Skip Navigation

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:

  • The timing and amount of the runoff

  • The pollutants contained in the runoff 


Graphic of stormwater runoff traveling from the roof of a house across a front yard and into a storm drain.


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 damage 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.


Pollutants contained within the runoff.  Examples include:

  • 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

  • Uncontrolled and polluted stormwater runoff negatively affects the health of all County watersheds!





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

There are many other examples but water that falls on these surfaces is unable to soak into them. 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, similar to a “snowball effect”. 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.


A property suffering damage due to erosion.

Image of a flash flood on the tributary

Erosion caused by stormwater threatens a property.
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.


Graphic from the Maryland Department of the Environment Stormwater Design Manual

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.



Environmental Protection Agency Stormwater Program

Figure 1.1 Maryland Department of the Environment Stormwater Design Manual



Help Prevent Stormwater Runoff

Everyone has a part to play in preventing stormwater runoff. The easiest way to minimize stormwater runoff is to leave land undeveloped, especially near streams. 


  • Build a RainScape on your property. Homeowners, nonprofits, businesses and homeowners associations can install RainScape techniques, including rain gardens, conservation landscaping, rain barrels, green roofs, permeable pavement and dry wells.  The County provides technical assistance and rebates to eligible properties. 
Image of rain barrel.


  • Install and maintain a stormwater facility on your property.   Stormwater management facilities are structures that are used to remove pollutants, prevent stream damage and erosion, prevent flooding, and protect public health. They include large scale infrastructure projects, environmental site design and , underground stormwater storage. (RainScapes are stormwater managment practices as well). 
Image of a Department of Environmental Protection staff member performing maintenance on a stormwater facility.



Image of a stream full of pollution.


Take Advantage of County Resources (and Financial Incentives)

Along with the satisfaction of helping to prevent pollution, erosion and stream damage, there are other benefits to helping to prevent stormwater runoff. 

  • The RainScapes Rewards Rebate Program offers financial incentives in the form of rebates to property owners who install RainScapes. In 2014, residential properties can receive rebates up to $2,500 per parcel and c‚Äčommercial, multi-family and institutional properties could receive $10,000 per parcel.

  • Credit on the Water Quality Protection Charge.  If your property or homeowners association has RainScapes or stormwater management facilities, you may be eligible for a credit off your annual Water Quality Protection Charge.  


How Does the County Manage Stormwater?

The County uses several approaches to best manage stormwater and polluted runoff.  Properly managing this volume and runoff involves a large coordinated effort that includes several agencies. 

The Department of Environmental Protection (DEP) handles many of the programs that assess and address existing impacts from stormwater and water pollution. It is also the lead agency for coordination and reporting related to the County's Stormwater Permit. These programs include:

  • Watershed Restoration: Oversees stormwater management and stream restoration projects,to improve local stream water quality.

  • Stormwater Facility Inspections and Maintenance: Program that inspects, maintains, and improves the County’s current stormwater facilities infrastructure.

  • RainScapes: A rebate program offered to residents and businesses who install environmentally friendly landscaping and small-scale stormwater management practices.

  • Public Outreach and Stewardship Program: Educates various groups in the County about the potential risks related to stormwater and how they can get involved to prevent it.

Image of DEP staff leading a restoration education event at Booze Creek.

  • Environmental Compliance: Water quality and illegal dumping enforcement and illicit discharge detection and elimination.
  • Solid Waste Services: Responsible for a number of programs related to proper trash management and for education and implementation of the reduce, reuse, and recycle programs. 


Along with administering these programs, DEP coordinates with several other County agencies to reduce the impact from stormwater and polluted runoff.


  • Department of General Services (DGS): DGS inspects and reviews stormwater prevention plans on Fleet sites and coordinates all SW prevention on capital improvement projects in the County.  

  • Department of Permitting Services (DPS): DPS handles stormwater permitting, storm drain mapping and the erosion and sediment control program.

  • Department of Transportation (DOT): DOT and DEP are coordinating to add stormwater management for road maintenance and rehabilitation projects.  MCDOT also administers the street sweeping program, cleans storm drain inlets and maintains storm drains.

  • Montgomery County Public Schools (MCPS): MCPS reduces and prevents stormwater pollution through the design, construction and maintenance of stormwater best management on their property. 

Outside of these efforts, there are several other agencies in Montgomery County that administer programs focused on reducing stormwater pollution in our streams.

While these agency’s environmental programs contribute significantly to reducing stormwater pollution in our streams, their protection and ability to thrive ultimately starts with our residents.  Find out how you, your family and neighbors can help reduce stormwater impacts in your community.


Human Health and Drinking Water Concerns

In the more urban and suburban areas of Montgomery County, the water from the faucets in your home or business is piped from a water filtration plant operated by one of the local water providers. The County's public water supply comes from the Potomac River or the Patuxent River. The source water from those rivers goes through a filtration and treatment process to ensure that it meets federal drinking water standards.

Depending on the property location, you may receive water from the Washington Suburban Sanitary Commission, the City of Rockville, or the Town of Poolesville.

In the western and northern areas of the County, many areas are served by private wells. In those areas, it is up to residents to periodically test their drinking water to assure that no contamination is occurring.

The Water Supply and Wastewater section of the DEP website has additional information on how public water and septic systems are managed and how water gets to our homes.