Stormwater Glossary
Use our glossary to define words like impervious cover, TMDL, and more
A
Atmospheric deposition
The process whereby airborne pollution (e.g. vehicle exhaust, power plant emissions) moves from the atmosphere to the earth’s surface in the form of wet deposition (rain, snow, fog) or dry deposition (dust, gas), eventually making its way into stormwater runoff. This is a major source of nitrogen pollution to the Chesapeake Bay.
B
Base flow
The normal, consistent flow of a stream or creek, fed by groundwater between rain events. Restoration to a baseflow involves techniques used to return a stream to its natural, steady flow (usually through reconnection to groundwater or floodplain).
Berm
A raised mound or ridge of compacted soil used to redirect or contain stormwater, often used in erosion control.
Best Management Practices (BMPs)
Engineered or natural practices used to treat, infiltrate, and reduce stormwater runoff and pollution.
Bioretention (rain garden)
An engineered soil practice of a shallow, landscaped depression that filters stormwater through soil media to remove nutrients, sediment, and other pollutants.
Bioswale
An engineered shallow, vegetated channel that moves water slowly across the land, allowing it to soak into the ground.
C
Check dam
A small barrier built across a ditch or swale to slow water flow and reduce erosion.
Conveyance
Structures or landforms (pipes, channels, culverts) that move stormwater from one place to another.
D
Daylighting
The process of uncovering a previously buried stream, river, or creek and restoring it to a more natural, open flow above ground.
Design Storm
A specific rainfall event (e.g., 1-inch, 24-hour storm) used to size stormwater practices in accordance with state or local regulations.
E
Embankment
An earthen berm, built to contain, slow, and filter rainwater, often as part of stormwater ponds or dams and designed to gradually release water through controlled outlets. These require specific vegetation and maintenance as part of a drainage system.
Environmental Site Design (ESD)
A planning approach that integrates natural hydrology and green infrastructure into development to mimic pre-development conditions.
Estuaries
Coastal water bodies where freshwater mixes with saltwater and supports rich ecosystems.
F
Flood maps
Areas identified as being susceptible to flooding during high‑water events (e.g., FEMA flood maps).
Floodplains
Flat land alongside a river or stream that naturally absorbs floodwaters during heavy rain.
Floodway
The area of a river or stream that carries the majority of floodwaters during high-flow events.
Forest cover
The amount or percentage of a land area covered by trees; this cover supports high infiltration and reduces runoff.
G
Gravel lens system
A layer of gravel installed in a pond or infiltration system to filter and cool stormwater as it moves underground.
Green gentrification
The process where environmental improvements (like green stormwater infrastructure) can inadvertently raise property values and displace long‑time or lower‑income residents.
Green Stormwater Infrastructure (GSI)
Stormwater management practices that use natural systems (plants, soils, wetlands) to manage and clean runoff while providing community and ecosystem benefits.
Grey infrastructure
Traditional engineered systems for water management, like concrete pipes, culverts, and treatment plants, rather than natural systems.
H
Headwater stream
A small, first- to third-order stream at the uppermost part of a watershed. These streams make up the majority of rivers in the U.S. and often have low flow, typically around one cubic foot per second on average.
Herbaceous plugs
Small, young plants (usually native grasses or wildflowers) used to quickly establish vegetation in restoration projects.
I
Illicit discharge detection and elimination (IDDE)
Illicit discharge refers to any non‑stormwater discharge (e.g. sewage, wash water) that enters into storm drains. IDDE programs are part of the MS4 permits focused on identifying, tracking, and removing these discharges.
Impervious surface
Hard, non-vegetated areas such as concrete or asphalt that don’t allow rainwater to soak into the ground, causing runoff.
In-stream habitat
Features in a stream (like logs, rocks, or plants) that support fish, insects, and other aquatic life.
Incised stream bed
A stream that has cut down far below its natural level due to erosion, often needing restoration.
Industrial permits
Permits that regulate stormwater discharges from industrial sites to minimize pollution.
Infiltration Rate
The speed at which water enters the soil, typically measured in inches per hour; a key design parameter for infiltration-based BMPs.
L
Levee
A long earthen embankment built alongside rivers or streams to prevent flooding of adjacent land.
Low Impact Development (LID)
A site-design approach that minimizes disturbance and manages stormwater close to its source using infiltration, evapotranspiration, and reuse.
M
Minimum control measure (MCM)
Required MS4 program elements (like public education or erosion control) that address stormwater pollution.
Municipal Separate Storm Sewer System (MS4)
A publicly owned system of storm drains, pipes, and ditches that collects and conveys stormwater to local waters; regulated under federal stormwater permits.
