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The Washington state stream temperature standards were established for the protection of designated beneficial uses – particularly for the protection of freshwater spawning, rearing and migration habitat for salmon. The standard is based on the moving average of the daily maximum stream temperature and is calculated using continuous (every 15 minutes) measurements collected at temperature gauges placed at routine monitoring locations. These gauges are operated by King County, the U.S Geological Survey and the Seattle District of the U.S. Army Corps of Engineers.

Most of the streams monitored within King County fall within the "Core Summer Salmonid Habitat" Aquatic Life Use Category, with a maximum 7-day average temperature maximum allowance of 16 °C. Many of these streams also have Supplemental Spawning and Incubation Criteria applied to specific months of the year. A few stream and river stations in King County are categorized as "Spawning and Rearing Habitat" with a 7-day average temperature maximum allowance of 17 °C. To find out what temperature criteria apply to streams we monitor, please view the "Stream Information" link for that particular stream (found on our Streams List page). Below are charts of the 7- Day Moving Average of the Daily Maximum Temperature (7-DMADMAX °C) for selected years.

Exceedences of the stream temperature standard suggest impairment of designated uses. The Washington State Department of Ecology makes this determination under the Clean Water Act Sections 303(d) and 305(b). The result of Ecology’s assessment includes placement of stream segments in one of five categories that range from Category 1 (meets standards) to Category 5 (polluted waters that require a Water Cleanup Plan – also known as a Total Maximum Daily Load or TMDL). Stream temperature TMDLs typically include the collection of additional data and the development of a stream temperature model to establish the magnitude of impairment relative to an idealized condition. Then action is taken to restore riparian vegetation (and sometimes other factors) to its maximum historic potential.

Please select a year to view the image: 
Continuous Temperature Monitoring image


The moving 7-day average of the daily maximum temperature for King County stream and river sites were calculated for all years going back as far as 2000. These scores are shown for each year in the maps above. As shown, many streams and rivers throughout the county exceed the 16 °C standard established for the protection of core summer salmonid habitat, with the exception of a few streams found in rural areas and less developed areas within the urban growth boundary.

A stream temperature TMDL has been completed for the Bear-Evans Creek Basin and temperature TMDLs are under development for the Snoqualmie River, the Green River, Newaukum Creek, and Soos Creek.

Influencing Factors:

Extensive development can substantially alter the extent of riparian shade that moderates daily peak stream temperatures. Development can also alter summer low flows through reduced groundwater recharge from impervious areas and by water management activities within the basin such as groundwater extraction and export via potable water supply and regional wastewater conveyance systems. Development induced increases in high flows combined with the loss of riparian tree cover can also cause the stream to become wider and shallower, which also contributes to higher peak stream temperatures. Climate change, particularly predicted increases in air temperature are expected to result in warmer stream conditions without substantial investment in restoring riparian shade and summer flow conditions.

What We Are Doing

King County has a range of regulatory, educational, and on-the-ground programs to reduce the impacts of development on streams and protect and restore riparian vegetation. More attention is also being paid to how development and basin water management activities affect summer stream flow and approaches are being explored to restore and improve flows in streams where historical flow declines have been observed.

The potential extent of impairment of streams for the designated use as core summer salmon habitat highlights the need for a more comprehensive and coordinated approach to identifying stream reaches that would most benefit from measures such as riparian shade restoration and improved summer stream flows. King County will work with Ecology, Puget Sound Partnership, and other regional stakeholders to advocate a regional scale water quality assessment, cleanup planning and implementation effort.

Data source: The data source for this indicator comes from King County DNRP/WLRD Scientific and Technical Support Section, the U.S. Geological Survey, and the Seattle District Corps of Engineers