Urbanization and the Health of Our Streams
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
What happens to precipitation when it falls on the ground? It follows various paths. Some of it soaks into the ground replenishing the groundwater. Some of it goes back into the atmosphere through transpiration by plants and evaporation from the ground surface (evapotranspiration). And finally, some of the precipitation becomes surface runoff and flows into our streams and lakes.
One of the main factors which determines the amounts of evapotranspiration, infiltration, and surface runoff is land use. Land development through urbanization plays a significant role in changing the hydrologic balance in our watersheds. For example, in a woodland where the natural landscape is not disturbed, precipitation turns mainly into infiltration and evapotranspiration. As more and more natural forests and rural farmlands are converted into residential and commercial communities, more trees are removed and more permeable (porous) surfaces are turned into less permeable or impermeable surfaces.
The net outcome of increased imperviousness is a significant increase in surface runoff in terms of rate, volume, and frequency. Streams, the main recipients of stormwater runoff, make room in the channel by eroding the banks and the bed. The result is a wider and deeper channel. As the stream erodes vertically and laterally, sediment is washed downstream where it eventually reaches the Potomac River and then the Chesapeake Bay. Sediment, and the nutrients attached to the sediment particles, impair water quality.
An unstable stream channel also decreases biological productivity and threatens the infrastructure (sidewalks, streets, and buildings) that borders the stream channel.
Stream degradation will continue as long as land development within the watershed continues.