Only Rain down the Drain!

Leaving grass clippings on the lawn when you mow is a great way to recycle their nutrients and doesn't contribute to thatch formation. But clippings that land on driveways, streets, and sidewalks as you mow can get washed into storm drains or surface waters, where their nutrients act as pollutants that fuel the growth of algae and degrade water quality. Orient your mower so that clippings are ejected back onto the lawn as you cut the grass. If any clippings land on paved surfaces, be sure to sweep them back onto the lawn or gather them up and add them to your compost pile. Remember -- only rain down the (storm) drain!

Read more from the national gardening association.