California adjoins the Pacific Ocean, Oregon, Nevada, Arizona, and the Mexican state of Baja California. With an area of 160,000 square miles (414,000 km2), it is the third-largest state in the United States in size, after Alaska and Texas. If it were a country, California would be the 59th-largest in the world in area.

In the middle of the state lies the California Central Valley, bounded by the coastal mountain ranges in the west, the Sierra Nevada to the east, the Cascade Range in the north and the Tehachapi Mountains in the south. The Central Valley is California's agricultural heartland and grows approximately one-third of the nation's food. Divided in two by the Sacramento-San Joaquin River Delta, the northern portion, the Sacramento Valley serves as the watershed of the Sacramento River, while the southern portion, the San Joaquin Valley is the watershed for the San Joaquin River; both areas derive their names from the rivers that transit them. With dredging, the Sacramento and the San Joaquin Rivers have remained sufficiently deep that several inland cities are seaports. The Sacramento-San Joaquin River Delta serves as a critical water supply hub for the state. Water is routed through an extensive network of canals and pumps out of the delta, that traverse nearly the length of the state, including the Central Valley Project, and the State Water Project. Water from the Delta provides drinking water for nearly 23 million people, almost two-thirds of the state's population, and provides water to farmers on the west side of the San Joaquin Valley. The Channel Islands are located off the southern coast.