Army Corps of Engineers approves request to allow more winter storage in Lake Mendocino

The U.S. Army Corps of Engineers (Corps) will allow additional water to be stored in Lake Mendocino during this winter’s rainy season to improve water supply reliability and environmental conditions in the Russian River, while continuing to ensure flood management capacity of the reservoir. The decision would allow the Corps to use modern weather prediction technology to operate the reservoir with more flexibility to store more water when no major storms are foretasted.

A group of scientists and engineers from local, state and federal agencies, including representatives from the U.S. Army Corps of Engineers, has been developing a proof-of-concept demonstration project for Forecast Informed Reservoir Operations (FIRO) since 2014. Last year, the group, the Lake Mendocino FIRO Steering Committee, filed a request with the Corps to allow a deviation from its established flood control operating rules. The deviation request was supported by a Preliminary Viability Assessment, which contained detailed modeling, analysis and scientific research. The assessment demonstrated that FIRO can provide water managers the information they need, with adequate lead time, to selectively retain or release water from reservoirs. That request was approved earlier this month.   

“The ability to leverage newer technology and knowledge base as it pertains to weather forecast enhances our ability to safely deliver the multiple missions at Lake Mendocino,” said Nick Malasavage, chief of Operations and Readiness Division for the U.S. Army Corps of Engineers San Francisco District. “In particular, the steps we are now taking to further develop and incrementally implement the FIRO concept adds an additional tool to maintain our primary responsibilities for flood risk management.”  

Under the approved request, a maximum of 3.8 billion gallons (11,650 acre-feet) of additional water could be stored in the reservoir between November 1 and February 28, which is enough water to supply approximately 97,000 people for a year.  

Lake Mendocino, located near the city of Ukiah, is operated jointly by the Corps and Sonoma Water. The Corps manages the flood control operations at the reservoir, or the water in what is referred to as the “flood pool.” Sonoma Water manages the water stored expressly for water supply, known as the “conservation pool” and is also responsible for maintaining minimum in-stream flows in the Russian River below Lake Mendocino.  

Studies show that about 50 percent of the rainfall and 80 percent of the floods in the Russian River watershed are due to atmospheric rivers – long narrow bands of warm, moisture-laden air that carry huge amounts of water vapor propelled by high winds.  

“We know that a majority of our rain each year comes from these atmospheric rivers,” said Sonoma Water Chief Engineer and co-chair of the steering committee Jay Jasperse. “Because we now have the technology to better predict the timing and intensity of these storms, it allows us the opportunity to manage our water supply more efficiently and maintain flood management capacity in Lake Mendocino.”  

A dramatic illustration of the potential benefits of FIRO occurred in December of 2012 when a large atmospheric river storm filled the available water supply space in Lake Mendocino and filled about 25,000 acre feet of the flood pool that is normally kept empty to take the crest off of floods. Operating under the Corps procedures, which dictate that water in the flood pool be released as soon as possible to make room for the next storm, dam operators followed the operations rules and released the water from the flood pool, even though no storms or flooding was forecasted in the near future. But no additional storms occurred, and the next winter was the beginning of a severe and extended drought. If improved forecasts had been available and used in 2012 and atmospheric river storms were not predicted to occur, and operation rules were more flexible, the water that had been released could have been put to beneficial uses just as the region entered a drought.   

As noted by F. Martin Ralph, Director of the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography and co-chair of the FIRO Steering Committee, “The FIRO effort that has led to this approval by the Corps is the result of a highly collaborative effort between engineers, physical scientists, biologists and forecasters. Sonoma Water and the Corps are to be commended for their leadership and innovation on FIRO at Lake Mendocino, which is setting the stage for further exploration of this promising approach.”  

“This collaboration will have far-reaching benefits for the resiliency and reliability of our water supply system in the face of a changing climate,” said James Gore, Chair of Sonoma Water’s Board of Directors. “Improved forecasting provides us with the ability to store more water and still maintain the flood protection benefits of our reservoirs. This is another great example of the benefits of a multi-agency partnership that addresses our most challenging issues.”  

The Lake Mendocino FIRO Steering Committee consists of representatives from Sonoma Water (Sonoma County Water Agency), the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography (Scripps), U.S. Army Corps of Engineers (Corps), National Oceanic and Atmospheric Administration (NOAA), U.S. Geologic Survey (USGS), U.S. Bureau of Reclamation and the California Department of Water Resources (DWR). The deviation request was submitted on behalf of steering committee members from Sonoma Water, Scripps, the Corps, NOAA and DWR.