Urban areas are characterized by impervious surfaces such as roads, driveways, sidewalks, and building roofs. Stormwater occurs when precipitation runs off these impervious surfaces. Stormwater can present both water quality and water quantity issues in urban watersheds. Water quality of local waterways is threatened when stormwater carries pollutants to streams and rivers; increased water quantity in these local waterways can damage streambanks, cause flooding, and create more water quality problems.