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The Lake

Lake in Transition

(Aerials Provided by Jerry Klein)


Lake Sammamish is a shallow, mesotrophic lake, approximately 8 square miles in size, draining a 98 square mile watershed, which is bisected by 13 miles of I-90.

The Lake Sammamish watershed stretches from Redmond through Bellevue, and Issaquah to Preston and Hobart, and empties into the lake through numerous small streams and rivulets. The largest of these, Issaquah Creek, provides about 70% of the inflow to the lake. This water system is in continuity with groundwater in the Issaquah aquifer.

Historically, the large volume of rain in this watershed (nearly twice the 35 inches a year that falls on Seattle) has been absorbed by the surrounding forests and, like a well-regulated reservoir, loose, sponge-like forest soils retained and then slowly released stored rainwater into the aquifer and surface streams.

Such a natural system prevents flooding in winter but continues to supply clean, cold water to streams and aquifer in the dry months.

Where forests have been replaced by impervious surfaces–roofs, roads, parking lots–the loss of rain storage capacity as well as a more rapid rate of runoff, can lead to flooding. The increased frequency of “100-year floods” over the years despite the absence of “100-year storms” is an identifiable consequence of land conversion from forest to subdivision.

A lake reflects its watershed. Conversion of the watershed from forests to cities causes deterioration of water quality. That means the problem is all of us, everyone who lives in the watershed or even drives through it!

Clearly we all have to be part of the solution.  We must rise to the challenges of protecting Lake Sammamish as an invaluable amenity in our community life!

Pair of adult Kokanee

Adult pair of Kokanee returning to spawn, photo by Roger Tabor USFWS​

Seven species of salmonid, both native and hatchery, and including commercially-important runs of coho and chinook from the Issaquah Hatchery spawn in and inhabit Lake Sammamish and its tributary streams. Bald eagles and a great blue heron rookery are dependent upon this ecosystem. Numerous migratory waterfowl use the lake and many species of wildlife are found around its shores. 

Salmonids, particularly, depend on a specific fresh water environment. They require cold, well-oxygenated water and loose, clean gravels to spawn successfully and for their eggs to incubate. Seven species of salmonid are found in Lake Sammamish, but at least one, a sub-species of kokanee peculiar to Lake Sammamish, is on the verge of extinction; shoreline spawning sockeye and steelhead have also disappeared from this system.


Lake Sammamish is a significant regional recreational resource with 1.6 million visits per year to the State Park (the most heavily used park in the State’s system), at the south end and heavy usage of Marymoor and Idylwood Parks at the north end.

On the eastern shore, Sammamish Landing Park is the newest park on the lake.

On the western shore Timberlake Park is popular as is Weowna Park. The city of Bellevue completed this park as a terminus of its completed Lake-to Lake trail linking Lakes Washington and Sammamish.

The Mountains to Sound Greenway prominently features Lake Sammamish. 


Of the many issues facing the lake, flood destruction of property is only one of many problems. Erosion is another.

Once vegetation has been removed, rain easily washes the exposed soils and sediments downhill into creeks and into the lake.  

Soils in this watershed are high in phosphorus, a fertilizer. Deltas of mud and rock form in the lake at outfalls of creeks and drainage culverts, smothering gravels with sediment, filling coves, and turning lake water murky.

This process can be dramatic, for example a single storm in January 9, 1990 transported over 40,000 tons of soil, rock and sediment into the lake at the mouth of Lewis Creek, following clearing of only 2% of the North Village project on Cougar Mountain; this despite developer projections that the entire project would result in the transport of less than 10,000 tons total of material into the lake.




The developed shoreline of Lake Sammamish is divided into 929 tax lots and zoned residential.  These lots have changed dramatically in character in the last 20 years.  The rapid urbanization of both the shoreline and the basin has led to increased phosphorus loading to the lake from non-point sources– soil erosion, garden fertilizers, detergents, pet wastes and failing septic systems.


Phosphorus is a naturally occuring fertilizer that exists in the native soils.  The increased load of phosphorus from other sources and from the soil itself stimulates growth of aquatic plants, including dense algae blooms that turn the lake a cloudy green and form surface scums.

As the algae die and decay, the lake looks and smells bad, repelling swimmers and boaters. The decaying algae also rob the water of the dissolved oxygen needed by fish, sometimes precipitating dramatic fish-kills.

Other components often found in run off are herbicides and petroleum products.

The cleaner the water is going into the lake the better it is for everyone and everything.

Flooding and Erosion
Phosphorous and More
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