foaming incidents and complaints in March 2003, not only at this site but in many different warm water river systems. A drier than usual winter and early spring may have contributed to a build up of the natural factors that cause foaming. There was no conclusive evidence linking the suds to sanitary sewer or combined stormwater runoff and there was no correlation at all to the very limited CSO/SSO's this spring around the River Raisin.

Section Acknowledgement:
Gerald Blaha of the Minnesota Pollution Control Agency for making available a list of websites for information relating to sources and consequences of foam.