What Is Stormwater?

Drain with caution sign

Whenever there’s precipitation, there’s stormwater. Stormwater is rain or water from melting snow that falls on or washes over pervious and impervious surfaces. Click here for our infographic.

  • Pervious surfaces allow stormwater to be absorbed by the ground (e.g., forests, grass, gardens and other undeveloped land).
  • Impervious surfaces don’t allow stormwater to be absorbed (e.g., roofs, parking lots, streets, and other hard surfaces).

As we build the homes that we live in; the buildings where we work, shop, and play; and the roads that we drive and bike on, our forests, woodlands, and grassy areas change. So where does all of the stormwater go?

Stormwater runoff is excess water that can’t be absorbed by pervious surfaces, or that flows off impervious areas. This runoff enters Howard County’s stormwater drainage system, where it’s carried to local waterways—and eventually the Chesapeake Bay.

Along the way, stormwater runoff picks up anything in its path—including oil from cars, fertilizers, litter, pet waste, and sediment—and carries it to the streams and rivers in your backyard. These pollutants can harm plant and animal life. In addition, this runoff can cause erosion, flooding, and property damage if not properly managed.

Stormwater runoff is responsible for 20 percent of the Chesapeake Bay’s pollution.

Our History

Click here for the Chesapeake Bay Water Quality Timeline.

Did You Know?


  • Almost 80 percent of rainwater runs off a lawn—it does not soak into the ground.
  • 1 gallon of oil can contaminate 1,000,000 gallons of water.
  • There are 1,000 miles of streams in Howard County.
  • The Patuxent River originates in Howard County.
  • The South Branch of the Patapsco River originates in Howard County.
  • Stormwater causes 20 percent of the pollution to the Chesapeake Bay, and it is the only pollution source that continues to grow.
  • After just one inch of rainfall, the runoff from a one-acre parking lot could fill a backyard swimming pool with 26,000 gallons of water.
  • One inch of rain during a storm generates more than 600 gallons of water running off of a 1,000-square-foot roof.
  • During the summer, outdoor water use amounts to 40 percent of household use.
  • Everyone lives in a watershed.
  • The first inch of rainstorm runoff generally carries 90 percent of the pollution.
  • Each day in Howard County, over 6,200 lbs of dog poop is generated, polluting our streams when it becomes part of stormwater runoff.
  • Yard fertilizer from residential areas is estimated to be responsible for one-third of the excess nitrogen polluting our waterways.
  • Over 1,000 tons of debris is kept out of streams each year by street sweeping in Howard County.
  • In Howard County, there are more than 750 miles of pipes and swales that transport stormwater from roads, rooftops, and driveways to our local streams.
  • 16 times more stormwater runoff is produced by a one-acre parking lot than is produced by a meadow of the same size.
  • Howard County's anticipated permit from MDE will likely require stormwater treatment for 20% of the currently untreated impervious area countywide.

For common definitions and terms please take a look at our glossary page.