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:
Results:
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
