South Fork Walla Walla River Floodplain Enhancement II

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.


This project proposed a new large wood structure and selective removal of flood debris, primarily woody material, from the existing wood structure. The new structure was placed on the right bank point bar just upstream of the channel split. The intention of the structure is to help direct water towards the side channel and provide additional roughness and habitat improvements during high flow events. PCSRF funds were used to purchase materials and construct the structure designed to maintain full functionality of the 2018 Project.


5 engineered log structures were built at the upstream end of the SFWW RM 5-6 floodplain restoration project to enhance in-stream habitat, maintain floodplain function, and mitigate unintended impacts on infrastructure. Each structure was composed of 6 logs with rootwads and 5 vertical members. The array of log structures is intended to encourage the thalweg of the South Fork Walla Walla River away from the county road, scour pools and provide juvenile and adult salmonid spawning and rearing habitat, and encourage sedimentation on the river right 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 5 large wood structures on the previously-completed South Fork Walla Walla River RM 5-6 floodplain restoration project. This adapative management action is taken in accordance with the site-specific monitoring and adaptive management plan and is intended to enhance instream fish habitat while maintaining floodplain functionality and mitigating unintended impacts on the adjacent county road. The project was constructed as planned and without incident.


Note: Projects 22-Umat-01 and 22-Umat-05 addressed the same effort 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.