Lower River Restoration
Walsh Lake Restoration
From 1904 to about 1947, the town of Taylor, WA was a large mining and manufacturing community in the Cedar River Municipal Watershed. In the 1930’s, the City of Seattle constructed the Walsh Lake Diversion Ditch (Ditch) to divert the water contaminated by this community. The water was intercepted just above the natural confluence with Rock Creek, a tributary of the Cedar River in the Municipal Watershed, and conveyed 1.7 miles to a discharge point on the Cedar River downstream of the Landsburg Diversion Dam (the diversion point for Seattle’s Cedar River municipal water supply). Following abandonment and decommissioning of the Taylor townsite in 1947, water from the 4.3 sq mi Walsh Lake Basin cleansed to the point where it is no longer considered polluted, making the Ditch obsolete.
View a slideshow on the confluence restoration project.
The Ditch and adjacent slopes were prone to periodic failures due to the Ditch’s problematic location in a steep ravine above Rock Creek. Most recently, a 300-foot section of the Ditch catastrophically failed during a January 2009 storm event, effectively disconnecting and dewatering the downstream section of the Ditch. Because the Ditch was no longer needed, SPU did not repair it, but rather, in 2012 restored the confluence of the Walsh Lake and Rock Creek stream systems. This was accomplished through:
- Removal of significant historic fills
- Reconstruction of functional floodplains
- Reconstruction/enhancement of the natural stream confluence conditions
- Dewatering, refilling, and slope stabilization of the Ditch
The project included removal of approximately 600 feet of road and three stream crossings. Complete excavation of the 40.5 Road as well as partial removal of the 18 and 40 Roads resulted in the removal of roughly 5,000 cu yards of fill from the Rock Creek / Walsh Lake confluence. Removal of road fill and reconstruction of low floodplains in this reach was designed using hydraulic models (HEC-RAS) with the objective of restoring natural geomorphic processes and self-sustaining channel attributes beneficial to the recolonization of anadromous salmonids (coho) in this system. Removal of two stream crossings on the 40 Road (Rock Creek crossing via three undersized culverts and the Walsh Lake Ditch bridge) also improved fish passage and connectivity to high quality habitat in the upper Rock Creek and Walsh Lake catchments.
To restore the confluence to a stable profile and pattern, filling sections of Walsh Ditch above and below the diversion structure was required. Approximately 100 feet of the Ditch above the water diversion structure (on the 18 Road) as well as 300 feet of the Ditch below the diversion were filled. Using these lengths, approximately 700 cu yards of fill comprised of alluvium and glacial outwash sands and gravel currently underlying the roads was placed within the Ditch. The third stream crossing structure, consisting of two pipes used to divert water into the Ditch below the 18 Road, was also removed and large wood placed to stabilize the site and provide fish habitat.
In addition to the removal of roads at the historic confluence, a portion of the 40 Road paralleling and bounding the Ditch west of the confluence was also decommissioned. To restore the natural hydrography with the least impact to the seeps and springs within and above the former Ditch, the Ditch berm was excavated at 27 locations along approximately 6,500 feet between the diversion structure and the 42 Road. This project element maintained proper steep-slope drainage and avoided impoundments by the perched Ditch. The project also scarified, and placed seed and straw on the road surface to support the planting and establishment of native plants.