Water Quality Improvement Project
Issaquah Creek Basin:
Fecal Coliform


The Issaquah Creek Basin is located on Washington State’s I-90 corridor at the south end of Lake Sammamish in western King County. The basin consists of the stream networks and watersheds of Issaquah and Tibbetts creeks, and contains most of the city of Issaquah. Streams in the Issaquah Creek Basin provide important resources for fish, primary contact recreation, education, and aesthetic enjoyment.

Water quality issues

Issaquah Creek Basin streams have too much bacteria, as measured by fecal coliform bacteria counts in the water. Because of this, these streams were placed on the state's 303(d) list of impaired waters for 1998. The federal Clean Water Act requires that streams on a state's 303(d) list have a total maximum daily load (TMDL) or other appropriate water cleanup plan developed. The TMDL or other appropriate water cleanup plan must determine the extent of the impairment, how much pollutant the water body can actually accept without violating the state's surface water quality standards (loading capacity), and how much the pollutant load must be reduced so that the water body meets the water quality standard for that pollutant (wasteload or load allocation).

Sources of bacteria contamination in the Issaquah Creek Basin include on-site septic systems; possible sanitary sewer line leaks; agriculture (commercial and small farms); landfills; and wildlife. Urban stormwater and stormwater runoff from roads and highways also carry bacteria to the streams.

Issaquah Creek, water intake facility, ¼ mile above the Issaquah Creek Salmon Hatchery, Washington State. Replaced an outdated dam and fish ladder. Photo courtesy of Larry Franks, FISH Member. Water intake facility that provides water for the Issaquah Creek Salmon Hatchery and a series of boulder weirs built in 2012. This replaced an outmoded dam and fish ladder built in 1937. © Larry Franks, FISH Member.

Why this matters

Fecal coliform is a type of “bacteria” common in human and animal waste. It indicates that sewage or manure is entering a water body. As the level of fecal coliform increases, the risk of people getting sick from playing or working in the water increases. Bacteria can get into our waters from untreated or partially treated discharges from wastewater treatment plants, from improperly functioning septic systems, stormwater drainage systems, and from livestock, pets and wildlife.

People can help keep bacteria out of the water. Properly collect, bag, and trash dog poop. Check your on-site sewage system to make sure it is maintained and working properly. Ensure livestock and manure are kept away from the water.

Status of the project

Washington State Department of Ecology (Ecology), along with local governments, citizens, and special interest groups, developed a water quality improvement report (also called a TMDL). This TMDL included a water cleanup plan that detailed suggested methods and initial efforts to reduce fecal coliform bacteria loading to the Issaquah Creek Basin. After addressing comments from public review, Ecology submitted the TMDL to EPA. EPA approved the TMDL in October 2004.

Technical information

Issaquah Creek Basin Water Cleanup Plan for Fecal Coliform Bacteria: Submittal Report (Ecology publication)

Related information

Focus on Bacteria in the Issaquah Creek Basin (Ecology publication)

City of Issaquah - Rainier Boulevard Street LID Improvement Project (Ecology publication)

WRIA 8: Cedar-Sammamish-Lake Washington Watershed Information (Environmental Assessment Program website)

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Last updated October 2015
  Water resource inventory area (WRIA) 8 map, Washington State.


WRIA: #8 (Cedar-Sammamish)
County: King

Water-body Names:
Issaquah Creek
N. Fork Issaquah Creek
Tibbetts Creek

Fecal Coliform

# of TMDLs - 3

TMDL approved by EPA

Contact Info:
Joan Nolan
Phone: 425-649-4425
Email: Joan.Nolan@ecy.wa.gov

Northwest Region
Department of Ecology
3190 160th Ave. SE
Bellevue, WA 98008-5452