California is sinking, but the movement is not uniform and occurs at highly variable speeds across the state. This phenomenon, known as land subsidence, is the lowering of the ground surface and is distinct from global sea level rise. The fastest and most dramatic rates of sinking are a direct result of human activity far inland. The primary zones of concern are the immense agricultural valleys where the land surface is dropping at rates that compromise critical infrastructure.
The Primary Driver of Rapid Sinking Groundwater Depletion
The most rapid rates of land sinking are directly linked to the widespread practice of groundwater extraction, particularly within the Central Valley. During drought cycles, when surface water supplies are diminished, agricultural and municipal users increase their reliance on underground aquifers. This extensive withdrawal lowers the water table, which reduces the hydraulic pressure supporting the weight of the overlying ground materials.
The mechanism involves a process called aquifer-system compaction, where the physical load shifts from the water to the granular structure of the sediments. The San Joaquin Valley’s geology is characterized by layers of compressible silt and clay, known as aquitards, interspersed with water-bearing sand and gravel. As water is drained from these fine-grained clay layers, they compress, similar to a squeezed sponge.
This compaction is largely irreversible. Even if water levels recover, the land surface will not rise back to its original elevation. The removal of water permanently alters the aquifer’s structure, causing a lasting loss of underground storage capacity, and the sinking accelerates during periods of drought when pumping intensifies.
Measuring Rates and Geographic Hotspots
The speed at which the land is sinking is measured using advanced satellite and ground-based technologies, providing precise data on vertical movement. Interferometric Synthetic Aperture Radar (InSAR), a satellite-based technique, is routinely used to create maps of surface deformation with centimeter-level accuracy. This remote sensing data is often combined with measurements from Continuous Global Positioning System (CGPS) stations to monitor ongoing changes.
These measurements reveal that the most dramatic sinking occurs in localized “subsidence bowls” within the San Joaquin Valley. Rates in key hotspots have been documented as high as 0.6 meters (nearly two feet) per year during severe drought periods. For example, the El Nido area experienced a maximum rate of 270 millimeters (about 10.6 inches) per year between 2008 and 2010.
On a broader scale, the average rate of sinking across the entire San Joaquin Valley reached approximately one inch per year between 2006 and 2022. Historically, some sections of the valley have subsided by as much as 8.5 meters (28 feet) since the 1920s. Significant subsidence has been observed near the towns of Corcoran and Chowchilla.
The Role of Tectonic Activity
In addition to rapid human-induced changes, California is subject to slower, natural vertical land movement driven by the interaction of the Pacific and North American tectonic plates. This long-term geologic process causes the Earth’s crust to either rise (uplift) or fall (tectonic subsidence) at a relatively constant rate. These tectonic movements are measured in millimeters per year, a magnitude far less significant than the localized effects of groundwater depletion.
Some coastal areas may experience uplift of several millimeters annually, while other locations near fault lines may sink at similar rates. Examples of natural tectonic subsidence are seen in certain estuaries, with average rates of 1.2 to 1.4 millimeters per year. This slow, background movement provides a long-term context for land elevation change.
Impact on Infrastructure and Water Conveyance Systems
The physical consequences of land subsidence are most apparent in the damage inflicted upon California’s public infrastructure. The sinking land damages roads, bridges, building foundations, and well casings, leading to substantial repair costs and community disruption. In some areas, the compacting clay layers have even caused groundwater well casings to buckle and collapse.
The state’s vast water conveyance network is particularly sensitive to these ground elevation changes. Critical components, including the California Aqueduct and the Friant-Kern Canal, have been severely impacted. When the land underneath a canal subsides, it alters the precise gradient required for water to flow by gravity, which reduces the canal’s capacity to move water.
Subsidence near Mendota, for example, has reduced the water conveyance capacity of the California Aqueduct by up to 46% in affected segments. Furthermore, the lowering of the land surface has exacerbated flood hazards by reducing the capacity of flood control channels, such as the Eastside Bypass. This diminished capacity restricts the movement of water during peak flows, increasing flood risk.