- Municipal watersheds attempt to balance growing and conflicting demands for water for human use and for ecosystems. The Mill Creek basin, a 295 km2 basin in southeast Washington, exemplifies these conflicts. Since the late 1800s, the City of Walla Walla has withdrawn water from Mill Creek for municipal use. However, current streamflow levels in Mill Creek do not meet minimum instream flow recommendations developed in part to protect federally listed threatened species, including bull trout and middle-Columbia steelhead. This study addressed the physical potential for changes in water withdrawals by the City to contribute to a Pilot Local Water Management Program Strategy focused on “flow from flexibility”, a set of innovative strategies adopted in 2009 to reduce conflict and improve instream flows in the greater Walla Walla Basin (RCW 90.92). Using records of water withdrawals by the City for 2001 to 2016, as well as climate and streamflow data, this study quantified the timing and magnitude of water withdrawals, their effect on streamflow in the basin, and other factors, such as climate and vegetation change, that might contribute to water conflicts.
On an annual basis over the period from 2001 to 2016, water withdrawals ranged from 16 to 25% of the annual discharge at the City Intake, and 11 to 19% of annual streamflow at USGS 14013000 (Kooskooskie), the point where compliance with instream flow recommendations is assessed. On a monthly basis, withdrawals ranged from 15 to 54% of streamflow at the intake gage, and 10 to 40% of streamflow at the USGS gage. In years with high discharge, more water has been withdrawn. Starting in 2010, water withdrawals have increased in the winter, but summer water withdrawals have not decreased, and even increased slightly. When the water withdrawn by the City in July through October is added back to the stream at the intake, and projected to the Kooskooskie gage, median daily streamflow in July through October would meet or exceed the minimum instream flow recommendations. Annual precipitation has not declined significantly since 1950. Over the period from 1940 to 2017, annual water yield declined by less than 0.01%, but August streamflow declined by 17% over the same period. Declining streamflow also may be due to increasing evapotranspiration. However, visual analysis of aerial photography from 1939, 1959, and 2015 reveals relatively little vegetation change in the watershed. Increased withdrawals from groundwater and surface water by private land owners along Mill Creek may also account for declining streamflow. The results of this study provide some insights into the available options for the City to contribute to “flow from flexibility.” This study indicates that “flow from flexibility” and other water- sharing or water conservation measures have potential to mitigate conflicts among alternative uses of water in the Upper Mill Creek Basin.