METROPOLITAN WATER DISTRICT: Subsidence of the California Aqueduct in the San Joaquin Valley

Plus a brief update on the voluntary agreements, and good news on the Colorado River

Recently, the Department of Water Resources released a report to supplement the 2017 California Aqueduct Subsidence Study (CASS) report that addresses the specific issues within a 10-mile-wide study corridor centered on the California Aqueduct south of San Luis Reservoir to the southern San Joaquin Valley and looked at predictions of future subsidence.

At the February meeting of Metropolitan’s Water Planning and Stewardship Committee, Ted Craddock, DWR Assistant Deputy Director of the State Water Project, provided an overview of the report, detailing the subsidence problem on the California Aqueduct, some of the recent work that DWR has been doing to assess the subsidence, and what the plans are for moving forward.

The State Water Project is an important part of the state’s water supply system that delivers water to over 27 million Californians and is significant to the state’s overall economy.  The California Aqueduct is the principal water conveyance structure for the State Water Project, beginning at the southern Delta, running south through the San Joaquin Valley, and over the summit of the Tehachapis, where it divides into the east and west branches with the west branch terminating about 444 miles from the Delta.


Since the completion of construction, the Department has had a program for routine monitoring and inspection of the aqueduct, so they were alarmed when during the drought years between 2013 and 2016, there was an increase in the rate of subsidence along the California Aqueduct that was up to three feet in some areas.

The slide illustrates the key areas of subsidence along the California Aqueduct from Merced County just south of San Luis Reservoir to the foot of the Tehachapis.  Mr. Craddock noted that the colors on the chart illustrate the amount of subsidence that occurred during the recent drought period with the red areas experiencing up to 6 feet of cumulative subsidence; the orange is about a foot of subsidence, and the blue and green areas of little or no subsidence.

The key areas where the most subsidence has been observed:

  • The Panoche Bowl, which is approximately 47 miles of the Aqueduct from Pool 15 to Pool 18; this area has experienced maximum subsidence of approximately 5.5 feet since 1967.
  • The Los Gatos Bowl, which is about 27 miles long and encompasses Pools 19, 20, and the northern part of Pool 21; it has had maximum subsidence of approximately 6 feet since 1967.
  • The Kern Bowl is a 32 mile reach between the southern part of Pool 23, all of Pools 24 and 25, and the northern half of Pool 26 has had two maxima of 3.0 feet and 3.3 feet of total subsidence that are separated by a subsidence minimum or “divide” that has exhibited approximately 1.0 foot of cumulative subsidence since 1967.
  • There are two smaller areas further south – an approximately 13-mile reach encompassing the southern end of Pool 30, all of Pools 31 and 32, and the northern end of Pool 33 which has experienced about 1.6 feet of subsidence since 1967, and a 9 mile reach that includes the southern part of Pool 34 and all of Pool 35 that has experienced about 1.7 feet since 1967.

Mr. Craddock said that when they saw the subsidence trend increasing, the Department embarked upon a study to get a better understanding of the causes, which resulted in completion of the California Aqueduct Subsidence Study in 2017.  In 2019, a supplemental report was released that that found that the subsidence that was occurring was caused by deep water pumping along the aqueduct; the report also looked at historical and present land use and determined a driving factor appeared to be the conversion of temporary crops to permanent crops in these areas that were requiring additional groundwater pumping during the drought period.

What we found in a few key locations is that the capacity of the aqueduct has been reduced to about 20% and with that reduction in capacity, we’re seeing reduced flexibility in the way we can operate the system,” Mr. Craddock said.  “That has led to some additional operations and maintenance costs as well.”

He presented a slide to illustrate the effect that subsidence has on the aqueduct.  The top figure shows what the original section of the aqueduct would have looked like and the bottom figure illustrates what happens with subsidence of about three feet.

Our canals are gravity canals and so the water level of the hydraulic grade line basically needs to stay the same to move water, but with the settling, that surface has now encroaching upon the freeboard of the aqueduct liner,” he said.  “As a result, we either have to operate at a lower capacity or take some additional risks on in terms of encroaching on the liner surface.  As we take on moving water at a higher surface, some additional risk has resulted in increased O&M costs and we’ve had to move to flatter schedule.  When we had a little bit more freeboard, we were able to decouple our pumping plants and operate at times of the day to take advantage of the different costs of energy.  We’re not able to do that as much as we used to in the past.

The slide on the lower left shows some of the damage.  As subsidence is occurring, it results in differential settlement of the aqueduct, so the concrete lining is being damaged and depressions can cause small collapses, so they have to make repairs as part of the routine operations and maintenance activities.

The photos on the slide on the right show the gate structures where the entire structure has settled, resulting in the water surface being higher.  The pictures show how the attached beam is actually below water as there should be some gap between the water and the top of the structure.  With subsidence, those flowways are now totally submerged and that has resulted in flow constrictions throughout the system which is a contributing factor to the reduction in capacity of the aqueduct.


The Department has embarked upon a program called the California Aqueduct Subsidence Program to rehabilitate and restore the aqueduct with the key goals being to restore the delivery capacity of the aqueduct and regain the operational flexibility of the system.  The program is a partnership between the Department of Water Resources, the State Water Contractors, and other State Water Project contractors including Metropolitan.  Since a significant section of the subsided area is in the San Luis Canal, which is a joint state-federal reach of the Aqueduct and a shared responsibility, the Bureau of Reclamation and the Central Valley Project contractors are also partners in the program.

We’re very cognizant that affordability is a key consideration for all the folks and organizations that receive water from the State Water Project, so as we plan this out, that’s something we’re aware we need to be considerate of,” said Mr. Craddock.  “So as a result, we are working with our partners to pursue supplemental funding beyond just the funding from the State Water Project contractors and organizations like yours that would support that.”

