California’s fifth largest reservoir — San Luis — is now at 16 percent of capacity.

The nation’s largest off-stream reservoir with the ability to hold 2,041,000 million acre feet exists to divert excess winter and spring river flows headed for the Pacific Ocean. As such it underscores the fact the entire Central Valley, the southern Sierra, the North Bay, and the East Bay are in exceptional drought — the worst designation of the United States Department of Agriculture drought monitor.

Historically on Aug. 16 San Luis would be at 36 percent of capacity.

As of Monday the reservoir 50 miles southwest of Manteca nestled on the eastern slope of the Diablo Range behind the nation’s fourth largest embankment dam had 335,300 acre feet of water. Those skirting its southern flank via Highway 152 on a daily basis are seeing the reservoir level drop by more than 2 inches a day.

The reservoir is filled primarily in the winter and spring with excess water diverted northeast of Tracy by pumping stations that send it south into the Delta-Mendota Canal and the California Aqueduct. The San Luis Reservoir storage is drawn down as needed by Central Valley Project and State Water Project contractors consisting of urban and agricultural users.

The Bureau of Reclamation in 2020 proposed raising B.F. Sisk Dam by 10 feet to increase San Luis storage by 130,000 acre feet. The increased storage would be used to address wildlife refuge water needs as well as existing contracts.

Four of State’s Big 5 reservoirs

now below 50% of normal

mid-August storage capacity

San Luis is in the worst shape of the state’s five largest reservoirs that have a combined water storage capacity of 14.8 million acre feet of water.

But in the overall scheme of water storage it is in a better position than the two largest reservoirs — Shasta and Oroville.

Shasta with storage of 4.5 million acre feet is at 29 percent capacity. Oroville with storage of 3.5 million acre feet is at 23 percent capacity. As such, they are in worse condition than at any point in the previous drought that ended just over two years ago

As alarming as those percentages may be, the critical data — historic average capacity on a given date — paints an even worse outlook.

The five largest reservoirs as of Monday based on historic levels for Aug. 16 were at:

*43 percent of average for Shasta.

*34 percent of average for Oroville.

*49 percent of average for Trinity.

*66 percent of average for New Melones.

*16 percent of average for San Luis.

It is why the California Department of Water Resources and the Bureau of Reclamation are curtailing contracted water deliveries to farms and reducing urban deliveries including 10,000 less acre feet of water going to Tracy.

The 2.4 million acre foot New Melones on the Stanislaus River basin that the South San Joaquin Irrigation District relies on to supply irrigation water to 50,000 acres of farmland and 210,000 residents in Manteca, Lathrop, and Tracy is at 40 percent capacity and 66 percent of historic average  as of Monday.

Don Pedro with 2.03 million acre feet of storage is the sixth largest reservoir in the state and serves Turlock and Modesto irrigation districts. It was at 54 percent capacity as of Monday and 74 percent of historic average.

Because water levels were less anemic behind New Melones, the Bureau reduced flows out of Shasta that would have gone toward assuring Delta water quality and placed the burden on the Stanislaus River watershed.

While the more robust flows have been a big plus for rafters and other recreational uses of the Lower Stanislaus, it is likely to set the stage for water delivery reductions for SSJID and Oakdale Irrigation District if a third straight dry year occurs.

New Melones has never been used for Delta water quality. It has been designated as the regulator for water quality at Vernalis on the San Joaquin River as well as Stanislaus River fish flows.

The Bureau,  after management plans in place did not slow down the draining of storage facilities north of the Delta for river flows and Delta environmental needs soon enough when a dry winter pattern took root in mid-January and February, was forced to look at alternatives to avoid major water quality issues for the water behind Oroville and Shasta sent through the Delta to reach the California Aqueduct where it travels as far south as San Diego.

New Melones water

being used to assure

SoCal water quality

So in order to push back saltwater intrusion from weakening the quality of water destined for large farms in the Southern San Joaquin Valley and urban uses in the Southland, the Bureau ramped up releases from New Melones at four times the normal flow for summer to take pressure off Shasta.

Given such use of New Melones water was not authorized in operating agreements both the SSJID and OID have expressed concerns if it were to become the norm inferring they would consider taking legal steps if necessary to protect their water rights.

That’s because under the agreement the two districts had with the federal government allowing them inundate the Melones Dam the two agencies built in 1925 that allowed New Melones to be built, they are entitled to the first 600,000 acre feet of water the basin generates each year as well as a carryover conservation account.

There is now a concern that the districts may now face a worse water year than natural conditions would create next year depending on how New Melones is now being operated.

SSJID between tighter operations, a plea for urban and farm users to conserve, and ending the irrigation season earlier anticipates reducing water use this year by 30,000 acre feet. Even so they will likely need to tap into 15,000 acre feet of the water conservation account.

The increased flows are likely to ultimately come at the expense of storage in the conservation account imperiling any cushion for a third dry year.

If the SSJID imposes mandatory cutbacks next year it will be shared equally percentage-wise between farm and urban users in Manteca, Lathrop, and Tracy.

To reach Dennis Wyatt, email dwyatt@mantecabulletin.com