Water Quality Improvement Project
Cottage Lake Area:
Total Phosphorus


Cottage Lake is located in King County and south Snohomish County, near the city of Woodinville, Washington State. It's drainage area is within the upper Bear Creek basin, which is a tributary to the Sammamish River. The lake receives surface water from Daniels Creek, including Crystal Lake, and Cottage Creek drainages. The lake also receives water from surrounding land areas. (See Study Area map)

Water quality issues

For a number of years Cottage Lake experienced chronically elevated phosphorus concentrations that have led to increased growth of algae (blooms) during the summer months. Cottage Lake was listing on Washington State's 1996 and 1998 303(d) lists for exceeding the state's water quality standards for total phosphorus. These conditions undermined the beneficial uses of the lake for activities such as fishing and swimming, and reduced Cottage Lake’s overall aesthetic value and biological health.

Sources of phosphorus inputs into the lake included residential and commercial development and maintenance, as well as animal husbandry.

Why this matters

Excessive nutrient inputs to lakes provide aquatic plant and algae dominance, making lakes unsuitable for recreation or viewing enjoyment. Phosphorus is one of the nutrients. When it is discharged into a waterbody, like a lake, it fuels the growth of algae.

Excessive algae growth reduces water clarity, increases oxygen demand in the bottom sediments severely impacting coldwater aquatic habitat, and can, depending on the dominant algae present, pose a human health risk.

Status of the project

The Department of Ecology (Ecology), with help from local citizens, developed a Total Maximum Daily Load (TMDL). Ecology submitted the finalized report to EPA, which approved it on September 2004. Ecology then developed a more detailed implementation plan, which was submitted to EPA in 2006.

Technical information

Cottage Lake Total Maximum Daily Load Analysis: Submittal Report (Ecology publication)

Total Phosphorus Total Maximum Daily Load Water Quality Implementation Plan (Ecology publication)

Related information

Cottage Lake: Watershed Education Helps to Protect a Valuable Resource (Ecology publication)

WRIA 8: Cedar-Sammamish Watershed Information (Environmental Assessment Program web site)


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Last updated May 2015
  Map for water resource inventory area (WRIA) 8, Washington


WRIA: #8 (Cedar-Sammamish)

Water-body Name:
Cottage Lake

Total Phosphorus

# of TMDLs: 1

Approved by EPA
Has an implementation plan

Contact Info:
Tricia Shoblom
Phone: 425-649-7288
Email: Tricia.Shoblom@ecy.wa.gov

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