South Fork Walla Walla River Floodplain Enhancement I

A large flood event that occurred in February 2020 caused changes to several previously completed floodplain and fish habitat restoration projects in the South Fork Walla Walla River. These changes have triggered adaptive management as defined in the Monitoring and Adaptive Management Plans developed for those previously completed projects.



The adaptive management plan identified the proximity of the river to the roadway at the location of this project as a risk that needs to be actively monitored and immediately addressed if the river were to encroach too close to the road, as the road is the only access to the homes upstream of the project site. The adaptive management plan identifies a 40-foot minimum buffer distance between the edge of the channel and the edge of the roadway. The channels proximity to the roadway and risk of further bank erosion triggers immediate adaptive management actions to protect this infrastructure. Direct bank stabilization was not a goal, but in order to ensure the continued success of the 2015 Project we proposed actions to encourage sediment deposition on the right bank and shift the thalweg away from the roadway. The intent of this adaptive management action is to increase habitat complexity and pool habitat availability while also protecting the roadway from being impacted from future channel migration. This will be achieved by strategic placement of large wood to encourage sedimentation behind the structures on the outside of the bend.



The overall purpose of the project is to improve conditions for ESA-listed and non-ESA-listed fish species while mitigating the embankment erosion on the outside of the river meander. This project proposed a series of large wood structures on the outside of the meander to encourage the thalweg away from the road. The design was not intended to create an armored bank.



This adaptive management project was completed between July 15 and August 4. A contractor was solicited and selected to provide materials, equipment, and labor to install one large wood structure on the previously completed South Fork Walla Walla River RM 6-7 floodplain restoration project. This adaptive management action is taken in accordance with the site-specific monitoring and adaptive management plan and is intended to act in concert with the existing project features to increase flow volumes and velocities into a constructed side channel inlet. The project was constructed as planned and without incident.



One engineered log structure was built at the upstream end of the SFWW RM 6-7 floodplain restoration project to enhance flow into a constructed side channel inlet. The structure was composed of six logs with rootwads and five embedded vertical members.



Note: Projects 22-Umat-01 and 22-Umat-05 addressed the same work with metrics to be allocated between the two. 22-Umat-01 is accomplished with IIJA funding while 22-Umat-05 is with base funding.

Adding the large wood structures will continue the work from the previous 2015 and 2018 projects. It will continue to provide connectivity to the floodplain and offer a wide variety of habitats to native fishes. It will add roughness during high-flow events. The large wood debris will provide pooling habitat for different fishes. It will allow sediments to deposit behind the structures and encourage the thalweg away from critical infrastructure adjacent to the 2015 Project.