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The Lake Washington story

The Lake Washington story

King County, Washington

Watershed overview

Photo of 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,. 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

Photo of West Point wastewater 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.
  • Daphnia
    Daphnia, commonly called a water flea, is a filter feeding planktonic crustacean about 2 mm long. It suddenly became an important member of the zooplankton in Lake Washington in 1976, although it had been present in small numbers previously. Daphnia is an efficient filter feeder and can reduce algae populations quickly, thus increasing water transparency. While Daphnia can consume some kinds of filamentous plankton, Oscillatoria clogs its filtering apparatus so it is unable to feed. Thus, the increase in Daphnia coincided with the demise of Oscillatoria in the lake. Since Daphnia can reproduce quickly to exploit favorable conditions, its numbers can fluctuate dramatically through the year, although peak abundances occur in May and June when temperature and sunlight trigger increased activity.
  • Besides the decrease of Oscillatoria and the increase in Daphnia, there was a reduction in the population of possum shrimp. Neomysis mercedis, the possum shrimp, is a planktonic crustacean that can reach a total length of about 14 mm (0.5 inch). It has been shown to have a strong feeding preference for Daphnia and is the main predator on Daphnia. In the late 1970s, Paul Murtaugh of the University of Washington studied the gut contents of Neomysis and concluded that it is an especially potent predator on Daphnia, capable of strongly influencing Daphnia abundances. Neomysis is a native species that has been present in Lake Washington for many decades, but has been scarce since 1968. Coinciding with its decline has been a rise in the number of long-fin smelt, which were discovered in the lake in 1960 by Robert Dryfoos, a UW student at the time.
  • Lake Washington’s longfin smelt is one of two landlocked populations of this small anadromous fish; the other population is in Harrison Lake, British Columbia. The species is distributed on the Pacific coast from northern California to northern British Columbia. Studies from the UW and Washington Department of Fisheries have shown the longfin smelt to be highly selective in feeding on Neomysis, with some two-year olds having a diet composed of 96% Neomysis. Younger longfin smelt are less specialized, eating Neomysis, Daphnia, and other crustacean zooplankton. Reasons for the increase in these smelt are not obvious, but the increase may be linked to inadvertent improvements in breeding habitat in the Cedar River; where the vast majority of spawning takes place. Government agencies have been working to sustain salmon habitat and to control flood damage in the area, and the smelt may have benefited from these habitat improvements.
  • Species composition of the lake

    Now we can see how changes in the amount of phosphorus, mainly from sewage outfalls into the lake, affected the species composition of the lake. The cyanobacteria Oscillatoria were not able to thrive in the lake after sewage diversions decreased the level of phosphorus input, so the numbers of Daphnia increased. The Daphnia populations also increased because its main predator, Neomysis, was reduced by the longfin smelt. Filter feeding by Daphnia helped reduce the green algae populations, so water clarity and quality increased.

    Photo of female Sockeye salmon

    Sockeye salmon are another species whose numbers increased during the lake's period of eutrophication, although the increase was probably not directly related to the level of phosphorus in the lake. Sockeye salmon are unique among salmon in that the smolts have a yearlong phase in freshwater lakes before to their migration to the sea. Sockeye had been planted in the Cedar River in 1953 but were in relatively low numbers until the mid 1960s. In 1970 the fish were numerous enough for the state to permit commercial fishing in the lake. The increase in sockeye may be due to inadvertent benefits reaped to the spawning beds, when flood control measures and a halt to channel dredging (because of equipment failure) just happened to reduce the silt accumulation on the gravel spawning beds. The smolts from Lake Washington are the largest of their species, but numbers have been down in recent years. Research into the cause of the decline is under way by several agencies. It includes research on food supply, predation, and physical damage from the Government Locks during out-migration.

    Contrary to common notion, UW research has not shown northern squawfish to be preying substantially on sockeye in Lake Washington, but cutthroat and rainbow trout have actually been implicated as being predators on the sockeye. Northern squawfish, however, are certainly adaptable in their diet, readily able to shift to different prey items. Largemouth and smallmouth bass are potential predators on sockeye, and one theory is that an increase in the number of boat docks has resulted in an increase in habitat for the bass. However, the spatial overlap between them and the sockeye may not be sufficient for there to be much of an impact. The major food supply for sockeye fry in winter is unknown. They do feed on Daphnia, but the sockeye fry appear in the lake about March or April, a month or two before Daphnia becomes abundant.


    Diaptomus and Epischura are genera of calanoid copepods, present in the lake year-round. The population peaks occur in early spring (March and April) and again in late summer (August and September). In contrast, Daphnia peak abundances are generally in May and June. The role of the copepods in the food chain of Lake Washington is now being investigated, because of recent declines in sockeye fry and smolts. Before the increase of Daphnia in the mid 1970s, Epischura and a summer cladoceran, Diaphanosoma, had been the preferred prey of planktivorous fish in the lake.

    The Lake Washington story is an epic in the scientific literature, thanks to Dr. Edmondson and his contingent of students and researchers. His research went well beyond Lake Washington. The literature produced through his UW lab covered five decades, and the influences in science and lake management are enormous. Well known for his work in the Pacific NW, he was also an expert on rotifers (a group of invertebrates, mostly fresh water, with nearly 2,000 species), with publications going back to 1934. Much of the information in this section is garnered from Dr. Edmondson's work. For more information, see the following publication.

    • Edmondson, W.T. 1991. The uses of Ecology: Lake Washington and Beyond. University of Washington Press.

    Sally Abella and Arni Litt, retired co-workers in the UW Zoology Department, kindly reviewed a draft of this section. Sally Abella worked with the King County Small Lakes program.

    For questions about Lake Washington, please contact Debra Bouchard, Water Quality Planner or Curtis DeGasperi, Lead Hydrologist, King County Science and Technical Support Section.