Kentch Reach River Restoration

The Kentch Reach on the South Fork Walla Walla River has been subject to large-scale human disturbance for more than 100 years. Timber and agricultural uses involved straightening the river channel and isolating the river from its historic floodplain. The result was a fast-moving conduit that was virtually devoid of native fish spawning and rearing habitat.

The Confederated Tribes of the Umatilla Indian Reservation (CTUIR) purchased the site in 2003 for the purpose of restoring fish and wildlife habitat. The tribe hoped to restore the original river ecosystem along with its meandering course across the floodplain. To realize this dream, this project needed to focus not just on building habitat, but also on rebuilding the river’s original physical processes.

The fate of several at-risk fish species hung in the balance during project construction. The river contained multiple ESA-listed species, and historically low water flows in 2015 created an additional threat. A single misstep in design or implementation could have wiped out a downstream hatchery where wild fish had been temporarily placed for their protection.

Approach

• With no historical reference point to draw on, the GeoEngineers team used cutting-edge analytical and quantitative methods to reestablish the intricate physical and ecological processes that existed before humans intervened. The team predicted natural conditions at the reach using several methods that quantified discharge, slope, width, depth, planform, bed-material size, and riparian conditions. These calculations enabled the GeoEngineers’ design team to select the optimum channel alignment for the transportation and deposition of sediments—a key success factor for building fish habitat.
• The team used the HOME (Habitat Optimization Modeling of Ecosystems) model to visualize and validate how different river channel designs would increase habitat in the reach under different conditions. HOME is a GeoEngineers-developed software tool that combines hydraulic modeling and quantitative habitat data to predict how restoration alternatives will affect habitat for focus fish species and life history stages at a range of flows.
• The restoration included extensive revegetation along the new river channel, but the project team was concerned that dry conditions would limit plant growth. GeoEngineers took the unusual extra step of conducting a groundwater study to determine hyporheic connectivity and the planting depths required to reach the water table.
• Unlike many restoration approaches, which require ongoing maintenance, the GeoEngineers’ design allowed the river to respond and adjust to natural events like flooding and debris accumulation in a way that preserves habitat.
• To protect the sensitive fish populations downstream, the team went to great lengths to limit turbidity, matching the site construction with the fish-treatment schedule and designing a silt fence at the hatchery intake. These measures worked so well that the CTUIR eventually removed the silt fence during construction, and none of the critical hatchery fish were lost during the project.

Results

The project reclaimed and restored 47 acres of floodplain, increased channel length from about 3,500 feet to 4,300 feet, increased side channel length from about 160 feet to 3,500 feet, and roughly doubled available habitat. With sensitive fish populations hanging in the balance, the GeoEngineers team took every precaution during construction to avoid affecting fish hatchery operations downstream.

The positive impacts of the restoration were immediate. Juvenile fish-rearing habitat exceeded expectations and spring chinook were documented spawning in the rebuilt river immediately after construction was complete. Thanks to cutting-edge analysis and modeling, the team created a sustainable river design that is restoring fish populations now, and will adapt to natural ecosystem changes to provide a stable habitat for the long term.