Department staff is also engaging with the Groundwater Sustainability Agencies that cover the four critically overdrafted basins along the aqueduct’s path:  the Delta Mendota Basin, the Kern Basin, Westside Basin, and the Tulare Lake Basin.

In December, we provided some preliminary comments on their plans,” he said.  “We’re exchanging information with them that we’ve obtained from our study and working with them to learn a little bit more about what they are doing, but ultimately our position as we engage with them is that we need to protect the aqueduct from further damage.”


The program is divided into two tracks.  The first would be near-term rehabilitation efforts, which will cost a few hundred million dollars over the course of the next three to five years and involve projects such as raising portions of the aqueduct, repairing siphons and gate structures, and in some instances, raising bridges over the aqueduct where there are crossings.  These efforts are primarily focused on keeping the aqueduct whole so there isn’t any further degradation in conveyance capacity of the aqueduct.

The second track would be a long-term assessment.  They are currently in the feasibility stage for the longer term assessment to look at the range of options available, from minimal repairs to fully restoring the aqueduct.  This will result in a joint state and federal EIR-EIS in about three years and then ultimately some recommendations for the long-term plan to repair the system.


During the discussion period, Director McKenney (Orange County) noted that one of the slides said that some of the subsidence is not recoverable, so if some of it is recoverable, is that being factored into the plans and at whose cost?

We have seen a little bit of rebound in a few areas, primarily the blue and green that were shown on the slide but as we looked at the water tables in some key areas, they have gone pretty low from where they were in the late 1960s, so as a result, we expect that with those changes, there is some subsidence that is not recoverable,” Mr. Craddock said.

In terms of recoverable, that is something that we would factor into our plans. We don’t want to overbuild any repairs if they are not needed, so we need to be sure that we’re work we’re doing is truly needed,” Mr. Craddock continued.  “In terms of damage, we’re currently assessing our options for how we specifically address damage for other entities that may have contributed to the damage.  Our engagement with the GSAs is the first step in that process.  Right now, we’re focused more on the future of protecting the aqueduct, but we do also have an effort underway to assess what are some options to address the prior damage.  I don’t have an answer to that now.”

Director Russell Lefevre (Torrance) asked if they anticipated getting funds from the legislature for this?

At this point, I don’t believe I can answer that specifically,” said Mr. Craddock.  “What I can say is we’re working with representatives of different water agencies currently on a range of possible alternative or supplemental funding sources ranging from state appropriation to federal appropriation.”

We will be working closely with the Department on the range of funding, but absent supplemental state or federal funding, obviously the contractors will be paying for a lot of these transportation facilities and the repairs to them,” added General Manager Jeff Kightlinger.

Director Steve Blois (Calleguas) asked, if we know the facilities are being damaged because of subsidence, and we know that pumpers are likely the cause of that subsidence, how close are we to identifying which pumpers those are, and in what relation?

“Our studies up to this point have been a little more global so not as granular,” said Mr. Craddock.  “We’re starting to move into the more granular look at specific causes right now.  I imagine it will take us on the order of a year of so to work through that.  Our engagement with the GSAs has been good.  Our team had a meeting on Friday with some of the GSAs to compare our groundwater models to their groundwater models.  Some of these GSPs that are being prepared are about 5 feet tall, so there’s a lot of data to wade through so it’s definitely a focus for us to understand the causes better so we can address that question.”


During the Bay Delta report, Assistant General Manager Roger Patterson reported on the recent progress with the voluntary agreements.  “The state was looking at what was on the table as far as the voluntary agreement proposal, and basically their conclusion is that they needed to have a more robust proposal.  So they have suggested how much of an increase they want to see.  It’s roughly an extra billion dollars of habitat, mostly for floodplain work in the Sacramento Valley but scattered around the watershed.  I would say I like the fact they are focused on the floodplain in the Sac Valley because we believe that’s a very limiting factor on the fisheries.  On the water side, they want to see an additional 100-200,000 acre-feet a year, that’s up about 25% from the proposal we had on the table before.  That’s going to be a stretch for the water users but we’re going to get at that next to see how we can generate that.  The good news on that is we need about 100,000 in dry years and a couple hundred when it gets a little wetter in normal years, so that’s a little easier to do.  It was a good positive step.”

This was again developing an alternative to provide to the State Board so they can analyze it as part of their water quality control plan update, so even if in the next month or two, we can flesh out the details on this, it’s a year to a year and a half for the State Board going through the SED before they would finally get to a decision, but if we have a strong proposal here with good stakeholder support, the chance of it getting it finished definitely increases.”


Bill Hasencamp, Manager of Colorado River Resources, had some good news for the Colorado River and Lake Mead.  California’s Colorado River water use in 2019 was the lowest in 70 years of record; prior to 2002 when the QSA was signed, California used 5.4 MAF.  Last year, California only used 3.8 MAF or a drop of about 30% less water last year than water use in 2002, a drop of more than 1.5 MAF.  The savings is attributed to not only Metropolitan’s efforts, but also those of IID, PVID, and the Yuma Project.

It’s a pretty significant drop, considering in 2002 we were trying to figure out how to get down to 4.4 MAF, and now we’re much below that,” Mr. Hasencamp said.  “As a result of the low water use in California and with the other states, Lake Mead is at it’s highest level in 6 years.  I won’t say Lake Mead is rocketing up in elevation, but we’ve had the exact same inflow to Lake Mead over the last six years and we’ve been able to stabilize it and actually have it go up.  Lake Mead was dropping, dropping, but now for six years, we’ve worked together with out partners, the DCP was part of it but also before the DCP with other efforts, and Lake Mead now is slightly creeping up and we actually are 20 feet above shortage.  And we thought last year at this time, we were headed to a shortage, so I think it’s a good news story.”


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