History of California from 1825 onwards from Wikipedia

Starting about 1825 the Mission Indian population started decreasing rapidly, as Indian deaths far exceeded births. The various acquired diseases and abuse of the Mission Indian population caused them to decline from over 80,000 in 1820 to only a few thousand by 1846. This process was sped up when in 1834–1836 the Mexican government, responding to complaints that the Catholic Church owned too much land (over 90% of all settled land in California), secularized (dismantled) the Missions and essentially turned the Indians loose to survive on their own. Most of the Indians went from doing unpaid labor at the Missions to doing unpaid labor as servants in the pueblos or workers on the ranchos. Other Indians returned to small Indian settlements in the sparsely settled Central Valley and Sierra Mountains of California. As the Mission Indians rapidly declined in population and the Missions were dismantled, most of the agriculture, orchards, vineyards, etc. which had been raised by the Mission Indians rapidly declined. By 1850 the Hispanic (Spanish speaking) population had grown to about 9,000 with about 1,500-2,000 adult males. By 1846 there were about 2,000 emigrant non-Hispanics (nearly all adult men) with from 60,000 to 90,000 California Indians throughout the state. Beginning in about 1844 the California Trail was established and started bringing new settlers to California as its relative isolation started to break.
The Mexican–American War began in May 1846, and the few marines and bluejacket sailors of the Pacific Squadron and the California Battalion of volunteer militia had California under U. S. control by January 1847, as all the pueblos in California surrendered without firing a shot. In February 1848 the war was over, the 25 years of Mexican rule with over 40 different Mexican Presidents was over, and the boundary disputes with Texas and the territorial acquisition of what would become several new states were settled with a $15,000,000 payment agreed at the Treaty of Guadalupe Hidalgo.
The California Gold Rush, beginning in January 1848, increased California's non Indian, non-Hispanic population to over 100,000 by 1850.This increased population and prosperity eventually led to the Congressional Compromise of 1850 which admitted California in 1850 as a free state—the 31st.