Sandy-Salmon Floodplain Reconnection
AT A GLANCE
This project removes strategic portions of the existing levee, followed by installations of large wood structures that mimic natural log jams in the opened side channels.
418
600
524
The Site
The confluence of the Sandy and Salmon Rivers represents one of the largest and most productive restoration opportunities in the Sandy basin. An initial opportunity analysis and conceptual design process in 2011 identified the confluence floodplain as the highest potential restoration opportunity in a 13-mile reach between the former Marmot dam site and the Zigzag River.
The floodplain extends nearly a mile, encompassing approximately 418 acres, and is owned primarily by the Bureau of Land Management and Clackamas County. Like much of the Sandy, the floodplain is isolated behind levees built after the record 1964 flood. Because of this, several historic side channels and backwater ponds are disconnected from the Sandy, except in extreme high flows.
Breaking the Levee from Sandy River Watershed Council on Vimeo.
The Project
- Slowing down the river's energy by spreading high water events across the floodplain.
- Installing large wood structures, riffles and gravel bars to mimic natural log jams.
- Restore the frequency of river water present in the floodplain
- Maximize habitat productivity prioritized for this reach and fish species using it
- Restore native vegetation
Construction for the two phase project began in summer 2019. SRWC is working with the Bureau of Land Management, Clackamas County, Clackamas Soil and Water Conservation District, Portland Water Bureau, National Fish and Wildlife Foundation, and local residents to reconnect the Sandy-Salmon floodplain and extend the Sandy’s corridor of healthy wild fish habitat.
Phase I (2019)
We installed more than 1,000 wood pieces in the reconnected channels in 4 island-and-terrace logjams. 524 rootwads and logs were either purchased by SRWC or donated by Clackamas County, and the rest of the wood was harvested on site. These large wood structures will enhance migrating and rearing habitat, assist with gravel recruitment, and disperse erosive energy during high water events. 600 feet of levee was removed, totaling 7,200 cubic yards.
Project results include the following:
- Floodplain acres restored: 418
- Stream miles enhanced: 2,700’ of clear – groundwater side channel and backwater habitat within a 1.2 mile reach
- Large wood placement: Over 1,000 pieces were installed in the reconnected channels and log jams. This will enhance migrating and rearing habitat, assist with gravel recruitment, and disperse erosive energy during high water events
- Partial levee removal: 600 feet of levee removed and 0.75 acres of floodplain contoured to reconnect historic habitat and restore hydraulic function.
- Community engagement: Pre and post project tours have featured over 50 participants. With the help of contract crews, future planting events aim to involve at least 100 volunteers.
Trail Impacts?
At project's end, parts of the existing levee will be gone to allow flow from the Sandy into side channels that are currently behind the levee, so within these areas the trail will descend to the level of the river and then will reconnect to un-impacted parts of the trails. Project plans also include building log jams at the channel openings, but while potentially rerouting small sections, those log jams will not prevent trail access either.
In higher river flows, we expect water to be running into those side channels, as indicated on the sketch on the sign we've posted. Much of the time, particularly in summer and autumn flow levels, as with the channel that has formed naturally in the upstream portion of the floodplain, the restored connections will be passable by foot. And we plan to work with the agencies that own the land around the project, Clackamas County and Bureau of Land Management, both to replant the forest around the project site and to ensure trail access comes back once the levee is breached and channels are restored.