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Overview

What is Sediment?

In the context of river and floodplain management, sediment refers to the soil, mud, sand, and gravel moved by rivers. Erosion and sediment movement in rivers are part of a natural process, but modifying rivers with flood control structures, such as levees, changes how sediment moves through rivers.

Tolt River Sediment

The Tolt River levees restrict the river and prevent sediment from being deposited in the adjacent floodplain. As a result, the Tolt River is experiencing a buildup of deposited sediment in the river channel, which takes up space for water flow and increases flood hazard and risk.

The Tolt River Sediment Management Feasibility Study will evaluate sediment management alternatives, including sediment removal and levee setbacks, to determine which alternative is a feasible and effective flood risk reduction tool for the lower Tolt River. The study will also consider how much sediment is being produced in the upper river watershed, moving down the river, and depositing in the lower river channel.

The study will focus on the lower Tolt River south of the City of Carnation between River Miles 0.0 and 1.8.

Tolt River Sediment Management Feasibility Study Fact Sheet - 1.9MB PDF

Map showing the tolt river sediment management study area
Project Area
Photo showing gravel bars in the Tolt River - May 2017
Gravel bars in Tolt River looking upstream from Snoqualmie Trail Bridge (2017)

Goal

  • Determine if sediment removal is a feasible and effective flood risk reduction tool for the lower Tolt River between River Mile (RM) 0.0 and RM 1.8.
  • Update analysis of sediment production in the upper Tolt River basin and analysis of sedimentation rates in the lower 1.8 miles of the Tolt River.
  • Engage with tribes, residents, landowners, and other interested parties to seek input about sediment management alternatives in the lower 1.8 miles of the Tolt River.
  • Integrate the Tolt River Sediment Management Feasibility Study with two other concurrent studies on the Tolt River: the SR 203 Bridge Improvements Feasibility Study and the Tolt River Level of Service Analysis.

Timeline

Benchmark Date
Project planning Early 2019
Technical studies and alternative development Summer - Fall 2019
Evaluate alternatives and cost-benefit analysis Early 2020
Draft study report Summer 2020
Finalize study report and share with public Fall - Winter 2020

Links

Tolt River Corridor Planning and Capital Investment Strategy website

About the King County Flood Control District

The King County Flood Control District is a countywide special purpose government created to protect public health and safety, regional economic centers, public and private properties, and transportation corridors.  The members of the King County Council serve as the governing body for the District. The Water and Land Resources Division of the King County Department of Natural Resources and Parks carries out the approved flood protection projects and programs for the District under an interlocal agreement. Information is available at the King County Flood Control District.


For more information about the Sediment Management Feasibility Study, please contact, Teresa Lewis, Project Manager, King County River and Floodplain Management Section, Water and Land Resources Division, Department of Natural Resources and Parks.