Understanding Stormwater Terminology


A layer of sand, gravel and clay below the earth’s surface with enough water for people to withdraw for use (through wells and springs). Rainwater soaks into the ground and fills aquifers.

Best Management Practices (BMPs):

A Best Management Practice is a behavior or action that a person performs that protects the health of the environment. Learn about simple BMPs you can do every day.


A shallow ditch with gently sloping sides and various layers of soils beneath intended to slow stormwater runoff and direct it to an area where it can soak in. Learn more about bioswales.


Water that exists underground in the cracks and spaces in soil, sand, and rock. Groundwater is stored in aquifers.

Illicit Discharge:

Any discharge to a municipal separate storm sewer system (MS4) that is not composed entirely of stormwater, with some exceptions. Learn about examples of illicit discharges and what to do when you see one.

Illicit Discharge Detection and Elimination (IDDE):

 A program whose purpose is to find, fix and prevent illicit discharges through a series of techniques and awareness campaigns.

Low Impact Development (LID):

 Techniques and design considerations that help manage the rainwater that falls on your property by allowing some to evaporate back into the air, some to absorb into the ground, some to be captured and used later as needed, and the rest to slowly pass into the stormwater system and into nearby streams. Learn about low impact development techniques in Bothell.

Municipal Separate Storm Sewer System (MS4):

A storm system that flows through its own set of pipes rather than being combined with the sewer system. Bothell has its own storm system which does not receive treatment before it discharges into streams, lakes, and rivers.  

National Pollutant Discharge Elimination System (NPDES):

A permit program, created in 1972 by the Clean Water Act (CWA), that helps address water pollution by regulating point sources that discharge pollutants to waters of the United States. Learn more about the NPDES permit program.

Rain Garden:

 A bowl-shaped shallow planted area in the landscape where rain water collects and absorbs back into the soil. Learn more about rain gardens.


Water from rainfall that flows into surface water including drainage facilities, rivers, streams, lakes, or Puget Sound.

Stormwater Engineer:

A person who designs solutions for problems created by surface water runoff and pollution.

Stormwater Pollution:

Anything in our stormwater that makes it unclean.

Stormwater Runoff:

Rain that falls on streets, parking areas, sports fields, gravel lots, lawns, rooftops or other developed land and flows directly into nearby creeks, lakes, rivers, and Puget Sound. This runoff carries pollutants to these waterways.

Surface Water:

Water found above the land, including oceans, estuaries, lakes, rivers, streams, and ponds.


The entire land area from which water drains into a particular surface water body such as a lake, stream, or river. Learn more about watersheds.