For questions about the King County  Major Lakes Monitoring Program, please contact Sally Abella (206) 477-4605, or Debra Bouchard (206) 477-4650.

Lake Washington

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. Historic Resources in Lake Washington.

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. Find out more about this fascinating recovery of Lake Washington - The Lake Washington Story

The basin of Lake Washington is a deep, narrow, glacial trough with steeply sloping sides, sculpted by the Vashon ice sheet, the last continental glacier to move through the Seattle area. The lake is 20.6 feet above mean lower low tide in Puget Sound, to which it is connected via Lake Union and the lake Washington Ship Canal, constructed in 1916. The Ship Canal is the only discharge from lakes Sammamish and Washington via the locks and dam at the western end. Prior to construction of the canal, the only significant inflow was from the Sammamish River in the north. Construction of the canal resulted in the lowering of the lake 9 feet to its present level, leaving the Black River dry and the Cedar River diverted into Lake Washington. Mercer Island lies in the southern half of the lake, separated from the east shore by a relatively shallow and narrow channel, and from the west shore by a much wider and deeper channel. Lake Washington elevation (external link)

In comparison to Lake Sammamish, Lake Washington is about twice as deep, four times the area and flushes about as frequently. Both lakes experienced diversion of wastewater in the 1960's. With effluent diversion, nearly three-fourths of the phosphorus income was diverted from Lake Washington (Metro 1983). Water quality improvements in Lake Washington were much more dramatic than the improvements observed in Lake Sammamish. The differences in the relative sources of nutrients and in lake basin morphometry resulted in the differences in the response to diversion of each of the lakes.

Physical Characteristics

Lake Washington and its Drainage Basin

Drainage area, including Mercer Island 357,760 acres (559 miles2) 1,448 km2
Lake area 21,500 acres 87.6 km2
Lake volume 2,350,000 acre-ft 2.9x109 m3
Mean depth 108 feet 32.9 m
Maximum depth 214 feet 65.2 m
Flushing rate 0.43 per year
Depth of epilimnion 39 feet 12 m
Epilimnion:Hypolimnion ratio 0.387
Length 22 miles 35 km
Main inflows Cedar River (57%)
Sammamish River (27%)
Main outlet Ship Canal to Puget Sound
Typical period of stratification Late March to early November
Trophic state mesotrophic
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Updated: 3/12/2014 12:00:00 AM