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,. 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.
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. In comparison to Lake Sammamish, Lake Washington is about twice as deep, four times the area and flushes about as frequently.
Factors influencing water quality
Water quality is greatly influenced by human activities, but other seemingly subtle biological activities also have great significance. Lake Washington is an interesting example of how human influences and biological processes can alter water quality.
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.
The late Dr. W.T. Edmondson, former professor of zoology at the University of Washington, (external link) studied the biology and chemistry of Lake Washington for many decades. The 1955 discovery of the cyanobacteria Oscillatoria rubescens (formerly called a blue-green alga) in the lake, by oceanographer George Anderson, led to further research and predictions that nutrient conditions would soon be stimulating nuisance algal conditions, as had been documented in Lake Zurich in Switzerland.
Unlike algae, which assails the eyes and the nose, some factors of water quality are invisible and can be measured only in the laboratory. Dr. Edmondson's studies implicated phosphorus from sewage as being the element from treatment-plant effluent that fertilized algae in Lake Washington. Phosphorus was found in concentrations of 70 parts per billion in the 1960s. That was enough to feed the significant growth of algae that darkened the water and washed ashore to rot and smell. This finding had major implications for industry, and great political discussion resulted.
Lake restoration and the Westpoint sewage treatment plant
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. Metro was established in 1958 and entrusted with the task of diverting sewage from the lake. Between 1963 and 1968, more than 100 miles of large trunk lines and interceptors were installed to carry sewage to treatment plants built at West Point and Renton. Treated effluent ended up in Puget Sound, where currents and tidal action diluted it. At the time, the $140 million dollar campaign was considered the most costly pollution control effort in the country. 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.
Just as Dr. Edmondson had predicted, completing these major facilities brought dramatic results. Effluent, which was at one time entering Lake Washington at the rate of 20 million gallons per day, was reduced to zero discharge in February 1968. After the last lakeshore treatment plant was closed, the concentration of phosphorus dropped quickly to about 16 parts per billion, a level maintained into the 1990s and beyond. The lake's transparency, as low as 30 inches in 1964, reached 10 feet in 1968. Water quality would continue to improve: in later years the transparency would reach depths of 17 to 20 feet, with a maximum depth of nearly 25 feet in 1993.
Changes to algae, zooplankton and fish in Lake Washington
Improvements to transparency after 1976 increased beyond what could be accounted for by the measured amount of phosphorus. This increase came about with changes in the composition and relative abundance of the algae, zooplankton, and fish.
During Lake Washington’s period of eutrophication in the 1960s, the cyanobacteria Oscillatoria rubescens was a prominent nuisance, forming thick masses near the surface of the water. This species is relatively long and filamentous, generally unsuitable food for grazing zooplankton. Oscillatoria has inhibitory effects on other algae through the physical impact of shading and through biochemical means. Since phosphorus is a necessary nutrient for Oscillatoria, it was able to thrive in the phosphate-rich lake water. With the sewage diversion from the lake and resulting decrease in available phosphorus, conditions were no longer ideal for Oscillatoria, and it diminished entirely in 1976.