Walla Walla Basin Habitat

Conservation Planning

Habitat Conservation Planning: Evaluating the Thermal Impacts of Streamflow and Irrigation Bypass in the Walla Walla River

The Walla Walla River Basin is an important aquatic resource that empties into the Columbia River downstream of the Snake River confluence and upstream of McNary Dam at approximately River Mile 315 (RKm 507). The main stem of the Walla Walla River originates in the Blue Mountains of eastern Oregon before crossing the state line into Washington near the towns of Milton-Freewater, Oregon and Walla Walla, Washington. Because of increasing concerns for resident bull trout and anadromous salmonid species in the basin, fish and water management agencies have begun to examine steps needed to protect several important rearing and spawning reaches in the river system. One issue concerns the impact of water temperatures in the main stem of the Walla Walla River. However, because natural flow and temperature conditions likely restricted the usability of the lower river, the primary focus area is the reach below Grove School Bridge in Milton-Freewater and upstream of the Mill Creek confluence (See Bower, 2007).

Traditional use of all the water in the river became a more pressing legal concern once bull trout were listed in 1998 and steelhead in 1999 as threatened species in the Walla Walla River under the Endangered Species Act. Federal agencies considered legal intervention and conservation groups considered lawsuits against the irrigation districts to enforce the law. But something different happened in the Walla Walla Basin. Irrigators, whose livelihoods depend on water for their farms, stepped forward and negotiated a compromise solution.

Beginning in 2000, three irrigation districts pledged to keep a minimum water flow in the river and signed an agreement to this effect with the U.S. Fish and Wildlife Service. Two of the irrigation districts that signed the agreement divert water in Oregon (Hudson Bay District Improvement Company and Walla Walla River Irrigation District). During the driest part of the season, they left 13 cfs (0.368 m3/s) in the river. The third district (Gardena Farms Irrigation District #13) is located in Washington, west of the City of Walla Walla, and left 10 cfs (.283 m3/s) in the river. The instream water improved flows and helped provide a continuous flow for passage to upriver for any bull trout and other fish in the mid to lower river sections.

This agreement was further amended in 2001 and then in 2002, when it was found that high channel bed infiltration rates in Milton-Freewater’s flood control lower levee reach allowed very little water to make it to the end of the levee system. In order to get enough flow so that the entire river was at least “wetted”, further increases in bypassed flows were negotiated and them monitored. Starting in 2001, the Oregon irrigation districts bypassed 18 cfs (0.5097 m3/s) and the Washington irrigation district bypassed 14 cfs (0.397 m3/s). Subsequently in 2002 the agreement was revised further with the Oregon districts leaving 27 cfs (0.7641 m3/s) from January 1st to June 30th and 25 cfs (0.706 m3/s) from July 1st to December 31st each year. The Washington district increased its bypass to 18 cfs year throughout the year. They have also reduced river fluctuations caused by their operations to prevent the stranding of fish.

Shortly after the start of the USFWS-Irrigation Districts agreements the irrigation districts also made the decision to enter into a Habitat Conservation Planning (HCP) process. Under this federal mitigation process, the irrigation districts would be protected against the threat of any further legal actions if they agreed to and implement a set of restoration steps to mitigate for any effect that their irrigation practices have on the Basin’s ESA listed fish species. In the HCP process, the impacts of the HCP participants needed to be assess with their relation to how much impact or “take” they actions (in this instance, irrigation withdraws) had on the listed species. In order to assess the amount of take for irrigation diverters, a technical assessment of how past, current and potential flow conditions effect the ESA fisheries both directly through adequate passage flows and indirectly through impacts to overall habitat conditions.

Fortuitously, during the time of these agreements, the State of Oregon Department of Environmental Quality and the Walla Walla Basin Watershed Council were collaborating on a required Total Maximum Daily Load (TMDL) assessment for temperature. As a part of this process a temperature model (Heat Source) of the Walla Walla River was developed to simulated changes to river temperatures based on changes in the various habitat components, including flow.

This HCP project utilized a combination of field information, flow balance calculations and the above-mentioned, predeveloped temperature model in order to estimate changes to flow and habitat conditions for this HCP process. The output from this project was then used by fisheries scientists at HDR Inc. to assess the impacts to the fisheries based on changes in habitat conditions. Two life cycle or population response models were developed by Keith Underwood at HDR and calibrated for habitat conditions using the information provided by this HCP project along. These specifics of these two models are described in Walla Walla River Bull Trout Population Response Model (HDR, 2007), and Walla Walla River Steelhead Population Response Model (HDR, 2007).

This project was made possible by funding and support from numerous partners and agencies. Funding specifically was received from the Oregon Department of Environmental Quality and its EPA 319 Grant program, the United State Fish and Wildlife Service’s support of the Habitat Conservation Planning process for the Walla Walla basin, and the Walla Walla Basin Alliance through their Natural Resource and Conservation Service funds. Additional support for the Walla Walla Basin Watershed Council was received from the Oregon Watershed Enhancement Board. Other contributors to project included the Oregon Water Resource Department, Washington Department of Fish and Wildlife and the Washington Department of Ecology. Special thanks to Don Butcher, Oregon Department of Environmental Quality for his strong support and guidance in the model development for the Heat Source models and to the Walla Walla Basin Watershed community and board members.