What is Stormwater?


As the County has become more developed, we’ve replaced our natural landscapes with asphalt, concrete, buildings and roadways. Before development, when it rained or snowed, the resulting water runoff would be naturally absorbed into the soil or flow over the ground to a nearby stream. Development has disrupted this natural cycle of water flow.

During rain and snow storms, runoff can pick up substances like oil, grease, litter, pesticides, or fertilizers and become polluted. The polluted runoff (commonly called stormwater runoff) carries these substances as it flows into our streams, lakes, ponds and rivers making them unhealthy.



Most of the stormwater runoff from roads, driveways, rooftops, and parking lots is not treated before it reaches our waterways. Urban stormwater runoff is the only major source of pollution that is increasing.


There are two major concerns with stormwater runoff:

1. The timing and the amount of the runoff:
  • "Flash floods" are caused by stormwater runoff moving at very high speeds either across the land or through the piped storm drain system after it collects from the surrounding area.

  • Periodic, fast-flowing stormwater damages streams by causing  erosion. Insects, larvae, fish, and amphibians are directly impacted by high volumes of this moving water and indirectly impacted when stream habitat features (such as pools, and riffles) are destroyed.

  • People can also be affected when this fast flowing stormwater damages or destroys property especially homes and buildings.


2. Pollutants contained within the runoff:
  • Sediment from construction sites or exposed soil and from the sides of streams (streambanks)

  • Excess pesticides/fertilizers applied on home landscaping, gardens, nurseries, agricultural fields and golf courses.

  • Trash and debris left on roads or sidewalks

  • Oil, auto fluids, and soapy water from car washing

  • Pet waste that has been left on the ground

  • Salt from winter weather road treatments


Impervious Surfaces

An impervious surface is any surface in which water is unable to penetrate. These hard surfaces interfere with the natural drainage patterns for local waterways. Common surfaces that are considered impervious include:

  • Rooftops – Homes and Buildings (A study found that in typical urban residential areas 30-40% of runoff is derived from rooftops). 1

  • Roads and Driveways

  • Parking lots and sidewalks

  • Patios, baskeball and tennis courts

When there is no place for the water to soak into the ground, the water accumulates and flows across these surfaces with increased momentum. As the water concentrates, it moves faster and faster across the land. Depending on the volume, once this concentrated, fast-moving water flows off of these surfaces or reaches streams, flooding, heavy erosion or other devastating impacts can occur.

Environmental Protection Agency Stormwater Program


A property suffering damage due to erosion.
Erosion caused by stormwater threatens a property.


Image of a flash flood on the tributary
Stormwater flash flooding at the Alta Vista tributary


In an ideal natural environment without development, 10% or less of the water typically runs off the land. In highly developed or impervious areas, over 30% of the water may run off the land.

This increased runoff has led to many land planning and development challenges. Even though Montgomery County has required stormwater management since the 1970's, there are many older developments without it.

Today, stormwater is regulated on the State and Federal government level through the Stormwater Permit program and local governments like Montgomery County face a daunting task in implementing stormwater reduction and improvement programs.



Do You Contribute to Stormwater Runoff?

Note: One inch of rain that falls over 1 square feet of impervious surface creates .6 gallons of water.

So a roof of 2100 square feet would produce 1260 gallons of water per 1 inch storm.

2100 x .6 gallons = 1260 gallons!!!

Find out how you can reduce the runoff on your property .


Figure 1.1 Maryland Department of the Environment Stormwater Design Manual