For questions about the King County  Major Lakes Monitoring Program, please contact Sally Abella (206) 477-4605, or Debra Bouchard (206) 477-4650.

Lake Sammamish

Lake Sammamish has two 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. In addition, other sampling stations are distributed around the shoreline of the lake, primarily off the mouths of influent streams. Changes at these shoreline sites often occur more quickly and are often greater than those observed in the middle of the lake. Click on the sampling station to view data graphs for that site.

Lake Sammamish Monitoring Overview

Lake Sammamish is the sixth largest lake in Washington and the second largest in King County and is one of the major recreational lakes with high use by fishermen, boaters, water skiers, swimmers, and picnickers. There are both State and County parks along the shore, and the lake has been designated a water of statewide significance.

The Lake Sammamish basin is a long, uniform trough with steeply sloping sides and a maximum depth of 32 m (105 ft). These characteristics are fjord-like, except the lake lacks the extreme depth exhibited by most fjord lakes ( Bathymetric Map). Annual average precipitation is approximately 90 cm, with about 75 percent occurring during extended periods of non-intensive rainfall events from October through March. Land use changes in the watershed alter the quantity, quality and timing of rainfall runoff. As forests are cleared and impervious surfaces (paved surfaces) increase, the water storage capacity of the soils decrease and the rate of runoff increases. These changes increase the high wet weather flows in the streams and reduce the summer low flows. The increased wet weather flows cause additional erosion and instability in the stream channels, and carry sediment into the lake. Decreased dry weather flows in the same streams reduce the amount and quality of in-stream habitat. Lake Sammamish experiences the cumulative impacts of all of the land use changes in the watershed and alteration to the influent streams. Lake Sammamish lake elevation (external link).

Physical Characteristics of Lake Sammamish and its Drainage Basin

watershed area 63,000 acres (98 miles2 ) 255 km2
lake surface area 4,897 acres 19.8 km2
lake volume 283,860 acre-ft 3.5x108 m3
mean depth 58 ft 17.7 m
maximum depth 105 ft 32 m
flushing rate 0.56 per year
depth of epilimnion 30 to 35 ft 10 to 12 m
Length 8 miles 12.9 km
main inflow Issaquah Creek
main outlet Sammamish Slough
typical period of stratification mid-May to mid-November
anaerobic period mid-July to mid-November
trophic state mesotrophic

The watershed of Lake Sammamish, while not nearly as developed as around lakes Union or Washington, has had some of the highest rates of development in the region. Historically, agricultural lands were scattered throughout the basin and urban and suburban uses concentrated in a few small communities such as Issaquah, Redmond, and east Bellevue. However, large areas of the basin have experienced rapid urban and suburban development as part of the overall growth in King County. New roads, housing developments, and urban areas are transforming the lake's forested watershed into urban and suburban land uses. These impacts, associated with changing land use, present a challenge to maintaining the high quality of this lake.

How the point and non-point pollutant loading impact the lakes and streams in the Sammamish watershed, and some of the proposed methods for addressing the problems were identified in the 1996 Lake Sammamish Water Quality Management Plan. (Click here to learn more about the Lake Sammamish Water Quality Story). The Plan identified three annual goals using water clarity (Secchi disk transparency), chlorophyll-a (a measure of algal biomass) and total phosphorus (important nutrient for algal growth). The following tables list how Lake Sammamish has measured up to these goals since 1997.

  • Mean summer (June - September) transparency of 4.0 meters or greater measured at mid-lake stations 611 and 612,
  • Mean summer (June - September) chlorophyll-a concentrations of 2.8 μ/L or less,
  • Annual mean volume weighted total phosphorus concentration of 22 μg/L or less.

Lake Sammamish mean annual volume-weighted total phosphorus, and mean summer epilimnetic total phosphorus, chlorophyll a, and Secchi depth collected at the north mid-lake station (0611) and the south mid-lake station (0612).

Station 611 Mean Annual Volume Weighted
Total Phosphorus (μg/L) Calandar Year
Mean Summer
Total Phosphorus (μg/L)
Summer Chlorophyll-a (μg/L) June-Sept Summer Secchi Depth (meters) June-Sept
Goal* < 22 < 2.8 > 4.0
1997 2013 2.4 5.6
1998 148 2.3 6.3
1999 1812 3.9 4.0
2000 1611 4.5 4.8
2001 1912 2.7 6.2
2002 1711 3.1 4.6
2003 1512 3.4 5.4
2004 2217 3.7 5.0
2005 1913 4.8 5.3
2006 22 11 4.3 4.3

Station 612 Mean Annual Volume Weighted
Total Phosphorus (μg/L) Calandar Year
Mean Summer
Total Phosphorus (μg/L)
Summer Chlorophyll-a (μg/L) June-Sept Summer Secchi Depth (meters) June-Sept
Goal* < 22 < 2.8 > 4.0
1997 1812 2.9 4.3
1998 138 2.7 5.7
1999 1610 3.6 4.2
2000 168 4.0 4.6
2001 1610 2.5 6.8
2002 1712 3.0 5.1
2003 1712 3.3 5.7
2004 2016 3.3 5.2
2005 1913 4.8 5.4
206 21 12 4.4 4.5

* As per the 1996 Lake Sammamish Water Quality Management Plan

Cells shaded blue indicate water quality goals have been met. Cells shaded green indicate goals were not met.

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Updated: 3/12/2014 12:00:00 AM