The majority of people now live in cities. These urban environments are characterized by suburbs that contain many acres of hard surfaces. Natural vegetation such as forests and fields slow rainwater down, allowing it to soak into the surface. In contrast, streets, roofs, parking lots and manicured lawns, all provide hard, impervious, surfaces that prohibit rain from soaking into the ground. Since the excess rain water can not soak into the ground, it accumulates and rushes into storm sewers and waterways.