Happily, the answer is that the bubbles are almost certainly caused by nature and not by irresponsible humans. The water in lakes and rivers always contains a certain amount of organic matter from algae, aquatic plants, and fallen leaves. When these break down, they release compounds similar to the fatty acids found in soaps, explains Dave Hollinger, a supervisor with the water resources unit at the Ministry of the Environment in Thunder Bay, Ont. Like soap, the compounds act as "surfactants," lessening the surface tension of the water and making it susceptible to foaming. So when wind, wave, and current action agitate the water, a sudsy foam often appears, frequently brown or greenish from the tannins and other compounds in it.