Impervious surfaces are any materials that do not allow water to flow through them. Examples include concrete, asphalt, and roofs.

Under natural conditions, land is covered with trees, grasses, and other plants. When it rains, most of the rainwater soaks into the soil or is taken up by plants. Relatively little of the rain flows over the surface. Rainwater mostly enters lakes and streams by moving laterally (sideways) through the soil. Moisture moves much slower through soil than it does on the surface, so stream levels would rise and fall gradually after it rains.

Urban development changed the natural water cycle. People removed vegetation to build homes, businesses, and roads. These surfaces (roofs, pavement, etc.) are “impervious,” meaning rainwater cannot soak through them. If the rainwater has nowhere to go, it will cause flooding. To prevent flooding, cities built storm sewer systems. These systems moved stormwater away from cities as quickly as possible through a system of storm drains and pipes. However, this “solution” caused other environmental problems, such as erosion and pollutants picked up by stormwater runoff and carried to rivers and lakes.

Resources:

• Impervious Surfaces and Flooding: USGS Water School resource about the impacts of impervious surfaces
• Causal Analysis/Diagnosis Decision Information System (CADDIS): EPA tool to help conduct causal assessments in aquatic systems
• Runoff: Surface and Overland Water Runoff: USGS Water School resource about the runoff
• The Prevalence and Cost of Urban Flooding: An analysis that combines insurance and FEMA claims data, property owner surveys, and GIS mapping of flooding in an urban environment.
• Polluted Runoff: Nonpoint Source (NPS) Pollution: An EPA collection of resources related to NPS and actions to reduce NPS
• Permeable Pavers: An overview of alternative paving materials

The environmental effects of impervious surfaces can be addressed by either reducing the amount of impervious surface (replacing pavement with native vegetation or permeable pavement) or capturing rainwater so it doesn't flow down the storm drain (installing a rain barrel and/or rain garden).

• Install a rain garden
• Install a rain barrel
• Plant native plants