Mill Creek Flow Restoration II

The City of Walla Walla depends primarily on Mill Creek, the second largest tributary to the Walla Walla River, for its municipal water source. Mill Creek historically provided critical spawning habitat for Mid-Columbia summer steelhead, bull trout, and spring Chinook salmon. However, since 1919, the upper Mill Creek watershed has been closed to all public access to protect the water source for the City of Walla Walla. The City diverts its water supply from Mill Creek more than 25 river miles above the City of Walla Walla itself, and diverts more than two-thirds of Mill Creek’s natural summer flow under its 1866, 28 cfs water right, which is both the largest and most senior water right on Mill Creek.



In an effort to provide system redundancy, the City began to add groundwater wells in the 1940s. Today, the City has 7 groundwater wells that can be used to supplement surface water diversions from Mill Creek. These wells both create water supply redundancy and can augment surface diversions from Mill Creek to help meet peak demands. Recognizing the importance of adding water storage capacity to strengthen the resilience of its water supply system, the City upgraded two of its wells to allow for injection in the early 2000s. This enabled the City to inject and store water diverted from Mill Creek during the winter, when flows were relatively abundant. This stored water could then be withdrawn from the aquifer when surface flows in Mill Creek were less abundant, such as the summer and early fall. Utilizing this water stored in the aquifer can also enable the City to reduce its diversions from Mill Creek, thereby restoring instream summer flows.



This project includes a source-switch for up to 28 cfs of the City of Walla Wallas municipal water supplies during the fish critical low flow months of August and September. Under this altered management approach, the City would partially meet its municipal water demands by supplementing its Mill Creek surface diversion with water pumped out of one of its deep basalt aquifer storage and recovery (ASR) wells. The City has a number of groundwater rights to the deep basalt aquifer, and two of these wells already have been converted to ASR wells. During winter high flows, the City diverts water for injection into two wells, and this water is available for later recovery to supplement the City’s municipal water demand.



However, when compared to the gravity diversion of surface flows, injecting and recovering water via ASR incurs substantial pumping costs. Further, because the City generates hydroelectricity from its surface diversion, utilizing ASR also results in lost revenue from forgone hydroelectric generation. Finally, because of the seniority of the Citys rights on Mill Creek, the reliability of summer surface flow availability typically is not an issue. As a result, the City relies almost exclusively on surface flows to meet its current municipal demands, despite its substantial groundwater rights.



By making ASR cost-effective, this project would enable the City to switch to groundwater to partially meet municipal demands during the dry, summer months. To prevent the depletion of groundwater reserves, this usage would then be replenished through the diversion and injection of Mill Creek water during winter high flows. While the City currently can use two of its groundwater wells for ASR, the ultimate intent is to ultimately expand this ASR capacity to all of its remaining wells.



Proposed Work:

Begin work to upgrade City wells #5, #2, and #7 to provide for injection and aquifer storage capacity. Upon completion of upgrades these wells will be used to store water for summer municipal use, allowing for the restoration of instream flows in Mill Creek during the low-flow season. 1 gauge will be installed under this project to measure discharge. Note: This is a multi-year effort so metrics have been divided accord

Benefits will be achieved through the restoration and protection of instream flows and the resulting habitat and water quality improvements. The impacts resulting from municipal water supply withdrawals that degrade aquatic habitats and limit fish production will be reduced. ESA species that will benefit include: Mid-Columbia summer steelhead and bull trout. Other species that will benefit include: spring Chinook, coho salmon, pacific lamprey, freshwater mussels, and other miscellaneous resident fish.