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Issaquah Creek Basin and Nonpoint Action Plan summary

Issaquah Creek Basin and Nonpoint Action Plan summary

Issaquah Creek Basin and Nonpoint Action Plan summary


The Issaquah Creek Basin and Nonpoint Action Plan describes the condition of streams and wetlands and proposes solutions to the problems of flooding, water pollution, and loss of habitat in the basin of Issaquah Creek and Tibbetts Creek. The creeks flow from steep headwaters in the southern basin into Lake Sammamish at the northern edge of the basin. Elevations range from more than 3,000 feet at the peak of Tiger Mountain to near sea level at the mouth of Issaquah Creek.

More than 80 percent of the basin remains in forest. The remainder is in pasture lands, dispersed residential lots, and a range of urban uses within the City of Issaquah, a community of 8,000 people. Continued population growth in the basin is expected to result in the expansion of the urban area and the conversion of forest lands in the upper basin to residential lots and subdivisions.

The plan was developed by a technical team staffed by the City of Issaquah and the King County Surface Water Management Division. The plan was prepared with the guidance of two committees, the interagency Watershed Management Committee and the citizen-based Basin Advisory Team. Funding was provided by the City, the County, and the Washington Department of Ecology.

Major surface water problems in the Issaquah Creek Basin

1. Flooding is widespread and recurrent in the lower Issaquah Creek basin. Much of the City of Issaquah is built on the floodplains of Issaquah and Tibbetts Creeks, and many homes and businesses in the city are flooded when the streams overflow their banks. Recent floods (in 1986 and 1990) caused extensive property damage and threatened to undermine several houses. Without further controls, future development in the upper basin will increase flood flows and aggravate downstream flooding conditions.

2. Additional controls on water pollution are needed to maintain the good water quality of Issaquah Creek. The water quality of Issaquah Creek is generally good despite localized pollution from urban sources, roads, and agricultural and forestry activities. This may change in the future, as urban areas expand and upland forests are cleared. Unless additional steps are taken to control pollution from future development, the amount of nutrients, sediment, and toxic materials that enter the stream system will increase substantially, degrading the streams and Lake Sammamish, which receives 70% of its inflow from the Issaquah Creek basin.

3. Habitat needs to be protected and restored to maintain populations of salmon and other fish and wildlife species. The habitat in the Issaquah Creek basin is unusually good for an urbanizing basin, and continues to support diverse and abundant populations of fish and wildlife, including six species of migratory salmon and trout. Some degradation in habitat quality is likely with additional development in the upper basin and within the City of Issaquah. Concerted actions to restore degraded areas and protect the areas that remain intact and productive will be needed to ensure that populations of fish and wildlife, including the salmon that spawn and rear in the stream, are preserved.

Solutions recommended in the plan

1. Reduce flooding and restore habitat by removing structures from floodplains. The plan recommends that the most severe flooding problems in the basin be resolved by removing some homes from the floodplain and restoring the ability of the channel and floodplain to safely carry floodwater. This solution has the added advantage of allowing the restoration of streamside habitat in the city. Landowner participation in the removal program will be purely voluntary. The scope of the program will be determined by the availability of funding and the willingness of landowners to participate. In order to estimate costs, the plan assumes removal of 27 homes on the lower segments of Issaquah Creek and the East Fork at a total cost of approximately $10 million. The removal program will be supplemented with floodproofing assistance for businesses and homes in less hazardous areas. Funding for both programs is expected to come from local, state, and federal sources.

2. Reduce future flooding and prevent damage to water quality and habitat by requiring new developments to retain forest land. In order to reduce surface-water runoff, pollution from sediment and nutrients, and loss of downstream habitat from upstream land clearing, the plan recommends that new developments be required to include areas that will remain in undisturbed forest. Where feasible, these forested areas will be maintained in tracts that are separate from developable lots. These requirements will be implemented through changes in rural residential zoning. When combined with other mitigation measures in the plan, this restriction on clearing is expected to dramatically reduce the future increase in flooding and pollution from continuing development in the Issaquah Creek basin.

3. Protect riparian and aquatic habitats that remain in good condition, and restore those that have been degraded. The plan proposes that several regionally significant stream segments within the Issaquah Creek basin, such as lower Carey Creek and North Fork Wetland 7, get additional protection from disturbance through higher standards for drainage facilities in new developments. These standards will be enacted through amendments in the King County Surface Water Design Manual. These areas will also be targeted for capital improvement projects to restore habitat.

4. Address individual drainage and habitat problems through capital improvement projects. The plan recommends the construction of more than 40 capital improvement projects to solve individual drainage and habitat problems. Twenty-nine of the projects, with a cost of $6 to $7 million, are identified as particularly important to address immediately. Funding for these projects will come from City and County surface water management fees, other agency budgets, and other sources. Approximately $2.5 million for capital projects in the Issaquah basin is included within the 1995-97 SWM bond proposal.

5. Reduce pollution from all existing and predicted sources through regulation, monitoring, enforcement, and education. The plan recommends a variety of education and monitoring programs to address water pollution from dispersed, nonpoint sources. Where sources of pollution can be readily identified, such as runoff from I-90 into the East Fork, the plan recommends capital improvement projects. Pollution from additional development in the basin will be minimized through the clearing restrictions and drainage system requirements mentioned previously.


The Issaquah Creek Basin and Nonpoint Action Plan was adopted in 1995 and is described in chapter 20.14 of the King County Code.  

For additional information about the Issaquah Creek Basin and Nonpoint Action Plan, contact Tom Beavers, Issaquah-Bear Creek Basin Steward.