Environmental Management Division

Watersheds

Follow these links to jump to more information on the page below:

• What is a Watershed?
• Rockville's Watersheds
• Threats to Watersheds
• Stormwater Management in Rockville
• Watershed Management Plans

What is a Watershed?
The U.S. Environmental Protection Agency (EPA) defines a watershed as "the area in which all water, sediments, and dissolved materials flow or drain from the land into a common river, lake, ocean, or other body of water."  A watershed can be very large (e.g. draining thousands of square miles to a major river or lake or the ocean), or very small, such as a 20-acre watershed that drains to a pond. Very large watersheds are typically called "basins", such as the Potomac River Basin, while a smaller watershed that nests inside of a larger watershed is sometimes referred to as a subwatershed.  A typical watershed may have dozens of these smaller subwatersheds.

Rockville's Watersheds
The City of Rockville is located within the headwaters of two watersheds: Cabin John Creek and Watts Branch.  Rockville also contains several major tributaries to a third watershed, Rock Creek, and contributes a negligible drainage area to a fourth watershed, Muddy Branch. All of Rockville's watersheds are part of the greater Potomac River Basin, which itself drains to the Chesapeake Bay.

Rockville's Three Major Watersheds

For more information on a particular watershed, please go to these watershed-specific webpages:
Cabin John Creek Watershed                    Rock Creek Watershed                    Watts Branch Watershed

From a watershed planning perspective, Rockville is very fortunate in its location: because we contain only headwaters and tributaries, all streams flow out of Rockville – nothing flows in.  We thus do not inherit the results of another upstream jurisdiction’s watershed protection efforts (or lack there of)—the City of Rockville alone is responsible for the conditions encountered in our streams and wetlands.  This grants us the somewhat unique opportunity to exert a large degree of control over the environmental health of our watersheds.

Threats to Watersheds
The general outlook for all streams located within urban or urbanizing areas is not good, due largely to the increase in impervious surfaces.  What are impervious surfaces?  Impervious means the condition of preventing or severely inhibiting the downward passage of water.  Impervious surfaces are thus features such as buildings, paving, compacted soil or gravel, or any other surface that prevents rainfall from infiltrating into the ground.  There is a direct correlation between increase in the amount of impervious surfaces in a watershed and decreasing stream health with regards to water quality, stability, habitat availability and biodiversity. 

An increase in the amount of impervious surfaces within a watershed increases both the volume of stormwater and the speed at which that water reaches the stream.   Because impervious surfaces prevent or inhibit the flow of water into the earth, more water is left on the surface as runoff.  Impervious surfaces are typically also much smoother than natural pervious surfaces, and without impedance the runoff quickly flows into storm drains and then into the streams.  The combination of an increased stormwater volume rapidly entering into the stream causes stream erosion as evidenced by channel widening and down-cutting.  This in turn leads to loss of trees, loss of in-stream habitat, damage to infrastructure (particularly sewer mains which may then leak untreated sewage into the stream), and in some cases loss of private property as streams erode into property lines.  Additionally, because impervious surfaces block rainfall from entering into the groundwater table, streams receive minimal groundwater recharge.  This leads to very low water levels between rainfall events, when naturally the water levels would have been maintained by groundwater.

Increased impervious surface area also decreases the water quality of the stormwater reaching the stream.  Runoff from these areas is much more likely to carry pollutants such as oil, grease and grit from roadways, fertilizers, herbicides and pesticides from lawns, and bacteria from pet waste.  The temperature of impervious surface runoff is also generally increased relative to runoff from natural surfaces.  Lastly, increased erosion leads to increased sediment loads in the streams.  All of these confounding factors lead to streams impaired in their ability to support life, and unhealthy conditions for humans and wildlife interacting with the streams.

Stormwater Management in Rockville
Stormwater management (SWM) structures are designed to collect and attenuate rainfall in order to separate pollutants that are collected as rain passes over impervious surfaces and to reduce the velocity of collected runoff to protect receiving streams from erosion.  The City of Rockville began its stormwater management program for new development in 1978.  Today, the City requires all new development to meet State of Maryland guidelines for water quality and quantity treatment.  The City's Stormwater Management law (Chapter 19 of the City Code) and the Department of Public Work's Stormwater Management Regulations describe the requirements for new development, which is administered through the City's development review and permitting process.

To address deficiences in stormwater management and stream protection caused by previous development, the City conducts periodic watershed management plans.  The purpose of these plans is to develop a comprehensive watershed rehabilitation and protection plan, coming up with a list of priority projects for each watershed which then get incorporated into the City's Capital Improvements Program (CIP) for implementation.

Watershed Management Plans
Watershed Management Plans are reviewed and updated approximately every ten years.  The current active plans are:

• Watts Branch Watershed Study and Management Plan adopted in August 2001
• Rock Creek Watershed Management Plan adopted in April 2000
• Cabin John Creek Watershed Management Plan adopted in February 1996

Watershed Management Plans attempt to balance competing interests to achieve a net enviromental benefit for the watershed by considering the following areas: