The stormwater issue is not just about the water itself, but what stormwater run-off carries with it. You don’t really see it because we are so used to it, but traffic bearing paved surfaces are filthy. Paved surfaces have a coating of tire residue (including copper and the tire preservative 6PPD that has been proven to decimate salmon in the Northwest), hydrocarbons from our cars, pulverized organic matter, dirt (both fine and coarse), and so on. Just look at your local white shiny crosswalks if you want to see the amount of dirt and residue that covers our city streets. A lot of this stuff is adhered to the surface of the pavement. It is stuck. It tends to become unstuck when it rains.