Most foam is natural. Foam is created when the surface tension of water is reduced and air is mixed in, causing bubbles. Many natural organic compounds will reduce surface tension, including those from decomposing algae and fish. In a lake, these organic compounds are mixed with air by wind and currents to produce foam. Large quantities of foam are often found on windward shores, coves and in eddies.
Natural foam has a somewhat earthy or fishy aroma.
Detergent foam, in contrast, will have a noticeable perfume smell.