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Water quality assessment & 303(d) list

Every few years, we perform a water quality assessment to track how clean the rivers, lakes, and marine waters are in Washington. We place assessed waters in categories that describe the water quality status. The assessment helps us to use state resources more efficiently by focusing our limited resources on waters that need the most work.

What is the water quality assessment?

The water quality assessment is our process of collecting data and assessing the quality of surface waters in the state. This includes all rivers, lakes, and marine waters. The assessment helps us use state resources more efficiently by focusing our efforts on waters that need the most work.

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

Read about EPA’s approval of the latest Water Quality Assessment​

How are waters 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 identify the water quality status of waterbodies. The decisions that we make must balance the risks of incorrectly identifying waters as impaired when they really are not, versus mistakenly identifying waters as not impaired when they really do have a problem.

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

303(d) list

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

Waters whose beneficial uses (such as for drinking, recreation, aquatic habitat, and industrial use) that are impaired by pollutants are placed in the polluted water category (category 5) of the water quality assessment. The 303(d) list, so called because the process is described in Section 303(d) of the Clean Water Act, lists waters in the polluted water category.

To view list, go to the water quality assessment page and select Category 5 (under Water Quality Status).

What happens to polluted waters?

If waters 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.