In the wake of one of the Mother Lode’s wettest winters in memory, some people may be wondering where all that water is going and how long local reservoirs will stay full or near-full.
Because the Mother Lode’s major dams and reservoirs are operated by more than seven agencies, there is no single easy answer to these questions.
Simply and generally, some of that water is getting captured behind dams on reservoirs, and some dam operators will try to keep reservoir levels up through September.
Most dam operators are required to make room for more rain and runoff before the next wet season kicks in this fall. But right now most Mother Lode reservoirs are full or near-full to the brim.
Pardee and Camanche reservoirs on the Mokelumne River were 93 and 99 percent full respectively as of Thursday. New Hogan on the Calaveras River was 74 percent full.
Donnells and Beardsley on the Stanislaus River were 89 and 99 percent full. New Melones was 91 percent full and Tulloch was 98 percent full.
Hetch Hetchy on the Tuolumne River in Yosemite National Park was 99 percent full. Cherry was 78 percent full. Don Pedro was 99 percent full. McClure on the Merced River was 98 percent full.
Making room for winter
Asked about reservoir level decreases at Camanche and Pardee later this summer and fall, Nelsy Rodriguez with East Bay Municipal Utility District responded, “We will be releasing water all year due to the abundant amount of rain and snow that came in this winter.”
East Bay MUD releases will increase by late-October so dam operators on the Mokelumne River can make room for more rain and runoff in the coming fall and winter, “and to feed to the salmon that are returning to their birthing grounds to spawn,” Rodriguez said. “We release the water they were born in, they smell and follow it back home and regenerate the salmon population in the Mokelumne River.”
By Nov. 5, East Bay MUD must have 200,000 acre-feet of storage space combined between Pardee and Camanche reservoirs, Rodriguez said.
“Because we operate Pardee and Camanche in tandem, the levels may change daily, but combined we must have that space for more water,” Rodriguez said.
Pardee is the East Bay area’s drinking water reservoir for 1.4 million people who live and work there. Camanche is East Bay MUD’s flood control reservoir. East Bay MUD also generates hydroelectric power with partners Pacific Gas & Electric. This past winter, East Bay MUD generated more than 215,000 megawatt hours of hydropower.
The highest water consumption dates are in summer, Rodriguez said. When it’s hot, people use more water. Maximum use can occur in June, July or August, depending on factors like weather, heat waves and cold spells, and social impacts like economy and drought rebound.
The snowmelt season at New Melones is defined as April 1 to Sept. 30, said Louis Moore, a Sacramento-based deputy public affairs officer with the federal Bureau of Reclamation.
“Oct. 1 we’ll start preparing for the next wet season,” Moore said. “We'll see how the next wet season proceeds before we decide how much water we want to release from New Melones. We have to look at real time conditions before we can make a schedule.”
Currently, the Bureau of Reclamation is releasing about 2,800 cubic feet of water per second from New Melones, and the inflow is slightly lower than that at 2,600 cfs, Moore said.
“We are working to maintain storage for the summer months and will shift to increasing releases to prepare for the rain season that starts in October,” Moore said. “This is pretty normal for wet year operations.”
On Thursday, New Melones was holding 2,186,600 acre-feet with a surface elevation of 1,068 feet, which is 20 feet lower than when the reservoir is at full capacity with 2,419,500 acre-feet of water, Moore said.
The lowest elevation at New Melones in the past two decades was 797.4 feet in November 2015, when long-submerged relics reappeared, including the old Parrotts Ferry bridge upstream from the new bridge, as well as remnants of the old town of Melones.
Since the Bureau of Reclamation began filling the reservoir in the early 1980s, the lowest level recorded at New Melones was 721.15 feet in October 1992. That was low enough to expose the old bridge at Melones, which connected Calaveras and Tuolumne counties on old Highway 49.
“We are pleased to have a wet year that has provided ample water supply and to see New Melones with plenty of storage,” Moore said.
Asked for a projection of what New Melones will look like come Oct. 1 or Nov. 1, Moore said, “Nobody knows that right now.”
Don Pedro operators are treating this water year as normal so far, said Calvin Curtin with Turlock Irrigation District Water & Power.
In a normal water year the reservoir fills sometime in the latter portion of June to the first week or so of July, then levels off for a period of time, Curtin said. Dam operators begin to draw the reservoir down slowly in August and September to reach an elevation level of 801.9 feet by Oct. 7. At that elevation the reservoir would be 83.25 percent full.
From June 3 to Sept. 8, Don Pedro’s maximum possible storage is the physical limit, full capacity of 2,030,000 acre-feet, at elevation 830 feet, Curtin said.
“As part of our flood control contract with the U.S. Army Corps of Engineers, the flood control space requirement increases daily from Sept. 9 to Oct. 7,” Curtin said. “From Oct. 8 to April 27 the maximum storage is 1,690,000 acre-feet, elevation 801.9 feet, which leaves 340,000 acre-feet of empty space for flood control.”
The water stored in Don Pedro is used for the irrigation of crops in the Turlock and Modesto Irrigation Districts, to supply drinking water to the town of La Grange and the city of Modesto as well as for environmental releases to the Tuolumne River, Curtin said. Releasing water from Don Pedro also generates hydropower at the dam and at the powerhouse at La Grange.
Lake McClure is projected to be at about 67 percent of capacity on Nov. 1, said Mike Jensen with Merced Irrigation District.
Representatives for the Army Corps of Engineers, Tri-Dam Project and the City and County of San Francisco were unable to respond in detail Thursday to requests for information about dams and reservoirs they operate on major Mother Lode rivers.