Walla Walla River Forks Floodplain Reconnection and In-stream Enhancement Implementation

The design for the Forks project has increased the channel complexity and morphology closer to the historical form and function. With the addition of multiple large wood structures, the instream velocity range is more diverse at high and low flows. This project gained different types of fish habitats and river complexity by adding wood structures instream and on the floodplain. The large wood structures create thermal diversity throughout the year and help with sediment transportation and redistribution. Removal and partial removal of a levee reconnected side channels and flood plain. The connection of the floodplain allows for the frequency of inundation. Riparian corridors were replanted with native plants and reseeded with native grass mix to gain better function.

This project will increase floodplain connectivity, increase in-stream habitat complexity, increase off-channel habitat availability, and remove barriers to fish passage in high quality spawning and rearing habitat for ESA-listed Middle Columbia Summer Steelhead as well as other native fish species. To the extent practical, our design focused on increasing the quality and quantity of habitat for juvenile steelhead and other fish species. Our analyses show that for the high-flow conditions, juvenile rearing habitat will experience the most substantial uplift owing to the added roughness features in the mainstem channels and reconnection of considerable floodplain habitat. Winter rearing conditions will also improve with the creation of pools associated with large wood structures.