Impervious Surface

An impervious surface is a hard area (such as a sidewalk, road or roof) that doesn’t allow water to seep into the ground. Instead, the water runs off the impervious surface, picking up many types of pollution in the process, and then flows into a storm drain or a nearby body of water.

How Impervious Areas Affect the Environment
Large amounts of impervious surfaces change the water cycle. The graphic below shows how impervious surfaces change the way rain moves through the water cycle. This and more information is available from the Federal Interagency Stream Corridor Restoration Working Group.
In a place with little impervious area, such as a forest, rain water is able to seep into the ground. It will then travel slowly through the ground, where it is filtered by natural processes, before reaching streams and creeks. This process helps keep the amount of water in streams and creeks from changing too much or too quickly. Research has shown that as the amount of impervious surface in an area increases, the amount of polluted runoff also increases. Since water cannot seep into the ground in areas with large amounts of impervious surface, more than 5 times as much water can quickly run off the land into nearby water bodies.
Illustration of Impervious Surfaces
Impact of Added Water
Since more stormwater is quickly reaching streams and rivers, there is an increased risk of floods occurring. The large amount of water also leads to streams flowing faster. This causes erosion and changes the shape of the stream. Both of these can be a serious problem as they lead to worse flooding and serious damage to wildlife habitats. Another problem associated with these changes is that there is less moisture in the ground because impervious surfaces do not allow water to filter into the soil. This means that plants (such as grass in lawns and ornamental plants) may die or need extra water from irrigation to survive.

Studies have shown that local water bodies are less healthy when as little as 10% of an area is covered in impervious surfaces. If more than 30% of an area is covered in impervious surfaces this can severely damage nearby streams, rivers, and lakes.