A very good question…
Foam forms when gases are trapped in water as bubbles and these bubbles are stabilized (do not readily burst).
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. The surfacants break the surface tension, which in turn allows air to more easily mix with water and creates bubbles that congregate as natural foam.
Only 1 percent of the foam you see on a water body is the actual foaming agent; the rest is air and water.
Trent Severn Water Way Lock 4 – Batawa
Fine particles of sediment may attach themselves to the foam bubbles. While this gives the foam a dirty appearance, naturally occurring foam is usually harmless.
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. Some man-made products, including detergents, can cause foam that is similar in appearance, but may be harmful to fish and other aquatic life.
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, and
- can occur over large areas and accumulate in large amounts, especially on windward shores, in coves and eddies.
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.