Lake Washington is a long, narrow lake located in an urbanized area close to Seattle. It covers an area of 88 km2 and is crossed by two floating bridges. The lake shore affords a good view of the snow-covered cone of Mt. Rainier. The climate around the lake is mild throughout the year with only sporadic snowfall in winter.

In 1916, a canal with locks and a fish ladder was made connecting the lake with the open sea to the west for navigation. The lake had been still oligotrophic in the late 1930's, but an increasing influx of urban sewage from the surrounding area caused rapid progress of eutrophication, as indicated by the frequent bloom of blue-green algae in summer since 1955. However, the construction of sewage treatment plants and the diversion of treated water through a 180 km-long pipeline, carried out in the five years from 1963 to 1968, have improved the quality of the lake water until it recovered almost completely in 1975. The chemical parameters of the lake water have remained quite stable for the past ten years, and the citizens of Seattle and neighboring cities enjoy swimming, fishing and sailing in the lake again.