While the vast majority of California, including its lengthy coastline and mountain ranges, sits well above the ocean, several large and economically important regions are significantly below the elevation of the Pacific Ocean. This situation results from a combination of ancient tectonic forces and modern human activities, particularly intensive agriculture and water management. The existence of these low-lying areas creates unique and costly challenges for infrastructure, water supply, and flood control across the state. Understanding the state’s true elevation profile requires acknowledging these deep depressions.
Defining California’s Sea Level Status
Large basins within California exist far below Mean Sea Level (MSL), the average height of the ocean’s surface used as the reference point for elevation measurements. While the state’s overall average elevation is high, this average masks deep depressions within its interior. The vast Central Valley and mountain ranges, such as the Sierra Nevada, are situated high above MSL. However, the basins that dip below MSL are not small, isolated pockets; they cover significant swaths of land. These large, below-sea-level areas are critical for agriculture and the state’s water system, necessitating constant management to prevent widespread flooding and protect resources.
The Geography of Submerged Land
Two major regions in California contain extensive areas of land below sea level. The first is the Sacramento-San Joaquin River Delta, a vast network of islands and tracts that forms the hub of the state’s water transfer system. Many Delta islands are protected by levees, with their interiors sitting between 10 and 25 feet below MSL in some locations. Locally, some areas are more than 26 feet below the surrounding water channels, creating significant hydrostatic pressure on the protective structures.
The second major depression is in the southeastern corner of the state, encompassing the Imperial Valley and the Salton Trough. This region contains some of the lowest points in the United States, including the floor of the Salton Sink, which is approximately 279 feet below sea level. The Imperial Valley is a heavily populated and farmed area existing almost entirely below the zero elevation mark, making it highly dependent on imported water and flood control measures.
Geological and Human Factors Causing Subsidence
The formation of these deep depressions is due to both natural geological forces and subsidence accelerated by human activity.
Tectonic Movement
The Imperial Valley’s low elevation is primarily the result of ancient tectonic movement. The region sits within the Salton Trough, a tectonically active pull-apart basin. This basin is a continental rift zone formed by the complex interaction and spreading of the Pacific and North American tectonic plates along the San Andreas Fault system. This geological spreading created a natural depression that continues to accumulate sediment.
Peat Oxidation
In contrast, the sinking of the Sacramento-San Joaquin Delta islands is largely due to human engineering and land use. The Delta’s original marshlands contained thick layers of peat, an organic soil drained for farming in the late 19th and early 20th centuries. Once exposed to oxygen, the peat decomposed rapidly through microbial oxidation, causing the land surface to shrink. This oxidation has caused the land to subside at average rates of one to three inches per year, with some areas having subsided over 25 feet since reclamation began.
Groundwater Withdrawal
A separate cause of subsidence, particularly in the Central Valley and coastal areas, is the excessive withdrawal of groundwater. Pumping vast quantities of water from underground aquifers removes the support structure from the saturated sediments. This reduction in pore pressure causes the fine-grained soils to compact. This compaction leads to a permanent, irreversible lowering of the land surface, further exacerbating flood risk and infrastructure vulnerability.
Infrastructure and Water Management Implications
The presence of vast, below-sea-level regions creates complex and costly demands on California’s infrastructure and water management systems. The Sacramento-San Joaquin Delta is protected by approximately 1,100 miles of fragile earthen levees. These levees are constantly stressed by the elevation difference between the water channel and the sinking islands. Failure from earthquakes or floods could lead to catastrophic flooding of agricultural tracts and critical infrastructure.
A levee breach would also allow saltwater from the San Francisco Bay to rush in, severely degrading the quality of freshwater exported through the Delta. This water supplies over 30 million Californians and millions of acres of farmland. Additionally, the sinking of coastal and valley areas due to groundwater pumping increases the risk of saltwater intrusion into coastal aquifers, threatening shallow infrastructure with brackish water and requiring mitigation efforts.