Corcoran is the proverbial canary in the mine when it comes to the invisible damage the current drought is likely to inflect.

Between May of 2015 and September 2016 areas near the South San Joaquin Valley community dropped upwards of 25 inches in the last drought due to land sinking with the removal of water from underground aquifers.

The subsidence was the end result of jacked up groundwater pumping due to the Bureau of Reclamation cutting off water deliveries as growers scrambled to protect hundreds of millions of dollars invested in cropland and orchards.

Corcoran is part of the Corcoran/Tulare Basin served by the Friant-Kern Canal.

It is part of the system of dams and canals that were put in place in the mid-20th century to partially stop the over-drafting of underground water supplies.

When 2017 delivered one of the wettest winters on record for the San Joaquin River water basin to break the back of the last drought, the Friant Water Authority could not use its canals at full capacity to deliver water to farmers.

That’s due to subsidence including one segment where the capacity was compromised by 40 percent with most of the loss occurring between 2016 and 2017. Subsidence just doesn’t damage and compromise canals. It also impacts bridges, roads, power lines, and even structures.

Today a 33-mile stretch of the 153-mile canal that delivers water to more than one million acres and 250,000 people has lost roughly 60 percent of its original capacity due to subsidence.

Its current flow capacity of 1,600 cubic feet per second is well below its original capacity of 4,000 cfs. The Bureau of Reclamation has said it will cost $500 million to restore the flow.

Subsidence has been

recorded just south

of Manteca & near Ripon

Subsidence, while not as pressing as an issue in the Northern San Joaquin Valley, is a still a growing threat in Stanislaus and San Joaquin counties.

Between 1949 and 2005 the California Department of Water Resources indicates a wide swath of western Stanislaus County as well as South San Joaquin County to within a mile or so of the 120 Bypass and East Highway 120 suffered an accumulative subsidence of up to 5 inches.

Much of the area is kept in check by aquifers under the Delta.

The issue of subsidence — or its root cause to be more precise — will create an entirely different set of problems for the cities of Manteca, Lathrop, Tracy, Stockton, and Ripon and surrounding farmland that, like the cities, rely 100 percent of partially on ground water.

Reduction of water flows into the Delta during times of drought leads to salinity intrusion into underground aquifers nestled under and near the Delta increasing as surface salinity makes further headway to the east.

Salts are a major issue in water as they can kill crops, slowly make land infertile and render water undrinkable. The increased salt volume in water was a major contributing factor to the demise of indigenous American irrigated civilizations in the Southwest during the 14th century as well as ancient farming civilizations in the Middle East.

The northern part of the valley near the Delta has the double whammy of not just land subsidence occurring in a drought with over pumping, but also the eastward push of salt water into potable wells.

Salt water intrusion

issue in wells south

of Manteca, in Tracy

Salt water during the 1986-1992 drought was detected in wells as far east as Jack Tone Road.

The intrusion of salt water into irrigation wells that growers had to press into service in South San Joaquin Irrigation’s Division 9 due to inadequate flows at times given they were at the end of surface water irrigation runs was one of the driving factors in pressurizing the Division 9 distribution system.

The state-of-the-art system has provided consistent water delivery and increased production and reduced water use by directing water to tree roots. It also eliminated ground water pumping for supplemental irrigation water and reversed the drop in the water table which in turn serves as an effective barrier to keep salt water at bay from domestic wells and those of farms not taking water from SSJID system.

Salt intrusion also had been recorded in drinking water pumped from aquifers in Tracy. The city of 95,000 also relies on surface water deliveries from the Central Valley Project and SSJID.

Groundwater management

may not help stem subsidence

until 2042 at the earliest

The Sustainable Groundwater Management Act adopted by the California Legislature that Manteca, Lathrop, Stockton, and SSJID leaders and their counterparts throughout the state are wrestling with will eventually help address subsidence.
The act primarily was passed to stabilize and stop the further depletion of groundwater used by 85 percent of the state’s population as well as agriculture.

Entities within individual basins must come up with plans to assure that no more water is taken out of an aquifer than is returned in a given year. The plans must be in place by 2025. But it doesn’t have to be fully implemented until 2042.

Based on the frequency of droughts in the last two decades (2007-2009. 2012-2016 and the current one that started in 2020), there could be three more droughts before the groundwater management plans are fully implemented.

That may not bode well for the 450 groundwater basins in California.

When over-drafting occurs, the loss of water empties the space between particles causing fine-grained sediments to compact.

That doesn’t necessarily mean those groundwater basins can’t be charged as most have coarser materials. Those water basins that have fine-grained particles are where compaction often can’t be reversed. That is especially true of clay

Subsidence from depleting groundwater is not limited to the San Joaquin Valley although that is where the biggest concerns exist today in California.

Subsidence caused 12-foot deep, 2,000-foot long fissures on runways at Edwards Air Force Base in the Mohave Desert.

Subsidence has reduced

California Aqueduct capacity

Back in the 1930s ground water pumping caused the ground beneath San Jose to sink to the point it was threatened with flooding during high tides in San Francisco Bay. The problem was resolved with the importation of water from the State Water Project.

Subsidence’s biggest threat is the continued delivery of water such as with the Friant-Kern Canal.

The 444-mile California Aqueduct that starts northwest of Tracy and supplies water to 27 million people as well as farms.

Prior to the aqueduct’s construction in the mid-1960s portions of the land it was built on has dropped 20 to 30 feet.

Subsidence stabilized for much of the first two decades the aqueduct was in operation. But then the 1976-1977 and 1986-1992 droughts hit. Increased pumping on nearby farmland has since significantly accelerated subsidence.

At one point from 2013 to 2016 sections of the aqueduct sunk nearly three feet.

Subsidence has reduced the capacity of segments of the aqueduct by 20 percent.

This has forced more water to be moved during peak energy times which only adds to California’s electricity woes.

A series of pumps are required to make it possible for 2.5 million acre feet of water each year to flow from the Delta to the Los Angeles Basin and beyond.

The biggest is the Edmonston Pumping Plant at the base of the Tehachapi range where 14 pumps move water over the 2,000-foot mountains.

It is why the biggest consumer in California of electricity is the movement of water via the California Aqueduct.

Infrastructure damage such as with the aqueduct caused by accelerated land subsidence threatens the effectiveness of many dams and canals sourcing surface water to the Central Valley.

Building foundations, pipelines, and other infrastructure may be at risk of damage as well. Areas with substantial subsidence may also be at higher risk of flooding.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com