California's Water Supply: Origins & Importance

Instructor: Sunday Moulton

Sunday recently earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

In this lesson, we'll look at water in the state of California. Learn where the state's water comes from and how they get it to people in the deserts of Southern California. You'll also see why water is so vital to California's thriving economy.

Water is Life in California

While the saying 'water is life' is true everywhere, in California, it is doubly true. There are expanses of fertile fields in places with limited rainfall and large cities rising out of the desert. Farming contributes heavily to making California the eighth-largest economy in the world. As one of the largest states for landmass, population, and agriculture, the water needs of California and the amount of naturally available water does not seem to match. So how is any of this possible?

Natural Sources of Water

Before we get into the complexity of California's irrigation efforts, let's look at the natural sources of available water. These can be categorized into two groups: surface water and groundwater.

Surface Water

Surface water is water flowing in rivers or collected in lakes and ponds, basically water on the surface of the Earth.

The San Joaquin River is a significant source of surface water in California.
San Joaquin

This surface water is replenished from precipitation in the form of rain or snow that melts in the spring. Most of the state's precipitation occurs in the northern 1/3 of the state, averaging 100 inches or more a year, where we find 75 percent of the available water. Unfortunately, 80 percent of the urban and agricultural demand for water happens in the southern 2/3 of the state.

This rain collects in ten major drainage basins. The state uses many of these basins as a collection point to begin diverting water to drier areas. The ten basins are as follows:

  • North Coast
  • Sacramento River
  • North Lahontan
  • San Francisco Bay
  • San Joaquin River
  • Central Coast
  • Tulare Lake
  • South Lahontan
  • South Coast
  • Colorado River

The Delta, an area where the San Joaquin River and the Sacramento River meet, is one of the largest regions for harvesting and exporting water to other parts of the state. This is where the State Water Project (SWP) pumps water to serve millions of customers in the San Francisco Bay Ares as well as people living in the San Joaquin Valley, the Central Coast, and many parts of Southern California.

The Colorado River is a second major source of water for people in California as well as people in six other states and Mexico. The long river stretches from Wyoming to the Gulf of California, traveling 1,440 miles from source to mouth. The Metropolitan Water District harvests the water to serve cities and towns across Southern California. This water also helps irrigate fields in the Imperial, Palo Verde, and Coachella Valleys.


Groundwater is all the water found below the surface and in the large and small gaps between rocks. Sometimes, the water collects in an area underground or flows through an underwater pathway. These collection areas are called aquifers. In a typical year, the water extracted from aquifers, through wells, supply 40 percent of the state's water needs.

Water flow cycle for groundwater
groundwater cycle

Developed Water Supply

The developed water supply involves all the water collected in reservoirs and moved through man-made supply systems. For California, this involves an extensive network of resources to meet the needs of all the farms and cities in the southern half of the state. In fact, California has more irrigated acreage of farmland than any other state.

Irrigation helps California to be one of the largest agricultural producing states.


Through the various water agencies and the state, water reaches Southern California through three major aqueducts, pipelines and channels created to artificially direct water.

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