Water quality assessment & 303(d) list

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The federal Clean Water Act requires states to perform a water quality assessment every two years to track how clean the rivers, lakes, and marine water bodies are. We place assessed water bodies in categories that describe the quality of the water and status of any needed clean up. The assessment helps us to use state resources more efficiently by focusing our limited resources on water bodies that need the most work.

View our online database for the latest assessment results.

View the candidate Water Quality Assessment

We held a public comment period on the draft Water Quality Assessment from April 8 to June 4, 2021. We then refined the assessment results based on comments received and submitted an updated "candidate" Water Quality Assessment to EPA for approval on Aug. 31, 2021. This version of the assessment can be viewed through the Draft Water Quality Assessment Review Tool or the Draft Water Quality Atlas, an online mapping application.

Read our response to comments on the draft assessment and supporting information.

See our timeline for how we developed this draft assessment.

Draft assessment documents

Public workshop

On April 20, 2021, we held a workshop to walk through the draft assessment. We highlighted the assessment process, key changes in this assessment, and provide a demonstration of how to use review tools and submit comments.

View presentation slides (pdf)

Next steps

EPA has 30 days from the time we submit the assessment to take action. We are working diligently with EPA to ensure a quick and smooth approval process. Following approval, the candidate list will become the new, approved Water Quality Assessment for Clean Water Act regulatory purposes. We will then announce a Call for Data for the next Water Quality Assessment cycle.

What is the water quality assessment?

The water quality assessment is our process of collecting data and assessing the quality of surface water in the state. This includes all rivers, lakes, and marine water. You can view the results of the assessment in our online database.

The federal Clean Water Act requires that all states restore their water bodies to be “fishable and swimmable.” Section 303(d) of the Clean Water Act establishes a process to identify and clean up polluted water.

What is the Water Quality Atlas?

We also have a mapping tool called the Water Quality Atlas. The Water Quality Atlas is an interactive search and mapping tool that includes additional layers of information in an easy-to-use mapping application.

Users can map the water quality assessment category results, view water quality standards for a location, identify areas addressed by Total Maximum Daily Loads (TMDLs), and see permitted wastewater discharge outfalls.

The current assessment

The current water quality assessment — called the 305(b) report and 303(d) list of impaired waters for the state of Washington — was approved by the U.S. Environmental Protection Agency (EPA) on July 22, 2016. See below for more information about the current process to update the assessment. 

Read the 2016 EPA approval letter.

Key changes that were made in 2016

  • Updated freshwater listings based on data collected between 2000 and 2010.
  • Updated how we map polluted rivers, lakes, and streams — moving from a land-based system (township/range/section) to a water-based (National Hydrography Database) system. The mapping change helps us be consistent with other agencies and states, and is strongly recommended by EPA.

The process to update the Water Quality Assessment

The following timeline provides information on each step of the current assessment cycle. Because we have been working on a large data automation project (see Assess the data section below) we have not submitted an assessment since 2016. However, this automation of our data should now allow us to meet our requirement for submittal every two years. This new assessment will serve as Washington's past due submittals.

The opportunity for early tribal review of the assessment is part of a 1997 cooperative agreement between the tribes, Ecology, and EPA that recognizes the government-to-government relationship between Washington and tribes in managing water quality.


Woman kneeling next to stream holding water sampling bottle

How are water bodies assessed?

We compile our own water quality data, and invite other groups to submit water quality data they have collected. We only assess data that meets high quality assurance and credible data requirements. Our goal is to accurately determine if water bodies are polluted based on the best available data.

The assessed water bodies are placed into one of five categories that describe the water quality.  Once the assessment is complete, we provide the public a chance to review and give comments. We submit the final assessment to the Environmental Protection Agency for approval of the category 5 listings, also called the 303(d) list.

Assessment categories


303(d) list

Water bodies whose designated uses (such as for drinking, recreation, aquatic habitat, and industrial use) that are impaired by pollutants are identified in the water quality assessment as "Category 5: Polluted water that requires a water improvement project." The 303(d) list, so called because the process is described in Section 303(d) of the Clean Water Act, lists water bodies in the polluted water category.

Federal laws, state water quality standards, and water quality assessment policy 1-11 guide our assessment of which water bodies to place on the 303(d) list.

To view the list, go to the water quality assessment database and select Category 5 (under Categories).

What happens to polluted water bodies?

If water bodies are polluted, we develop a water cleanup plan, also called a Total Maximum Daily Load or TMDL, to reduce pollution sources throughout the surrounding watershed. After pollutant controls are in place, we monitor in the watershed to see if the water meets state water quality standards.

Additional languages for the public review