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Lake Twelve is a small (43 acres), shallow lake located in the Cedar River basin of southeastern King County, approximately one mile northeast of the City of Black Diamond. Forestland and a large bog-type wetland comprise the majority of the watershed. In 1983, land adjacent to the western boundary of the watershed was cleared to make way for an open cut surface mining operation. In 1986, Pacific Coast Coal Company built a large noise berm between the lake and the mining site to limit the impact of noise.

The primary water quality concerns identified by lake residents in 1988 were increasing algal blooms and aquatic plant density, in particular, the presence of the invasive Eurasian Watermilfoil. An extensive limnological study of Lake Twelve was conducted between 1991 and 1992 resulting in a management plan completed in 1994. An integrated aquatic plant management plan (IAPMP) was completed in 1995. A major infestation of Eurasian Watermilfoil was documented during the production of the IAPMP. In 1996, a whole lake Sonar aquatic herbicide treatment was used to eradicate the invasive plant. Follow up surveys in 1997 and 1998 did not detect any Eurasian Watermilfoil. However, in 1999 the plant was again observed and removed by divers. In addition, the lake was treated in 1997 with Rodeo herbicide to reduce the lily growth in areas where they posed a problem. The lake will be surveyed again in 2000 to detect any further colonization of the invasive plant.

Lake Twelve has been part of the Lake Stewardship Volunteer Monitoring Program since 1994. Twice a month from May through September, volunteers collect total phosphorus, chlorophyll a, total nitrogen, and phytoplankton samples to send to the County lab. Additional measurements, such as temperature, Secchi depth, precipitation, and lake level are collected by volunteers weekly throughout the year.

Lake Twelve statistics

For questions about lakes in King County, please contact or call the Water and Land Resources Division front desk at 206-477-4800.