The following is from the King County Natural Resources and Parks website:

Lake Washington is the largest of the three major lakes in King County, and the second largest natural lake in the State of Washington. Lake Washington’s two major influent streams are the Cedar River at the southern end, which contributes about 57 percent of the annual hydraulic load and 25 percent of the phosphorus load, and from the north, water from Lake Sammamish via the Sammamish River contributes 27 percent of the hydraulic load and 41 percent of the phosphorus load. The majority of the immediate watershed is highly developed and urban in nature with 63 percent fully developed. The upper portion of the watershed is the headwaters of the Cedar River that lie in the closed Seattle Water Department watershed. Lake Washington is perhaps the best example in the world of successful lake restoration by the diversion of sewage, and has been extensively studied and researched.

The lake received increasing amounts of secondary treated sewage between 1941 and 1963, which resulted in eutrophication and declined water quality of the lake. Planktonic algae was dominated by blue-green bacteria (algae) from 1955 to 1973. Sewage was diverted from the lake between 1963 and 1967, with discharge of untreated effluent, except for combined sewer overflows (CSO’s) reduced to zero by 1968. Rapid and predicted water quality improvements followed, blue-green algae decreased and have been relatively insignificant since 1976. These changes have been well documented in the book The Uses of Ecology by W.T Edmondson.

For more information see The Lake Washington Story on King County's website (opens in new window).