Walla Walla County suffered considerable damage during the flood of March-April 1931, with Mill Creek’s discharge at 6,000 cubic feet per second or cfs.
As a result, the U.S. Army Corps of Engineers constructed a concrete diversion dam at Rooks Park and an off-stream earth-rock dam for Bennington Reservoir (completed 1942), and an artificial channel for Mill Creek (completed 1949).
This Mill Creek project saved Walla Walla during the December 1964 flood (Walla Walla River at 20,300 cfs), the January-February 1965 flood (Walla Walla River at 14,500 cfs), the February 1996 flood with Mill Creek at 6,400 cfs and the February 2020 flood with Mill Creek at 7,000 cfs, preventing potential damage of hundreds of millions of dollars. However, the Mill Creek channel can safely accommodate only about 3,800 cfs.
Wednesday at 5:30 p.m., the Corps is hosting a public meeting at the Courtyard by Marriott, 550 W. Rose St.
Questions need to be answered: What discharge can the gates and channel between Rooks Park and Bennington Reservoir safely accommodate? How long does it take the reservoir to fill with Mill Creek at 7,000 cfs? How long does it take to drain the full reservoir (with 250 cfs in the return channel and 100 cfs in Russell Creek) down to recreation level?
Global climate change will increase temperature and precipitation in Eastern Washington and Eastern Oregon, according to “Oregon 2100: Projected Climate and Ecological Changes,” published by the University of Oregon.
Evaporation will increase from warming oceans and from reduced sea ice. A warmer atmosphere will hold more moisture and more energy. As long as winter temperatures in the mountains are below freezing, snowfall may increase.
Warmer rains will melt snow faster. The ground, frozen as in 1996 or saturated as in 2020, will not allow infiltration, causing runoff down hillsides and into streams. The frequency and magnitude of floods may increase.
The Corps analyzed the 1931 flood using “a combination of maximum observed meteorological and hydrological conditions that contribute to a major flood within the basin.” Assumptions included: “The maximum observed storm occurred on the last of March and first of April 1931 …” “Maximum observed temperatures for the last of March occurred in 1934.” “Snow cover extending down to an elevation of 3,000 feet … maximum snow conditions on the first of April.”
Such conditions could result in a Mill Creek flood of 11,300 cfs.
Suppose the Blue Mountains have a deeper snowpack when rains come that are warmer and last longer. Suppose two floods arrive a week apart with snowfall between them, instead of the month apart in 1964-65.
Rain-on-snow events are not the only flood hazards in the foothills of the Blue Mountains. A flash flood in June 1903 was Oregon’s worst natural disaster; raging Willow Creek killed 247 people in Heppner.
Last week, a Walla Walla River levee was overtopped in Milton-Freewater. The Touchet River inundated parts of Waitsburg and Touchet. Every big flood backs up water behind some bridges, so small that flow is constricted.
What is the future for Walla Walla and vicinity in terms of floods?
In places, levees are too low. The Mill Creek project is old and not particularly fish friendly. Should there be a new, larger flood channel around southeastern Walla Walla from Bennington Reservoir to the Walla Walla River? What are possible other solutions?
Whatever is done to reduce the flood hazard in Walla Walla must include improving passage for anadromous salmon and steelhead.
Ideally, the work would be aesthetically pleasing. Weekend releases from Bennington Reservoir could allow for year-round kayaking between the Corps’ project office and Walla Walla Community College.
What will be the source of funding, not only for increased flood protection, but also for longer and/or higher new bridges?
In the 2016 presidential contest, the platforms of both Republicans and Democrats included billions of dollars for improving our nation’s aging infrastructure: Dams, levees, bridges, roads.
Yet, little has been done because of years of deadlock in Congress.
Bennington Reservoir was 95% full in 1996 and at 85% last week. Perhaps the most important question is how safe that earthfill dam is when the reservoir is near maximum capacity.