Although areas of red slime and oil-like films may look like pollution, they are caused by a natural bacteria reacting to the presence of iron and manganese in oxygen-deprived groundwater discharge to the stream. A red slime indicates where oxidizing bacteria have fixed oxygen onto iron, resulting in a red flocculant (floc) of oxidized iron. The oily sheen is a thin layer of oxidized manganese. These substances may be extensive in areas of shallow, slow-moving stream. Oily-looking films are not difficult to discern from petroleum products. The way to tell the difference between a bacterial film and oil floating on the water is to break the film. If the sheen breaks into pieces, it is bacteria, if the sheen does not break, it is petroleum. These bacteria are not human health hazards or water quality concerns.

Foam can be caused by natural sources, wastewater, chemical spills, or detergents. Natural foam is no cause for concern as it is caused by decaying leaves and organic materials. These materials release organic substances that produce foam when mixed with air. Natural foam can accumulate in areas of slow moving water and behind obstructions. Natural foam can be white, tan, or brown with an earthy smell. Detergent foam is a bright, clean white with a soapy smell.