On a soggy weekend, all eyes are on Lake Mendocino, the rapidly filling reservoir behind Coyote Dam north of Ukiah.
The surging east fork of the Russian River is fast filling up the lake, promising to end three years of drought conditions with current levels on Saturday reaching close to 100 percent of the target water supply level for the coming year, according to local and state water officials.
“The rainfall is phenomenal. It has filled a big hole in the local water supply,” said Sean White, the city of Ukiah’s water director.
The unexpected change is from a series of rainstorms that are expected to last into the coming week. By mid-January, Lake Mendocino’s water storage could be almost three times the amount measured just several weeks ago at the end of November.
The reservoir’s rising level is good news amid a series of rainstorms, power outages, high winds, downed trees, and local flooding.
“Nothing has gotten the lake to these levels in recent years. It is significant,” said White. Lake Mendocino’s capacity is 68,400 acre-feet of water. On Saturday, on the eve of a third expected major rainstorm, the lake’s volume measured about 63,000 acre-feet.
The Sonoma County Water Agency’s main water supply is Lake Sonoma in the Dry Creek region, which holds nearly four times the water supply and principally serves 600,000 users in Sonoma County and northern Marin.
A month ago, Lake Sonoma’s storage was at the lowest historical level ever at 96,310 acre-feet, but by mid-January, its level is predicted to rise to 217,803 acre-feet and still short of its overall capacity of 245,000 acre-feet.
Water from Lake Mendocino is key to the drinking water sources for the communities of Ukiah, Hopland, Cloverdale, and Healdsburg. Downstream flows support threatened Chinook salmon and steelhead trout during fall and winter seasons and provide irrigation and frost protection for thousands of acres of farmland between Ukiah Valley and Healdsburg. The reservoir also provides critical flood control to protect communities like Guerneville, which would suffer even more drastic flooding issues if Coyote Dam was not in place.
Andrea Rodriquez, a spokeswoman for the Sonoma County Water Agency which controls the bulk of water stored behind Coyote Dam, said Saturday the current level in Lake Mendocino is amazingly nearing an “adequate water supply level” for the New Year.
Yet there are concerns about how fast Lake Mendocino will rise, and when flood measure measures must be put in place.
“We are not there yet but we are nearing a tipping point where we stop managing the lake for water supply, and the U.S. Army Corps of Engineers steps in to oversee management for flood control,” said Rodriquez.
It is a stark change from three years of drought when Lake Mendocino’s water level dwindled to a record low.
Less than two years ago California Gov. Gavin Newsom and an entourage of water officials showed up at Lake Mendocino, held a news conference, and dramatically proclaimed a drought emergency because of severe conditions in the Russian River watershed. His proclamation on April 21 gave the state Water Resources Control Board authority to curtail water rights in the counties of Mendocino and Sonoma.
“We are most certainly in a better situation now than in the last few years, but honestly, ‘feast or famine’ at Lake Mendocino is not a new thing,” said White, a longtime North Coast water expert.
White said it is ‘still anyone’s guess how this ends up but there is no doubt we are going to be in a better position than we have been in the last couple of years.’
A major difference in this era is a new forecasting system for Russian River reservoirs, which has twice now allowed state and federal authorities to increase water supply benefits while managing flood risks.
It is called FIRO (Forecast Informed Reservoir Operations) which Rodriquez, White, and others say better informs decisions to retain or release water behind dams.
“It provides more flexibility,” said Rodriquez.
The Sonoma County water agency and the Army Corps learned a hard lesson in 2012 after storms pushed Lake Mendocino water levels into the reservoir’s so-called flood pool. The decision then was made to release water downstream but a severe drought lasting until 2015 arrived, resulting in serious reductions in water storage and river flows.
Following that, the Sonoma agency and the Corps engaged with the Scripps Institute of Oceanography to develop a more sophisticated forecasting system. After six years of extensive technical and modeling analysis and annual testing, the new FIRO system proved successful during the course of two very different water years: a wet 2019 and a very dry 2020. Even in the driest year, the system allowed the agency and the Corps to maintain a 19 percent increase in water storage by the end of winter.
The new forecasting system allows the Corps the discretion to hold back an additional 9,500 acre of surplus water through Feb. 15 unless another atmospheric river is forecast. The discretionary amount rises to 19,000 feet by March 1, absent any major storm activity, according to Sonoma County Water Agency fact sheets.
In May, the innovative forecasting system was recognized statewide for being able to better address climate change impacts, including more severe and prolonged droughts.
“The successful and collaborative work done on Lake Mendocino, utilizing science, technology, and advances in forecasting, is groundbreaking and serves as a model that can be tested at reservoirs throughout California,” said Pamela Tobin, president of the Association of California Water Agencies.