Imperviousness and Watershed Development

Roads and driveways don’t make very good sponges. Neither do houses, shopping malls, or parking lots. That’s a real problem for rivers and streams. To keep flowing in the hot, dry times of summer, rivers and streams need what is called base flow.

Base flow is water that percolated into the ground when it rained and moved slowly through the soils and wetlands of the watershed during the wetter times of the year, finally reaching the surface again when it encounters a stream or river. Watershed soils and wetlands act like a sponge, soaking up water during heavy rains or snowmelt and slowly releasing the water as base flow, even in drier times. Because this water moves slowly through the soil, rather than quickly running across the surface, it provides continuous flow, even in summer. Base flow is what keeps most of our smaller rivers and streams flowing in summer.

In watersheds where a good deal of the land is developed, and natural vegetation and soils are replaced with pavement and buildings, this natural sponge is diminished. Pavement and buildings are impervious and rainwater can’t reach the soil beneath them, so it runs off, into the gutter and on to the stream, then down to the river and quickly out to sea. As a result, less remains to provide base flow in summer, so rivers, streams, and wetlands dry up more frequently. Even lawns cause a significant increase in runoff when compared to natural plant communities, such as forests.

As the level of development increases, rivers and streams experience higher flows when it rains and lower flows when it doesn’t. When it rains hard in winter and spring, developed watersheds are more likely to experience floods. During the drier months of summer, rivers and streams in developed watersheds experience lower flows and are more likely to dry up.

Although watershed development and the increased imperviousness that accompanies it is a problem, it can be addressed by land use regulations and thoughtful site design.

For more information on imperviousness, stormwater runoff, and what you and your community can do about it, go to the Imperviousness section of our Resource page.