Lake Washington is the second largest natural lake in Washington state, USA, behind Lake Chelan, and the largest lake in King County. It is situated between Seattle to the west, Bellevue to the east, Renton to the south, and Kenmore to the north, and surrounds Mercer Island. It is fed by the Sammamish River at its north end and the Cedar River at its south, as well as a number of creeks, including Ravenna Creek/University Slough and Thornton Creek on the west and Kelsey Creek , Juanita Creek , and Coal Creek on the east.

Before construction of the Lake Washington Ship Canal in 1916, Lake Washington's outlet was the Black River, which joined the Duwamish River and emptied into Elliott Bay. When the canal was opened, the level of the lake dropped nearly nine feet (3 m). The canal to Puget Sound became the lake's sole outlet, causing the Black River to dry up and disappear, as a few years earlier, the Cedar River had been diverted to empty into Lake Washington instead of the Black.

Four bridges cross Lake Washington. The Evergreen Point Floating Bridge (officially the Governor Albert D. Rosellini Bridge--Evergreen Point) carries Washington State Route 520 from Seattle's Montlake neighborhood to Medina. The Lacey V. Murrow Memorial Bridge and the Third Lake Washington Bridge (officially the Homer M. Hadley Memorial Bridge) carry Interstate 90 from Seattle's Mount Baker neighborhood to Mercer Island. The East Channel Bridge carries Interstate 90 from Mercer Island to Bellevue. The Evergreen Point, Lacey V. Murrow, and Third Lake Washington bridges are the longest, second longest, and fourth longest floating bridges in the world, respectively.

Shoreline cities and towns (clockwise)

Seattle, Lake Forest Park, Kenmore, Kirkland, Yarrow Point, Clyde Hill, Hunts Point, Medina, Bellevue, Beaux Arts Village, Mercer Island, Newcastle, Renton

Statistics

Area of lake	21,500 acres	87.6 km²
Area of drainage basin	300,000 acres	1,274 km²
Volume	2,350,000 acre-feet	2.9 km³
Depth (mean)	108 ft	32.9 m
Depth (max)	214 ft	65.2 m
Length	13 miles	21 km
Height*	20.6 ft	6.3 m

* Above Puget Sound mean lower low tide