San Joaquin County is a county located in Central Valley of the U.S. state of California, just east of the San Francisco Bay Area. As of 2006, the population was approximately 620,000.The urban population is 610,783 people. The county seat is Stockton. San Joaquin County was one of the original counties of California, created in 1850 at the time of statehood. The county takes its name from the San Joaquin River. In the early 19th century Lieutenant Gabriel Moraga, commanding an expedition in the lower great California Central Valley, gave the name of San Joaquin (meaning Joachim) to a rivulet that springs from the Sierra Nevada and empties into Buena Vista Lake.