A watershed is an area of land that drains to a common body of water such as a stream, river or lake. Excluding water taken in by plants, all the rain that falls on Arlington eventually drains into the Potomac River. The 26 square miles of the County are a small part of the 64,000-square-mile Chesapeake Bay watershed.
Stormwater is rainfall that doesn’t soak into the ground. Instead it flows over land into the County’s extensive storm drain network of pipes where it’s released — untreated — into our streams.
Approximately 42 percent of the County’s land is impervious. Impervious surfaces, like roads, sidewalks and buildings, don’t allow water to soak into the ground.
Most buildings and roads in Arlington were built before environmental laws addressing stormwater quality and quantity, such as the Chesapeake Bay Preservation Ordinance and Stormwater Detention Ordinance, took effect. Therefore, most stormwater isn’t controlled or treated before it flows into local streams. However, we’re working hard to install sustainable stormwater management techniques to help our environment.
How does stormwater alter Arlington’s streams?
- Volume: During storms, a significant quantity of stormwater enters our streams, causing erosion that exposes infrastructure and undermines trees that line the streams themselves.
- Erosion: The high volume of water erodes stream banks, compromising trails and trees along our stream valley parks.
- Pollutants: Stormwater washes pollutants like nutrients (nitrogen and phosphorus), sediment, bacteria, petroleum, pet waste and trash into our streams, causing poor water quality.
- Temperature: During the summer months, stormwater heats up as it flows over hot pavement, which then increases the temperature of the stream water by as much as 10 degrees Fahrenheit, causing stress or death to aquatic organisms.
- Groundwater Recharge: During storms, less water is soaking into the ground to our groundwater. Groundwater helps our streams continue to flow, even during drought periods. Without recharging the groundwater, less water is available to maintain our streams and support aquatic organisms.