Foam on a stream. While it may be a sign of river pollution, foam can often occur naturally. A turbulent stream can cause a foam mat to form from decaying plant material. Natural organic compounds are released reducing the surface tension of the water. With low surface tension, air is mixed into the water and bubbles are formed, leading to foam. Without analysis of the water it is not always possible to be certain of its source. Photographed in Derbyshire, UK.