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Assessment Methodology Updates

U.S. Environmental Protection Agency regulations require states to describe the methodology, data and information used to identify and list water quality limited segments requiring TMDLs. Oregon's assessment methodology contains the "decision rules" DEQ uses to compare data and information to existing water quality standards. The methodology is used to develop the biennial Integrated Report pursuant to Clean Water Act Sections 303(d) and 305(b) and OAR 340-041-0046. The methodology builds on DEQ's protocols from previous 305(b) and 303(d) assessments. 

Before each reporting cycle, DEQ may update the existing assessment methodology and develop new methodologies. Oregon statute requires DEQ to publish the  methodology before drafting the Integrated Report

2024 Integrated Report methodology updates

The Assessment Methodology for Oregon's 2024 Integrated Report will include updates for inland, estuary and marine waters.

DEQ's Water Quality Assessment Program held two public processes for the Draft 2024 Assessment Methodology. The inland and estuaries draft Assessment Methodologies for Oregon's 2024 Integrated Report were posted for public comment Jan. 5, 2023, and accepted comments on the methodologies through Feb. 21, 2023. The draft 2024 Ocean Acidification and Hypoxia Assessment Methodologies for marine water were posted separately on May 31, 2023, and comments accepted through July 7, 2023.

Supporting documents for 2024 methodology updates and development.

DEQ collaborated with a technical workgroup to develop draft assessment methodologies to assess the impacts of ocean acidification and hypoxia on marine aquatic life.

Future Integrated Report updates

DEQ has set a strategic vision for the water quality assessment program composed of short-term and long-term priorities. Short-term updates will be taken on in the next two assessment cycles. Long-term updates will be developed in coordination with water quality program goals.

Freshwater biocriteria

​​DEQ is committed to making updates to the biocriteria methodology based on peer review recommendations received in 2018. DEQ has updated its reference screening protocols, identified additional reference site data, and incorporated new data into its reference screening process. DEQ is exploring the option of a new or revised model, multi-metric index, and additional supporting lines of evidence. This work fulfills DEQ's commitment to EPA to pursue additional steps and lines of evidence to increase confidence in DEQ's interpretation of its biocriteria narrative.​

Tissue toxics

Oregon's methodology for listing segments of waterbodies as “impaired" for toxics does not currently include a method for assessing toxics data in tissue other than mercury. Category 5 listings are based on either water column concentrations of toxic pollutants, or on fish advisories issued by the Oregon Health Authority. DEQ is developing a method to assess support of the fish consumption (fishing) beneficial use using data collected from aquatic tissue samples by interpreting Oregon's narrative criteria for toxic substances OAR 340-041-0033(1).​

Using benchmarks to assess toxics narrative criteria

DEQ will evaluate the use of guidance values, or concentrations for toxic compounds that do not have WQ criteria that DEQ may use in application of Oregon's Toxic Substances Narrative (340-041-0033(2)). ​

​Aquatic weeds

​​​The current assessment method of the Narrative Criteria ORS 340-041-0007(9) lacks specific details and protocols. To support a determination of impairment by aquatic weeds or algae, terms such as “excessive growths” and “dominate the assemblage” must be clarified and a weight of evidence approach defined. This update will add clarity to the existing methodology.

Excessive algae

​​DEQ historically listed waterbodies based on excessive amounts of algae (i.e. benthic/floating algae). DEQ is exploring multiple lines of evidence that may be used to assess attainment of this criteria. This update will provide clarity and a clear weight of evidence approach to remove “excessive algae” listings. Lines of evidence may include: methods for visual estimation of algae, periphyton biomass thresholds, algae community composition, and other supporting WQ information such as nutrients, diel dissolved oxygen and/or pH.


In conjunction with protocols for excessive algae, DEQ plan to develop a weight of evidence approach for nutrient impairment.  Possible lines of evidence could be diel DO/pH, phosphorus/nitrogen concentrations together with chlorophyll (water column and benthic), phytoplankton community structure, and macroinvertebrates.​

Assessment of TMDL benchmarks for Category 2

​​​DEQ is coordinating with EPA to determine, where no numeric criteria exist (i.e. phosphorus, nutrients, sediment), whether attainment of TMDL targets/benchmarks may be used to identify where waterbodies have been restored and beneficial uses are currently being supported.


Sedimentation is a significant cause of impaired waters in Oregon.  Development of a sedimentation benchmark and corresponding assessment methodology would require peer review. A methodology to assess suspended and bedded sediment would build on current knowledge and procedures for implementing the narrative criterion. ​