Nursery Bridge Reach Phase II Fish Passage and In-stream Habitat Enhancement III

The Nursery Bridge Reach of the Walla Walla River is one of two major fish passage bottlenecks in the Walla Walla subbasin that currently impact recovery of Middle Columbia River summer steelhead, Columbia River bull trout, and spring Chinook salmon. Over a century of impacts from urbanization, channelization, and agriculture has resulted in extreme constriction of the Walla Walla River, reducing its nearly 5-mile wide active alluvial fan to the current floodprone width of approximately 300 feet. The Nursery Bridge Reach is a highly altered portion of the Walla Walla River near Milton-Freewater, Oregon, that includes several road and railroad crossings, a significant point of diversion for senior water users of the Walla Walla River Irrigation District (WWRID), and an extensive US Army Corps of Engineers-authorized flood risk management project consisting of a levee system and a large grade control structure.



The Nursery Bridge Phase II Fish Passage and In-stream Enhancement Project (hereafter “the Project”) is intended to address the factors identified above that impact ESA-listed salmonids. The objective of this project is to provide stable fish passage connectivity to the Eastside Fishway at the GCS, reliable inflows to the WWRID diversion, and habitat for salmonids over a range of flows.



Fish passage stability will be achieved through a combination of designed features. An armored bar immediately upstream of the GCS is intended to realign the channel with the Eastside Fishway entrance and the WWRID diversion. The bar will be made of large material that will be less mobile in the high-energy, constricted channel to create a more stable alignment for the channel in a more favorable geometry for maintaining fish passage through the Nursery Bridge reach. In conjunction with the armored bar, a shallow notch in the GCS and an approximately 350-foot roughened riffle low flow channel will be constructed to maintain mobility of bedload through the reach and a stable low-flow channel directly upstream of the GCS across the Eastside Fishway and WWRID diversion entrances. The proposed channel alignment and designed structures will reduce the need for annual dredging and provide access to critical sections of the channel that would allow for maintenance without mobilizing equipment into the stream.



Instream restoration actions upstream of the GCS and roughened riffle include excavating approximately 3,258 linear feet of side channel to reinitiate perennial flow and installing 34 engineered log jams in the main channel and in side channels. Habitat enhancement features are designed to improve salmonid habitat for various life stages over a range of potential flows, as well as maintaining the current level of flood protection provided by the existing levees.



This project is being collaboratively developed with a diverse collection of stakeholders including subbasin comanagers at the ODFW, CTUIR Fish Passage Operations, the Walla Walla Watershed Council, the Milton-Freewater Water Control District, and the Walla Walla River Irrigation District. Each of these entities has a vested interest in the continued functionality of the reach for fish passage, water quality and quantity, efficient irrigation water delivery, and adequate flood risk mitigation for the community of Milton-Freewater, Oregon. Each stakeholder has reviewed each iteration of the design and provided comment on how to better achieve the shared goals of the project. Co-funding and consequent proposed metrics were reported for the previous year of this project (PCSRF Project# 20-Umat-02). This years project solely reflects work to be completed with 2021 PCSRF funds for this project, which will be spent on subcontract fees for constructing the roughened riffle structure to enhance fish passage at the GCS. The proposed metrics for the overall improvement on this reach were previously reported in Project #20-Umat-02, so

Limiting factors for Middle Columbia River summer steelhead in the Nursery Bridge Reach include: blocked and impaired passage, degraded floodplain connectivity and function, degraded water temperature, altered sediment routing, and hydrologic alterations (NMFS 2009). Due to the infrastructure constraints in the reach, it may not be feasible to address all of the identified limiting factors; however, this project intends to address fish passage and floodplain connectivity and function in the reach.



The proposed GCS notch and low flow channel in the immediate vicinity of the GCS will create a more stable channel adjacent to the entrance to the Eastside Fishway. Providing reliable fish passage at the Nursery Bridge GCS allows access to over 70 miles of some of the highest quality spawning and rearing habitat in the Walla Walla Subbasin, located within the North and South Forks of the Walla Walla River and Couse Creek. This project would increase the efficacy of the Eastside Fishway at the GCS and allow volitional passage for all life stages of ESA-listed salmonids to meet ODFW and NMFS fish passage requirements.



Degraded floodplain connectivity and function will also be addressed as a result of this project. Floodplain connectivity will be enhanced by reinitiating 3,258 feet of side channel which will increase floodplain activation at lower flows to provide off channel summer rearing habitat and low velocity refugia for winter rearing salmonids on the floodplain. Large wood additions will also increase complexity and functionality of the floodplain for juvenile salmonid rearing.



Due to the constrained nature of the flood control project in the Nursery Bridge reach, this project will likely not address the remaining limiting factors of degraded water temperature, altered sediment routing, and hydrologic alteration. These factors will be addressed in future phases of the Nursery Bridge reach project.