Most people are surprised to learn that washing a car can pollute their local streams, rivers, lakes, and Puget Sound. Car wash runoff not only contains soap, it's also loaded with the oils, greases, fuel, and road grime that were on the car. In most urban neighborhoods the dirty washwater flows down the driveway, into the street, and within a few feet it drops into a storm drain. After the dirty washwater falls through the slotted storm drain grate, it flows through underground pipes straight to the nearest creek - road runoff does not go to a municipal sewage treatment plant.
The soaps and detergents are harmful to fish and other aquatic organisms (like the insects fish eat) because they contain surfactants. These chemicals are designed to lift and coat the dirt and grime so it doesn't settle back onto your car, but surfactants are also great at coating gills and this prevents fish and aquatic insects from getting the oxygen they need.
The good news is that it is possible to have clean cars and clean streams.