Sewage effluent was diverted progressively from Lake Washington during 1963-1968, and the chemical conditions changed in close relation to the amount of sewage entering. The total phosphorus content of the lake decreased rapidly to 1971 after which year it varied around a value of about 50,000 kg (= 17 micro/l) with a slight decreasing trend. The lake has retained about 56% of the phosphorus that entered during 1971-1975. Winter means of nitrate and the annual mean total content of Kjeldahl nitrogen has decreased at a slow rate during the entire period. Phytoplankton as measured by chlorophyll in the epilimnion during summer dropped to a low value in close proportion to phosphorus during diversion, but has decreased faster than phosphorus during 1971-1976. A large increase in transparency occurred in 1976. A major change is taking place in the character of the zooplankton of Lake Washington in that Daphnia became very abundant in 1976. This event is probably not directly related to recovery from eutrophication, so the lake is entering a new phase.

Includes bibliographical references (pages 15-18).