Rappahannock Friends and Lovers of Our Watershed


What's that foam in the water?

What causes foam to appear on rivers, lakes and streams?

As with most liquids, water molecules are normally attracted to each other. This attraction creates tension at the surface of the water, often referred to as a thin "skin," which allows some insects to glide across it.

  • When leaves, twigs or other organic substances fall into water and begin decaying, they release compounds known as surfacants.
  • This interaction breaks the surface tension, which in turn allows air to more easily mix with water and creates bubbles. These bubbles congregate as natural foam.
  • However, not all foam is natural. Certain man-made products, including detergents, can cause foam that is similar in appearance, but may be harmful to fish and other aquatic life.

When am I most likely to see natural foam on a waterbody?

  • On a windy day, because foam occurs when air mixes with water to form bubbles.
  • During the fall when trees drop their leaves and aquatic plants begin to die back and decompose.
  • Throughout the spring as plants lose their buds.
  • When the outdoor temperature rises, because heat accelerates plant decay, which releases the organic substances that contribute to foam.
  • During soil erosion events or from human activities, such as gravel washing.

Is foam harmful?

  • Foam is usually harmless. In fact, only 1 percent of the foam you see on a waterbody is the actual foaming agent; the rest is air and water.
  • However, excess foam is sometimes the result of too much phosphorus in the water.
  • Although phosphorus in an important plant nutrient, it is not found abundantly in nature and too much of it is indicative of pollution from human activities.
  • Excessive phosphorous can result in nuisance algae blooms, fish kills due to low dissolved oxygen from decomposition processes, and irregularities with the water's taste and odor.

How can I tell what kind of foam it is?

Although it's difficult to know for sure, foam from various sources can have different characteristics.

Natural foam usually:
  • appears as light tan or brown in color, but may be white;
  • smells earthy, fishy or has fresh cut grass odor;
  • can occur over large areas and accumulate in large amounts, especially on windward shores, in coves and eddies; and
  • dissipates fairly quickly, except when agitated (as in high wind conditions).
Unnatural foam from human activity usually:
  • appears white in color;
  • gives off a fragrant, perfumed or soapy odor; and
  • usually occurs over small area, localized near source of discharge.

What should I do if I suspect a waterbody's foam is the result of a chemical release or spill?

If you suspect foam to be from unnatural causes, call the Department of Environmental Quality.  The telephone number for Rappahannock County is 703-583-3864. 

Where can I get more information?

For more information visit the Culpeper Districts website at www.culpeper.vaswcd.org and click on Reporting Pollution on the left sidebar menu.


from Indiana Department of Environmental Management, Office of Water Quality Assessment Branch/Surveys Section, with Virginia information provided by Greg Wichelns.