N
National Pollutant Discharge Elimination System (NPDES)
Federal permits under the Clean Water Act that regulate stormwater and wastewater discharges to protect water quality.
O
Organic detritus
Pollen, leaves, and plant clippings that are nutrient-rich, can clog storm drains and add nutrients to surface water.
P
Pervious surfaces
Ground surfaces (such as grasses, permeable pavers, vegetated areas) that allow water infiltration.
Pervious pavement
Special paving stones with gaps or permeable surfaces that allow water to soak through rather than run off.
Phase I community
A community (such as a city or county) regulated under a Phase I Municipal Separate Storm Sewer System (MS4) permit, typically serving a population greater than 100,000, with more extensive stormwater monitoring and reporting requirements than smaller MS4s.
Phase II community
A community regulated under a Phase II MS4 permit, generally serving a smaller population (under 100,000). Phase II permits require implementation of minimum stormwater management measures but typically have fewer monitoring and reporting requirements than Phase I permits.
Point and Nonpoint Sources
Point sources are single, identifiable discharge locations (e.g. pipes from a factory), whereas nonpoint sources are diffuse and come from larger land areas (e.g. agricultural runoff, urban stormwater), making them more difficult to track and manage.
R
Regenerative Stream Channel (RSC)
A stormwater restoration technique that rebuilds eroded streams with pools, sand, wood, and vegetation to mimic natural systems.
Reservoir
A large artificial or natural water body used to store water for drinking, irrigation, or stormwater control.
Retrofit
Upgrading an existing stormwater facility or landscape to meet new environmental standards or improve performance.
Riffle (grade control)
A shallow section of a stream with fast-moving water over gravel or rock, used to stabilize stream beds and oxygenate water.
Riparian buffer
A vegetated zone, often forested, along the edge of a stream or river that protects water quality by filtering runoff and stabilizing banks.
S
Sand-bedded pools
Shallow basins with sand bottoms used in stream restoration to slow water and trap pollutants.
Step pool stormwater conveyance system (SPSC)
A series of small pools and riffles created along a steep stream to reduce erosion, slow water, and improve aquatic habitat.
Stormwater Control Measure (SCM)
A term used interchangeably with “BMP,” emphasizing engineered practices that manage stormwater quantity and quality.
Stormwater outfall
The endpoint where stormwater drains into a stream, river, or pond—usually through a pipe or culvert.
Stormwater Pollution Prevention Plan (SWPPP)
A plan developed to reduce pollutants from many construction sites, industrial facilities, and municipal stormwater systems. Most plans include sources, best practices (silt fences, covered areas, seeding on bare soil), inspection and maintenance plans, spill response procedures, and training and responsibilities.
Stream corridor
The stream channel and adjacent land that together support water flow, wildlife habitat, and floodwater movement.
Stream grading
Shaping the stream bed and banks to control water flow, prevent erosion, and improve habitat.
Stream gully
A deeply eroded channel formed by water flowing with enough force to cut into the soil.
Stream network
The interconnected system of streams, rivers, and wetlands within a watershed that carries water through heavily populated areas.
Stream order
A way of ranking streams based on their size and hierarchy. When two first-order streams join, they form a second-order stream; when two second-order streams join, they form a third-order stream, and so on.
Stream restoration
The process of repairing a degraded stream to improve its ecological health, stability, and natural flow.
Street dirt
Trash, sediment, pollen, and leaf litter that accumulate on streets and can enter stormwater systems.
Surface runoff
Rainwater that flows across land into storm drains and waterways instead of infiltrating into the ground.
T
Total Maximum Daily Load (TMDL)
A regulatory plan that sets the maximum amount of a pollutant a waterbody can receive and still meet applicable water quality standards.
Turf cover
Grass‑dominated vegetated land cover that can slow runoff and support some infiltration.
U
Underdrain
A buried, perforated pipe that collects and conveys water from stormwater systems such as rain gardens or bioswales, acting as a backup drainage system for when the soil cannot reliably absorb and remove stormwater on its own, such as during heavy precipitation events.
W
Watershed
An area or ridge of land that separates waters flowing to different bodies of water, and channels precipitation into streams, rivers, reservoirs, bays, or the ocean.
Watershed Implementation Plan (WIP)
State-developed plans describing how pollution reductions required by the Chesapeake Bay TMDL will be achieved.
Wattle
A bundle of sticks or branches woven into fencing and placed on slopes or streams to slow erosion and hold soil.
Weir
A low barrier in a drainage system that controls the flow of water by forcing it over its crest, used for flood control and to help manage heavy rainfall runoff.