Cedar River Basin

The King County Flood Control District invests resources in the Cedar River Basin to protect public safety, the regional economy and critical infrastructure.

For the purposes of the Flood District’s budgeting and planning, the Cedar River Basin includes the Cedar River, Issaquah Creek, and the Sammamish River.

The majority of the Sammamish River lies within incorporated areas including Kenmore, Bothell, Woodinville, and Redmond. A wide variety of land uses are found along the river, which was once almost exclusively agricultural. In and around city cores, land uses include moderate-density residential, business districts, and some light industrial uses.

Information about individual rivers in the basin are available below.

Cedar River

The Cedar River flows 45 miles from its headwaters in the Cascade Range to its mouth at Lake Washington, ultimately draining to Puget Sound. Divided by several dams, the upper and lower river areas differ distinctly in character and management.

The City of Seattle owns and manages a 90,000 acre protected area along the upper river known as the Cedar River Watershed. The Cedar River Watershed provides drinking water for 1.4 million people in the greater Seattle area and hydroelectric power generation. Residences, commercial businesses, and industrial facilities are located along the lower part of the watershed, downstream of Landsburg. More than 60,000 people, live in and around this area, in the communities of Maple Valley, Ravensdale, and Renton. As the river enters Renton, one finds parks, single- and multi-family residential development, commercial development, and portions of the downtown business core.

During the winter flood season, shifting floodwaters can inundate, erode or shift areas of low-lying floodplain, channel banks, and active gravel bars. Steep landslide prone hillsides flank the Cedar River creating additional risks. Landslides can deposit a significant amount of material in the river, blocking a portion of the mainstem channel, backing up the river, and even rerouting its course, as was the case in February 2001 as a result of the Nisqually earthquake.

Issaquah River

Issaquah Creek headwaters originate from the steep slopes of Cougar, Squak, Tiger and Taylor mountains. The basin encompasses about 61 square miles and includes Holder, Carey, Fifteen-mile, and McDonald creeks as tributaries to Issaquah Creek. More than 75 percent of the basin is forested, with the remainder in wetlands and pastures. Over 40 percent of the land is in public ownership with Washington Department of Natural Resources, Washington State Parks, King County Parks, and City of Issaquah Parks. Less than 10 percent of the basin is comprised of urban and cleared areas, but this urbanized area is located in and around the floodplains of Issaquah and Tibbetts Creeks near Lake Sammamish.

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. Floods in 1986 and 1990 caused extensive property damage and threatened to undermine several houses.

Sammamish River

Investments in the Basin

Cedar River Capital Investment Strategy

A capital investment strategy proposes a sequence of coordinated projects to address the most critical flood and erosion risks and to restore habitat along a specified stretch of river or within a river basin.

The strategy highlights near-term, medium-term, and long-term projects and actions that were identified during an assessment of key problem areas and potential solutions.

Capital investment strategies are directed by the Flood Control District, completed by King County, and eventually approved as policy guidance by the District’s Executive Committee.

Cedar River Capital Investment Strategy

Current Basin Projects

Herzman Levee Setback and Trail Stabilization Project
The Herzman Levee Setback and Trail Stabilization project will provide flood and erosion risk reduction along a stretch of the Cedar River about four miles east of the City of Renton. The need for the project was identified in the Cedar River Capital Investment Strategy, completed in 2017, which identified flood and erosion risks and potential solutions throughout the lower Cedar River valley.

Jan Road Neighborhood Improvements
This project undertakes improvements to Jan Road to minimize flood risk. Alternatives include raising Jan Road to improve access during flood events; setting back or removing potions of the Jan Road and Rutledge-Johnson Levees in order to reduce potential damage to the downstream Cedar River Trail Levee, which protects portions of both the Cedar River Trail and the Maple Valley Highway; and acquiring at-risk homes. The project will be designed to ensure that equivalent or better flood risk reduction remains for the houses remaining behind the levees.

Maplewood Landslide and Flood Risk Reduction Feasibility Study
This project analyzes flood and landslide hazards and potential associated risk in the Maplewood neighborhood. It includes an analysis of existing levees to see if levee-modifications could reduce flood risks. If the landslide hazard assessment indicates the potential to increase flood risks, an additional feasibility study will be conducted to evaluate options to mitigate landslide hazards.

Sammamish River Capital Investment Strategy
This project develops a capital investment strategy for the entire length of the Sammamish River from Lake Sammamish to Lake Washington.

Willowmoor Floodplain Restoration Project
This project reconfigures the outlet from Lake Sammamish to the Sammamish River in order to maintain or reduce current level of flood risk in the downstream river channel and along the lake shore. The project will be designed in a manner that reduces flooding impacts and improves conditions for fish listed on in the Endangered Species Act and other wildlife in the transition zone between the lake and the river.

Completed Basin Projects

Belmondo Revetment Enhancement Project
Cedar Rapids Levee Setback Repair Project
Cedar River Corridor Planning
Cedar River Gravel Removal Project
Elliott Bridge Reach Mitigation Project
Rainbow Bend Levee Removal and Floodplain Reconnection Project
Riverbend Mobile Home & RV Park Acquisition