History of the Major Lakes Monitoring Program

The Major Lakes Monitoring Program is designed to protect the significant investment in water quality improvement and protection made by the people of King County. Sewage and wastewater used to be discharged directly into the county's lakes, but the construction and operation of the sewage collection and treatment system transferred wastewater effluent out of lakes Washington, Union, and Sammamish. Visit King County's Wastewater Treatment Division Web site for a complete history. Sewage and wastewater now enter secondary treatment facilities at West Point and Renton, from which clean, treated water is discharged into the well mixed waters of Puget Sound. The result was dramatic improvements in lake water quality. Monitoring water quality is still important, as several pump stations are on or near the shores of the lakes, miles of sewer pipelines lie on the bottom of lakes Sammamish and Washington, and most of the influent streams are crossed by one or more sewer pipelines. During certain strong storm events, combined sewer overflows (CSOs) still discharge some dilute sewage and storm water into Lake Washington, the Ship Canal, and Lake Union.

Sampling and flow monitoring sites are distributed around the lakes and streams to monitor the pipe and pump systems, as well as to monitor the long term environmental quality of these waters. All of the county's combined sewer overflows (CSOs) to Lake Washington are controlled to one event per year as agreed to with the Washington State Department of Ecology.

Lake Washington, Union and Sammamish are in a constant state of change. Water quality is influenced by activities and inputs from the watershed through the influent streams, lake nutrient cycles, ecological interactions, and seasonal or year to year variability in weather. Determining how these lakes respond over time is one of the goals of this monitoring program. Lake responses can vary from short term responses due to seasonal weather patterns, to long term responses due to watershed changes. The combination of short-term and long-term trends, seasonality, changes in land use, and ecological interactions, all of which occur simultaneously, make it difficult to detect or predict either the presence or direction of water quality trends in the lake.

Seasonal changes are accounted for by monthly or bimonthly sampling at all of the stations. Wet season, dry season, changes in sunlight intensity and day length all combine to generate seasonal cycles in the lake. These seasonal water quality cycles are not uniform at all depths in the lake, so at each station samples are taken from 1 m below the surface of the lake to just above the lake bottom.

Each of the lakes has one or more sampling stations located in the deep central basin of the lake, where the influence of the shoreline is muted by the surrounding water and mixing action of wind and waves. Changes observed over time at these sites reflect broad, large scale, or landscape scale changes in the watershed and the lake. Other sampling stations are distributed around the shoreline of the lake, primarily off the mouths of influent streams. Changes in water quality at these stations are more directly influenced by shoreline activities and by the quality and quantity of inflowing stream water. Changes at these sites often occur more quickly and are often greater than those observed in the middle of the lake.

Read about how we sample the lakes.