Central Valley, great trough of central Calif., c.450 mi (720 km) long and c.50 mi (80 km) wide, between the Sierra Nevada and the Coast Ranges. The Sacramento and San Joaquin rivers drain much of the valley before converging in a huge delta and flowing into San Francisco Bay; the delta is California's leading truck-farming and horticultural area. The Central Valley is California's agricultural heartland, although its urban and suburban areas have expanded dramatically since the 1970s. With its long growing season and fertile soil, the valley has the largest single concentration of fruit and nut farms and vineyards in the United States; cotton, grain, and vegetables are also grown. Precipitation ranges from 30 in. (76 cm) in the north to 6 in. (15.2 cm) in the south. Two thirds of the valley's agricultural land is in the south, while two thirds of its water is in the north. The Central Valley project sought to address this problem by bringing water from the Sacramento basin in the north into the San Joaquin Valley in the south, where Fresno and Tulare counties are the two leading U.S. agricultural counties. The dry, alkaline Tulare Lake basin in the extreme south is almost totally unsuitable for irrigation. Oil extraction and refining and petrochemical production are also important in the region. The Central Valley was seen by Spanish explorers in the 1500s but remained virtually uninhabited until the 1849 California gold rush. Irrigation was introduced in the 1880s.