Protecting Stream Buffers
The land bordering a stream affects its water quality. A vegetated buffer slows down stormwater runoff, filters pollutants, stabilizes stream banks, removes sediment, and removes nitrogen and phosphorous from surface flows and groundwater. In a forested buffer, tree roots remove nitrogen and the tree canopy moderates water temperature, which is critical to fish.
Because of the importance of stream buffers for the Chesapeake Bay's water quality, they are legally protected by Virginia law. Buffers within 100 feet of streams are designated as Resource Protection Areas, and are protected by the Chesapeake Bay Preservation Ordinance.

Before disturbing a stream buffer, check the Fairfax County website to learn what's allowed,
and what activities require approval.
Steps to Restore the Buffer
Don't mow the buffer, especially the 25 feet closest to the stream. The area adjacent to the stream is especially sensitive to potentially harmful activity such as chemical use or excessive removal of vegetation.
Don't remove existing native understory vegetation or trees.
Do remove exotic invasive plants, such as English ivy, that overtop, outcompete, and kill native trees and plants—but removing any vegetation within the buffer area first requires permission from Fairfax County.
Don't install structures or paved surfaces without approval from Fairfax County. This is a violation of the Chesapeake Bay Preservation Ordinance, and you may be required to remove them when the county finds out.
If it is not vegetated, plant the buffer area with native plants, including trees, shrubs, wildflowers, and grasses. The wider the area planted, the better it is for wildlife and water quality. You do not need approval to plant native plants. Here are some useful sources of information:
Plant NOVA Natives has a wealth of information about beautiful native plants for stream buffers, as well as a list of native plant sales and nurseries where you can buy them.
Virginia's Department of Conservation and Recreation's recommended native plants for Virginia riparian buffer zones.
The U.S. Fish and Wildlife Service's Native Plants for Wildlife Habitat and Conservation Landscaping.