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Stormwater and Watersheds

Stormwater Treatment Pond near highway 18
Stormwater treatment pond near
State Route 18.


WSDOT and Stormwater Runoff

Rain becomes stormwater runoff when impervious surfaces like streets, sidewalks, and parking lots prevent it from soaking into the ground.

Statewide, WSDOT operates and maintains about 40,000 acres of impervious surfaces including:

  • Highways
  • Rest Areas
  • Ferry Terminals
  • Park and Ride Lots
  • Maintenance Facilities

As stormwater runs off of these impervious surfaces and nearby properties, it can pick up pollutants like oil, fertilizers, pesticides, dirt, trash, and animal waste. If left uncontrolled and untreated, stormwater runoff can harm habitat, erode stream channels, and carry pollutants into lakes, rivers, and the Puget Sound.


Stormwater Regulations

WSDOT must comply with federal and state water quality laws. These laws require WSDOT to have permits for discharging stormwater runoff from our:

  • Stormwater drainage systems
  • Construction sites
  • Industrial sites
  • Sand and gravel sites


How does WSDOT protect and improve water quality?


Performs Actions for Total Maximum Daily Loads (TMDLs)

Ecology develops TMDLs (water cleanup plans) for water body segments that do not meet state water quality standards for a specific pollutant. WSDOT performs actions assigned to us in TMDLs to help improve the quality of lakes, rivers, and Puget Sound.


Designs effective stormwater management systems

WSDOT uses tools in our Highway Runoff Manual to design stormwater management systems for transportation projects. The manual outlines a process for selecting stormwater management systems which are approved by Ecology to control stormwater flow, treat stormwater pollutants, and when possible, mimic natural drainage patterns.


Finds and eliminates spills and illicit discharges

WSDOT finds and eliminates spills and illicit discharges and connections to our stormwater drainage system. This helps prevent pollution from reaching lakes, rivers, and marine waters.


Maps existing stormwater system and new facilities

WSDOT digitally inventories and maps its existing stormwater management system and new facilities built during transportation projects. This helps document facility locations, set level of service targets for maintenance activities, identify deficiencies and illicit discharges, and prioritize retrofits.


Controls erosion at construction sites

WSDOT controls erosion at construction sites to protect the environment and prevent erosion-related cost overruns and project delays.

WSDOT weekly inspects construction areas that have a potential to discharge stormwater carrying eroded soil or pH affected stormwater to water bodies.


Prevents and controls spills at construction sites

WSDOT requires contractors to make Spill Prevention Control and Countermeasure plans for every construction project. These plans help keep hazardous materials out of lakes, rivers, and marine waters. 


Retrofits existing highways to manage stormwater runoff

Many existing highways either do not have any stormwater flow control or treatment facilities, or do not have enough to manage the runoff from all impervious surfaces. WSDOT identifies, prioritizes, and implements retrofit solutions to manage stormwater runoff from these highways.


Maintains highways and other transportation facilities

WSDOT maintenance crews pick up litter and use mechanical street sweepers to pick up debris and sediment from highways and other transportation facilities. This helps keep pollution out of stormwater runoff.

WSDOT also routinely inspects and cleans ditches, culverts, catch basins, and stormwater management facilities to make sure they are working properly to control and treat stormwater runoff.

WSDOT's Stormwater Pollution Prevention Plans identify best management practices (BMPs) to help keep pollution out of stormwater runoff during everyday operations at rest areas, park and ride lots, ferry terminals, and maintenance facilities.


Monitors and researches stormwater quality

WSDOT monitors stormwater quality to help characterize pollutants in the stormwater that runs off of highways and other transportation facilities. Stormwater monitoring also helps us evaluate and understand the effectiveness of stormwater management systems and stormwater pollution prevention plans.

WSDOT uses stormwater research projects to help identify state-of-the-art, cost effective solutions for designing, constructing, and maintaining stormwater management systems.


Tracks actions taken to improve water quality

WSDOT's annual stormwater reports provide updates on what we are doing to protect and improve water quality in lakes, rivers, and marine waters. The reports include information on activities required by our municipal stormwater permit.

What can I do to help improve water quality?

  • Report spills and illicit discharges and connections to the appropriate authorities.
  • Keep your vehicle properly maintained. Fix drips and leaks and tune your engine.
  • Find alternatives to driving alone.
  • Use a vehicle trash bag and secure your loads to prevent litter on and near the highways.
  • Become an Adopt-a-Highway volunteer.
  • Pick up after your pet.