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King County Flood Control District issues study of Tolt River channel migration zone


View maps and ask questions about a six-mile channel migration zone along the Tolt River at a May 8 meeting with King County floodplain managers, King County Flood Control Supervisor Kathy Lambert and other County representatives.


The King County Flood Control District has issued aFlood Control Zone District Logo draft study of the Tolt River’s channel migration zone (CMZ), mapping those areas geologists consider at risk due to natural changes in the river’s location.

The Flood Control District and County officials will host a public meeting to discuss the migration zone from 6-8 p.m. Monday, May 8, at the Sno-Valley Senior Center at 4610 Stephens Ave. in Carnation. Flood Control District Supervisor Kathy Lambert will attend the meeting.

Channel migration zones are mapped and regulated by King County as critical areas. The natural movement of a river across its floodplain can occur progressively – when erosion on one bank is matched by deposition on the opposite bank – or suddenly, when a channel jumps from one alignment to another.

This draft study identifies areas potentially at risk of channel migration along the lower six miles of the Tolt River, starting from the upstream end of the Tolt River Road and extending to the river’s confluence with the Snoqualmie River. The Tolt is considered one of the most active rivers in the county.

“I encourage residents who live near the Tolt to attend this meeting, ask questions and learn more about the river’s natural channel migration hazards,” said Supervisor Lambert.

Josh Baldi, director of the County’s Water and Land Resources Division, said the CMZ study is based on detailed LiDAR images, aerial photographs and other new information that have enabled geologists to better understand the river’s high level of activity.

“The Tolt River is young, dynamic and powerful,” he said. “This is the kind of solid, science-based information that can help residents and communities make sound decisions and plan for their futures.”

The draft CMZ study and accompanying map for the Tolt River will undergo a 45-day review and public comment period that ends May 31. Once incorporated by amendment into King County’s channel migration public rule, new land development applications within the CMZ will be subject to the County’s existing zoning code regulations.

The May 8 public meeting will give participants a chance to learn more about the channel migration zone along the Tolt River and hazard-area regulations. The first 30 minutes will be an open house, where participants can view maps and talk to staff, followed by presentations by county representatives from the Department of Permitting and Environmental Review (DPER), which oversees permitting, and the Water and Land Resources Division, which mapped the migration zone. The open house will resume at 7:40 p.m.

Attendees can submit written or oral comments at the public meeting. Written comments can also be sent to John Bethel at or sent to his attention at King County Water and Land Resources Division, 201 S. Jackson St., Suite 600, Seattle, WA, 98104.

The draft CMZ report and map may be revised based on comments received. A final Tolt River CMZ map will be adopted by DPER, consistent with provisions of the public rule. In addition to regulating new land development within affected areas, the Tolt River CMZ map and study will inform implementation of Tolt River risk reduction projects.

The draft study and maps can also be reviewed at the Carnation Public Library at 4804 Tolt Avenue and at DPER’s office at 35030 S.E. Douglas St., Snoqualmie.

For more information about the draft Tolt River CMZ study, map or public meeting, contact John Bethel at If you have questions about the King County channel migration public rule or CMZ regulations, contact Steve Bottheim with DPER at 206-477-0372 or

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The King County Flood Control District is a special purpose government created to provide funding and policy oversight for flood protection projects and programs in King County. The Flood Control District’s Board is composed of the members of the King County Council. The Water and Land Resources Division of the King County Department of Natural Resources and Parks develops and implements the approved flood protection projects and programs. Information is available at