The Walla Walla River winds through the traditional lands of the Cayuse, Umatilla and Walla Walla people in Oregon and Washington, and supports a wide variety of salmon and trout species—the core of the Pacific Northwest’s lucrative fishing industry. Unfortunately, more than a century of human disturbance in the form of dams, irrigation systems, and flood controls caused profound damage to the watershed and its ecosystems.
The Confederated Tribes of the Umatilla Indian Reservation (CTUIR) has spent more than two decades systematically restoring aquatic habitat throughout the Walla Walla River watershed. The Walla Walla River Forks Project is part of this overall restoration program and one of the largest and most complex projects CTUIR has completed in the watershed.
GeoEngineers’ river science team has a long record of collaboration with CTUIR and was proud to continue the partnership by leading design for the Walla Walla River Forks project, which aimed to enhance fish and wildlife habitat near the confluence of the North and South Fork Walla Walla Rivers, about five miles southeast of Milton-Freewater, Oregon. The project area included the South and North Fork and continued downstream of the confluence into the mainstem Walla Walla River—about 4,000 linear feet of river channel in total.
The local agricultural industry depends on river water for irrigation. As a result, the rivers through this project area had been modified to block the floodplain and divert water to cropland throughout the valley. These modifications prevented natural river processes, threatened fish, and reduced habitat quality. The GeoEngineers/CTUIR team had to find sustainable solutions that would meet both water use and ecological objectives—and they did it in an extremely complex and dynamic watershed.
Reconnecting a Historical Floodplain
Well-intentioned changes to the watershed had reduced habitat quality and flood capacity over the years. Instead of a resilient, interconnected floodplain of wetlands and natural channels, the project was artificially restricted in unsustainable ways.
These changes ranged from minor channel alterations and temporary irrigation dams to an 800-foot containment levee built in the 1960s that cut the river off from a large swath of its historical floodplain. Habitat quality suffered, and downstream communities faced much greater flood risks as a result. These risks were demonstrated dramatically when a greater than 100-year flood swept through the reach in 2020 as the project was being designed. CTUIR and the project team knew that a reconnected floodplain would mitigate these risks by improving flood capacity.
A disconnected floodplain also meant fewer sheltering wood masses, low-velocity pools, riparian vegetation, and riffles (shallow, fast-moving areas of a stream). These are essential features for spawning and rearing fish, but they also promote healthy stream evolution by influencing channel currents and sedimentation. By adding large woody material (LWM) and riparian vegetation throughout the carefully designed channel and floodplain, the team built a sustainable ecosystem that will be maintained by natural watershed processes into the future.
The project team removed the levee, regraded the area, and added several pilot channels to the floodplain as a more sustainable means of flood control. In total, the team designed more than 2,200 linear feet of side channels through new riparian habitat, dramatically increasing juvenile salmonid rearing habitat. The reconnected floodplain will support fish and wildlife while attenuating flood risks to downstream communities.
Balancing Agricultural Needs and Fish Passage
Population levels of salmon, steelhead and other critical fish species have plummeted to crisis levels in the past 50 years, impacting overall ecological health and the Pacific Northwest’s culturally significant fish industry. Dams, undersized culverts, and other man-made infrastructure play a role in fish declines. Physical barriers and narrow channels with unnaturally rapid flow rates often prevent fish from accessing critical spawning and rearing habitat. Correcting these issues was a primary goal of the Walla Walla River Forks project
For decades, nearby farmers had been erecting temporary gravel push-up dams in the rivers to divert irrigation water during the spring and summer months. This irrigation strategy was expensive and time-consuming, costing farmers thousands of dollars and hundreds of man-hours each year. The dams weren’t ideal for river habitat either. They blocked fish passage, sometimes even diverting them into dry channels, and artificially altered channel behavior.
GeoEngineers’ project team collaborated with the landowners/water users to find irrigation designs that could maintain fish passage, protect aquatic habitat, and deliver water more efficiently and sustainably. The solution included two roughened channels, one each along the North and South Fork. These channels created naturally stable upstream pools where farmers can reliably divert irrigation water without blocking fish or damaging habitat. By respecting both the environment and local landowners, GeoEngineers found sustainable solutions that will attenuate flood risks while supporting a healthy ecosystem and agriculture production for generations to come.
When construction finished in fall 2022, the project reach had greater flood storage capacity, increased floodplain connectivity, significantly improved fish and wildlife habitat, unencumbered fish passage, and reliable irrigation water delivery. Riparian vegetation and a variety of specially engineered log jams and other LWMs are already protecting young fish and promoting healthy stream development.
As CTUIR and other organizations continue their long-term efforts to undo more than a century of environmental damage in the Pacific Northwest, GeoEngineers hopes that the Walla Walla River Forks Restoration Project can serve as an example of how even the most complex river ecosystems can be sustainably restored.