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The Lake

Lake in Transition

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. While the East Lake Sammamish Basin (16% of lake’s watershed) is one of the fastest urbanizing areas in King County, more than 80% of the Issaquah Creek basin (70% of the lake’s watershed) remains undeveloped. A large portion of the Issaquah Creek Basin remains in forest use, including DNR’s only “urban forest”—the Tiger Mountain State Forest, approximately the same size as the East Lake Sammamish Basin. 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. 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 has completed this latest 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. The developed shoreline is divided into 929 tax lots and zoned residential.

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.

In addition to direct property damage and delta formation, floods and erosion also damage the fisheries. 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.

Toxic wastes in non-point runoff harm water quality. Flooding increases the delivery of toxins from lawns, gardens and roads into the lake. Petroleum products and herbicides are serious concerns but phosphorus remains the number one threat for Lake Sammamish. This naturally occurring substance is necessary for life in very small quantities. However, rapid urbanization 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 fertilizer so it 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.

Studies of Lake Sammamish by government agencies and the University of Washington conclude that without reduction of phosphorus loading to the lake, its water quality will deteriorate to levels last seen when the city of Issaquah used to dump its sewage directly into the lake. Metro was formed in the early 1970’s, at huge public cost, to restore good water quality in Lake Washington and Lake Sammamish. Sanitary sewers and treatment plants were built to achieve the community’s clean water goal. But there is no easy “fix” for non-point pollution–prevention is the best and most cost effective technology available.