This summary from Maven's Notebook provides an excellent overview of the major water sourcing, storage and distribution projects throughout the Golden State.
California has built a water delivery infrastructure that is likely the most extensive anywhere on earth, capable of moving a drop of water that originates near the northern border all the way down south to the Mexican border. Through the development of this infrastructure, man has rearranged California's natural assets to meet societal needs, making the state unrecognizable from its pre-settlement history in the process.
California's climate and hydrology are unlike any other in the nation, with variability and uncertainty the main characteristics. In an average year, the total amount of precipitation is about 200 million acre-feet; however, the actual precipitation can vary anywhere from 100 million acre-feet to 300 million acre-feet, depending on whether it is a wet year, a dry year, or something in between. About half of the precipitation will evaporate, be used by vegetation, or sink into the subsurface, salt sinks, or flow to the ocean; the remaining half, known as ‘dedicated water' is what is available for use in cities, on farms, for the environment, or to be put in storage.
California has more than 1,400 named dams and 1,300 reservoirs that help with flood management, water storage and water transport. Hydropower from dams also provides a major source of electricity throughout the state and a source of revenue. Dams are owned, maintained and operated by federal, state and local agencies.
Groundwater is also an important part of the state's water supplies, comprising about 40% of water used in an average year, and 60% or more in a drought year. But groundwater is very much location dependent: some communities have no groundwater and rely solely on surface water while other communities may have only groundwater; other communities rely on a mix of imported water and groundwater, and even some rely solely on imported water. California has been identified as the heaviest groundwater user in the United States, with approximately 16% of the nation's groundwater supplies being extracted from the state's aquifers.
For details on where the water for your city or county comes from, check out this website from the Water Education Foundation: