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 Epilimentic Total Phosphorus (μg/L) June-Sept	Summer Chlorophyll-a (μg/L) June-Sept	Summer Secchi Depth (meters) June-Sept
Goal*	< 22		< 2.8	> 4.0
1997	20	13	2.4	5.6
1998	14	8	2.3	6.3
1999	18	12	3.9	4.0
2000	16	11	4.5	4.8
2001	19	12	2.7	6.2
2002	17	11	3.1	4.6
2003	15	12	3.4	5.4
2004	22	17	3.7	5.0
2005	19	13	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 Epilimentic Total Phosphorus (μg/L) June-Sept	Summer Chlorophyll-a (μg/L) June-Sept	Summer Secchi Depth (meters) June-Sept
Goal*	< 22		< 2.8	> 4.0
1997	18	12	2.9	4.3
1998	13	8	2.7	5.7
1999	16	10	3.6	4.2
2000	16	8	4.0	4.6
2001	16	10	2.5	6.8
2002	17	12	3.0	5.1
2003	17	12	3.3	5.7
2004	20	16	3.3	5.2
2005	19	13	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.