The Delta–Mendota Canal is a 117-mile-long aqueduct in central California, United States. The canal was designed and completed in 1951 by the U.S. Bureau of Reclamation as part of the Central Valley Project to supply freshwater to users downstream of the San Joaquin River.
Sinking Ground Puts San Joaquin Valley Water Systems at Risk
SACRAMENTO, Calif. – The Department of Water Resources (DWR) has released a comprehensive new assessment of water conveyance in the San Joaquin Valley – home to more than 4 million Californians and one of the nation’s most important agricultural regions. The San Joaquin Valley Conveyance Study examines the effects of land subsidence on the systems that move water across the region and evaluates the need for infrastructure improvements or expansions to support long-term water reliability.
The study, an action in Governor Newsom’s 2020 Water Resilience Portfolio, finds that the top priority for improving conveyance in the San Joaquin Valley is stopping or minimizing land subsidence, especially near State Water Project and Central Valley Project conveyance facilities. This can only be achieved over the long term by raising groundwater levels above critical thresholds. The study also finds that repair of existing conveyance infrastructure is more important than expanding or building new conveyance because the region has limited surface water supplies.
“This study makes the picture unmistakably clear: the most effective path forward is stabilizing groundwater levels and repairing the major canals that bring surface water to the San Joaquin Valley,” said Joel Metzger, Deputy Director of Statewide Water Resources Planning and Enterprise Project Management. “Taking action to support these goals is essential if we want to protect the Valley’s communities, support its agricultural economy, and maintain a functioning water system in a hotter, more extreme future.”
Sinking Ground, Shrinking Capacity
The San Joaquin Valley experiences some of the most widespread, severe land subsidence in the nation caused by decades of groundwater decline. As groundwater has been pumped faster than it can be replenished, the land has sunk permanently, reducing the capacity of California’s water infrastructure to move water efficiently.
Facilities like the San Luis Canal, California Aqueduct, Delta-Mendota Canal, and Friant-Kern Canal serve as the conveyance backbone of the State Water Project and Central Valley Project, but subsidence has limited their conveyance capacity. Some sections of the San Luis Canal have sunk by more than eight feet since it was placed into operation in the 1960s. 2023 levels of subsidence have resulted in a 44 percent reduction in California Aqueduct capacity. In 2017, severe subsidence — over 10 feet in some areas — reduced deliveries from the Friant-Kern Canal by about 300,000 acre-feet.
This loss of capacity limits California’s ability to capture and move floodwater in wet years, leaving communities reliant on already stressed groundwater supplies. If land continues sinking at the pace observed over the past decade, it could severely impact the state’s ability to deliver water, hurting California’s economy, public health, and overall safety.
Limited Surface Water Supplies
The study also shows that many conveyance facilities such as canals and aqueducts in the San Joaquin Valley are rarely filled to their maximum capacity — a sign that water availability, not conveyance capacity, is the limiting constraint. With limited surface water supplies, new or expanded conveyance facilities would provide limited benefits.
The San Joaquin Valley Conveyance Study follows the State Water Project Adaptation Strategy as the second report in a series of ongoing studies conducted by DWR that underscore the future challenges, opportunities, and solutions for water management in the San Joaquin Valley.
The next report, the San Joaquin Basin Flood-MAR Watershed Studies, assesses climate change impacts across the San Joaquin Basin’s five tributary watersheds and opportunities for groundwater recharge-related actions to support water supply, groundwater, flood management, ecosystem, and equity needs. The Watershed Studies are anticipated to be released in December 2025.
Contact:
Marina Gelpi Clay, Public Affairs, Department of Water Resources
media@water.ca.gov