Here's an experiment, borrowed from the United States Environmental Protection Agency's website, that helps demonstrate how detergents and other soaps affect our streams.
Sometimes detergents and other soaps get into our streams and lakes. This happens when people wash cars in their driveways and allow the dirty wash water to go down a storm drain. The dirty water travels through the storm pipes, untreated, and ends up in the nearest stream or lake. Once there, the detergents and soaps can do some real damage to the water's surface habitat.