Water Quality Improvement Project
Lake Sawyer Area:
Total Phosphorus


Lake Sawyer is located in the city of Black Diamond, Washington, and is a significant regional recreational resource for the area. Lake Sawyer is 280 acres in size, and its watershed encompasses approximately 8,300 acres. As part of the Big Soos Creek basin of the Green River watershed, Lake Sawyer serves as a pathway for a late winter run of Coho salmon. Coho salmon travel through Lake Sawyer on their way to spawning grounds in the Ravensdale and Rock Creek systems. Resident rainbow trout, cutthroat trout, kokanee, and various warm water fish species are present in Lake Sawyer as well. As the fourth largest lake in King County, Lake Sawyer is a primary recreation area for swimming, boating, fishing, and aesthetic enjoyment.

Water quality issues

The general health of Lake Sawyer and its watershed has been a concern for several decades. In the 1970s, evidence of failing septic systems in the watershed resulted in the decline of water quality of Rock Creek, Ginder Creek, and Lake Sawyer. To address this concern, the city of Black Diamond acquired funding from the Environmental Protection Agency’s (EPA) Innovative and Alternative Grants Program to develop a wastewater treatment plant. The treatment plant began operating in 1981, and was designed to discharge to a natural wetland next to Rock Creek, ultimately draining to Lake Sawyer. The innovative project soon failed, resulting in excessive nutrient loading to Lake Sawyer and prompting the Department of Ecology (Ecology) to set stricter effluent limits for Black Diamond. In 1992, all wastewater from the treatment plant was diverted to the Renton sewage treatment plant via King County’s (Metro) sewer line.

Lake Sawyer photo, Washington State.  Photo courtesy of Tricia Shoblom, Washington Department of Ecology.

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 water body, 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

In 1991, Ecology developed a Total Maximum Daily Load (TMDL), also referred to as a water cleanup plan, for Lake Sawyer. A water cleanup plan is a numerical description of the amount of a pollutant that a water body can accept and still meet state water quality standards (Washington Administrative Code 173-201A). The Lake Sawyer Total Phosphorus TMDL set the total phosphorus target concentration for Lake Sawyer at 16 µg/L. EPA approved the TMDL in 1993.

A 2002 Ecology assessment found that continuing phosphorus control is needed in the Lake Sawyer watershed to help the lake meet and maintain water quality standards into the future. Sources of phosphorus were identified as stormwater runoff, primarily occurring in the winter months; phosphorus-rich sediments being re-suspended into the water column during annual lake turnover; and aquatic plant decay, the majority of it occurring in the fall.

The diversion of Black Diamond’s wastewater treatment discharge from the natural wetland above Lake Sawyer to a sewer line successfully reduced the majority of nutrient input to the lake and significantly helped the lake to recover. Significant urban growth is planned in the Lake Sawyer watershed in coming years, which could have a negative impact on water quality.

Since re-directing Black Diamond’s wastewater away from Lake Sawyer, there has been a steady decline in phosphorus but the lake still has too much. Ecology developed the Lake Sawyer Total Phosphorus Total Maximum Daily Load Water Quality Implementation Plan, a follow up document to the 1993 TMDL. The plan provides a framework for corrective actions to address sources of phosphorus pollution in Lake Sawyer and the surrounding watershed. It also incorporates recommendations from the Lake Sawyer Management Plan and other documents developed for Lake Sawyer. Municipal stormwater permittees will implement actions to reduce phosphorus in stormwater to meet Lake Sawyer TMDL requirements. The Lake Sawyer Steering Committee will serve as sounding board for implementation projects, monitoring updates, and adaptive management. Black Diamond will continue to contract with King County’s Lake Stewardship Program to monitor phosphorus in Lake Sawyer to track trends in lake quality and enable adaptive management.

Ecology finalized the water quality implementation plan and sent copies to EPA in June 2009. Ecology will administer permits and manage TMDL implementation activities in Lake Sawyer watershed, with special attention to phosphorus levels. Ecology’s Northwest Region Water Quality Lake Specialist will continue to coordinate TMDL-related activities and provide technical assistance for the TMDL. Ecology will also conduct at least three sampling surveys in Lake Sawyer watershed to help further define pollution sources and promote source correction. Ecology grant programs will provide project funding opportunities, and Ecology's northwest region grant project managers will assist grant applicants to develop water quality improvement projects.

Technical information

Lake Sawyer Total Phosphorus Total Maximum Daily Load (Ecology publication)

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

Lake Sawyer-Black Diamond Waste Load Allocation Evaluation (Ecology publication, 1989)

Lake Sawyer Hydrogeologic Study--Black Diamond, Washington (Ecology publication, 1990)

Diagnostic Study of Lake Sawyer, King County, Washington (Ecology publication, 1991)

Lake Sawyer Effectiveness Monitoring for Phosphorus (Ecology publication, 2002)

Related information

Focus on Lake Sawyer (Ecology publication)

Focus on Soos Creek Watershed (Ecology Publication)

Overview of Ecology's TMDL process

Lake Sawyer Water Cleanup Plan (TMDL) for Phosphorus (Ecology publication)

WRIA 9: Duwamish-Green Watershed Information (Environmental Assessment Program web site)


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Last updated May 2015
  Water Resource Inventory Area (WRIA) 9 map, Washington State.


WRIA: #9 (Duwamish-Green)
County: King

Water-body Name:
Lake Sawyer

Total Phosphorus

# of TMDLs: 1

Implementation plan submitted to EPA

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