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.


Bio refers to living things. Bioinfiltration uses plants and soil to help rainwater soak in.


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.

Curb and Gutter:

Pathway along streets that directs stormwater to a storm drain and into a pipe to be transported to a stream or river. Curbs and gutters are often referred to as gray infrastructure.


The part of roof gutters that directs water away from a building. These are best directed onto planted areas to reduce the amount of water that runs off a property and to make good use of the rain.

Drinking Water:

Water that has been treated so it is safe to drink, cook with, etc. Facilities, staff, energy, and money are required to treat water. Conserving drinking water helps municipalities save money.


Community of living things (plants, animals, microorganisms) and nonliving things ) (water, air, soil) that interact to create habitats. When water is polluted, the ecosystem is impaired.


The process that moves material, especially soil, from one location to another. It is caused by the action of wind, water, or other forces working on the Earth's surface. Runoff water increases erosion.


Passing a fluid through a medium to separate solids such as when water passes through soil and solids are trapped by soil particles or plant roots. When water runs directly into a storm drain, it is not filtered as it is when allowed to flow through and soak into a planted area first.

Fresh Water:

Water that is not salty. Only about 3% of the Earths' water is fresh water. As Earth's population grows, the demand for fresh water increases.


Water that exists underground in the cracks and spaces in soil, sand, and rock. Groundwater is stored in aquifers. Much of the water used for drinking and irrigation comes from groundwater.

Impervious Surface:

Any surface that water cannot soak into such as streets, sidewalks, driveways, rooftops, and compacted soils. Urban areas have lots of impervious surfaces, so there is more stormwater runoff.


Process of moving water into the soil from the surface. Infiltration is the focus of many new stormwater management practices. These practices are referred to as green infrastructure.

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.

Nonpoint Source Pollution:

Pollution that cannot be easily traced to one source or property because small amounts come from many sources and properties. These numerous small amounts eventually accumulate to become harmful amounts.

Pervious (porous) Surface:

A surface that allows water and air to move into and through it (i.e. soil, porous pavers). A new type of pervious concrete is now being used to pave driveways and parking lots. 

Point Source:

Pollution that can be traced to a specific source or property such as a factory, and oil or chemical spill, or a wastewater treatment plant.

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.

Rainwater Harvesting:

Catching or collecting rainwater in a planted area (rain garden, green roof) or container (rain barrel, cistern) so less runs off. Harvested rainwater is often used instead of drinking water for irrigation.


Primarily eroded soil; also dirt from rooftops or paves surfaces. Sediment is often deposited in water bodies with runoff.

Storm Drain:

Openings built into curbs and streets that are and connected to a pipe to carry away stormwater. As urban areas grow, older storm drain systems can't handle all the runoff during heavier rainstorms and flooding can occur.


Water from rainfall and snow 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. This can include contaminants like soil, pesticides, litter, oil, grass clippings, tree leaves, and bacteria that are collected by stormwater flowing over a surface and then carried into surface water.

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. The more imperious surfaces there are, the more runoff there is and the faster it moves. Fast moving water can collect more pollutants and cause more erosion of soil and streambanks.

Surface Water:

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

Wastewater Treatment (Sewage) Plant:

Municipal plant the treats wastewater (water from toilets, washing machines, dishwashers, showers, etc.) before it is released into a stream or river. In most communities, including Bothell, stormwater is not treated before it is released into a stream or river.

Water Cycle:

The continuous movement of water through its liquid, gas, and solid phases above, on and below the Earth. The water on Earth today is the same water as when Earth was formed. New water is not added to the Earth when it rains.


The entire land area from which water drains into a particular surface water body such as a lake, stream, or river. We all live in a watershed, and we all live upstream or downstream from other watersheds. Learn more about watersheds.