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Impervious surfaces are land surfaces such as roads, parking lots and building roofs that repel rainwater and do not permit it to soak into the ground. Instead, it flows quickly into nearby streams, causing unnaturally large and sudden flows that contribute to stream erosion. As rainwater flows across impervious surfaces, it picks up pollutants such as oil and other engine fluids, and carries them into streams and marine areas, where they can harm aquatic plants and animals. Furthermore, when rainwater is diverted into storm drains, it is "wasted" in the sense that it is not available to nourish vegetation or to replenish groundwater. Groundwater is important for sustaining minimum flows in streams, and in some cases, it is a source of drinking water for people.

How do impervious surfaces change the flow of water?

In natural landscapes such as forests and meadows, spongy soil and plant roots enable water to infiltrate the soil. After water seeps into the ground, it may:

• Nourish plants, and cycle back into the air through plant processes
• Continue to flow just beneath the surface to nearby streams (as inter-flow)
• Continue into deeper groundwater

Physical and chemical processes accomplished by microorganisms and plant roots help to filter and purify this water. Large volumes of water are stored in the soil and in wetlands. Sudden rainstorms in natural areas thus cause only a gradual change in the water level of streams. Evaporation of stored water also helps to cool the air in natural areas.

A classic example of an impervious surface is paving, utilized to make roads and parking lots all over the world. Roofing and other building materials are also classically impervious. Routine human use of land can also create impervious surfaces; for example, dirt paths can develop highly compacted soil which is effectively impervious, and mismanagement of farmlands can also create compacted soil conditions.

Impervious surfaces are closely associated with humans, with the percentage of impervious surface expressed as a percentage of total land mass rising dramatically in more heavily settled areas like cities. In a rural community, the coverage might be lower than 10%, while in some cities, it can approach 90%.

In urban landscapes dominated by impervious surfaces, instead of infiltrating, rainwater flows across the impervious surfaces. A brief rainstorm over a large area urban area can cause a great amount of water to suddenly flow into the storm drains and streams. Since no water is stored in impervious surfaces, they quickly dry after a rainstorm. Warming from the sun therefore has a much greater effect; this is one reason why cities are often several degrees warmer than the countryside. 

What effects do impervious surfaces have on our community?

• Impervious surfaces can prevent the natural replenishment of groundwater, an important drinking water source in many areas. Unlike surface water, groundwater moves very slowly and requires decades to centuries or more to replenish depleted reserves.
• The high velocity and high volume of water flowing off impervious surfaces causes greater and more frequent flooding. The high flows also cause destruction of stream channel characteristics (stream side vegetation, pools and meanders) that in a healthy system help to reduce the energy of the water. The loss of these features in turn results in greater flood damage, in a cycle of destruction.
• Erosion from increased stream flows can cause property loss and damage that can amount to millions of dollars. Deposition of sediment downstream of eroded areas also damages property.
• Pollution and alteration of stream flows and channel characteristics can destroy the habitat of commercially important species such as salmon.
• Wetlands filter and purify drinking water. When they are filled in for suburban development or degraded by pollution from impervious surfaces, millions of dollars may be required to install a water treatment plant.
• Installing and upgrading water infrastructure in order to accommodate the large volumes of water that flow off impervious surfaces will cost huge sums of taxpayers' money.

What effects do impervious surfaces have on ecosystems?

• Replenishment of groundwater through infiltration is important for maintaining a base flow in streams throughout the summer. Impervious surfaces prevent much of the rainfall from replenishing the groundwater and this can cause the water table (the level of groundwater) to drop. During dry to normal conditions, a low water table may cause streams to dry up so they can no longer support fish and other aquatic species.
• During storm events, the high velocity and high volume of runoff from impervious surfaces can overcome the capacity of streams. This can cause stream banks to "blow out" and erode the sides and bottom of the channel. Aquatic habitat is lost and the resulting erosion carries sediment (sand, silt, mud) downstream and into our lake. 
• Excessive sediment from erosion can bury the habitat of bottom-dwelling plants and animals.
• Toxic chemicals are often present on impervious surfaces, and are carried directly into streams, wetlands and the ocean. For example: oils and gasoline are leaked from vehicles; heavy metals are deposited from the atmosphere in industrial areas; pesticides and fertilizers are washed out onto streets and sidewalks.

One of the most common issues associated with an impervious surface is flooding. If the water has nowhere to go, water levels can rise radically, even after a small storm. Impervious surfaces also inhibit groundwater recharge, generate a great deal of polluted runoff, and reduce the aeration of the soil. Furthermore, they collect heat, making the surrounding environment much hotter, and they inhibit the growth of trees and plants, which contributes to the development of heat by eliminating shade while also reducing air quality, as trees and plants normally act like giant scrubbers to pull impurities out of the air.

The conditions associated with impervious materials are often likened to those found in a desert. Many environmental agencies have advocated for changes in building policy to address this issue. For example, permeable and semi-permeable paving can be used to allow water to return to the Earth, or floodwaters can be collected in tanks and dispersed in a controlled way. Such measures would benefit the environment in addition to making communities safer by reducing flooding.

What can I do to reduce impervious surfaces?

The options for reducing impervious surfaces range from large-scale planning of new developments, to simple back yard modifications. The principle is to encourage water to soak into the ground so that pollutants are filtered by soil and vegetation, and the volume of water that flows into streams is reduced.

Large-scale planning and development options

• New developments can be "clustered" to reduce the area of impervious surfaces required. By doing so, it is often possible to create or preserve publicly-accessible green space, which may include a natural stream corridor or wetland. "Smart Growth" is an example of this type of planning.
• Existing wetlands can be preserved for their ecological importance and their ability to filter and store water. Constructed wetlands, can be built to mimic natural wetlands, and used to treat the runoff from impervious surfaces.
• Multi-story parking structures or underground parking can be built instead of sprawling one-level parking lots to reduce the area of land required for parking.
• In many places, swales are replacing the traditional concrete curbs and gutters for managing stormwater. Swales are gradually sloping depressions or trenches, often lined with gravel and/or planted with vegetation, that allow rainwater to infiltrate the ground.
• Narrower streets and green "islands" can reduce the paved surface area of streets and increase infiltration of rainwater. They also calm traffic and beautify the neighborhood.
• "Green roofs" (e.g. rooftop gardens) can transform an impervious surface into a beautiful amenity. By allowing rainwater to be partially taken up by plants, they reduce the amount of water that runs off the property. A green roof can also be a relaxation space, a food source, and an energy saver, since the layer of soil helps to insulate the space below.
• Alternative materials can be used in parking lots, driveways, sidewalks and roads to increase infiltration of rainwater.
• When community developments and redevelopments are planned on a "watershed scale," many of these technologies can be combined and huge cost savings in flood damage, water supply and pollution control can be realized.

• Plant a garden of native plants; studies show that lawns, while preferable to pavement, do not allow as much rainwater infiltration as native grasses, shrubs and trees. A "rain garden" can be created in a shallow depression where it will intercept the rainwater runoff from your roof or property. Native plants suited to the amount of water that flows off your property help to take up and filter the runoff, and create wildlife habitat.
• Rain barrels or underground cisterns can be placed under rain gutter downspouts to capture runoff from the roof; this water can then be used to water the lawn and garden.
• Replace asphalt driveways with interlocking pavers, gravel, permeable pavement or a "ribbon" style driveway (two strips of pavement with grass in between). Where possible, limit the length and width of the driveway.
• When designing a new walkway, consider using gravel, mulch, wood chips or stepping stones instead of asphalt. An "S" design can help to direct rainwater off to the sides of the path, instead of "funneling" it to one location.