Historian Benjamin Madley recorded the numbers of killings of California Indians between 1846 and 1873 and estimated that during this period at least 9,400 to 16,000 California Indians were killed by non-Indians, mostly occurring in more than 370 massacres.defined as the "intentional killing of five or more disarmed combatants or largely unarmed noncombatants, including women, children, and prisoners, whether in the context of a battle or otherwise".According to demographer Russell Thornton, between 1849 and 1890, the Indigenous population of California fell below 20,000 primarily because of the killings.According to the government of California, some 4,500 Native Americans suffered violent deaths between 1849 and 1870.Furthermore, California stood in opposition of ratifying the eighteen treaties signed between tribal leaders and federal agents in 1851.
The Mexican–American War ended on February 3, 1848, although California was a de facto American possession before that. The Treaty of Guadalupe Hidalgo provided for, among other things, the formal transfer of Upper California to the United States. The California Gold Rush began at Sutter's Mill, near Coloma. On January 24, 1848, James W. Marshall, a foreman working for Sacramento pioneer John Sutter, found shiny metal in the tailrace of a lumber mill Marshall was building for Sutter on the American River. Marshall brought what he found to John Sutter, and the two privately tested the metal. After the tests showed that it was gold, Sutter expressed dismay: he wanted to keep the news quiet because he feared what would happen to his plans for an agricultural empire if there were a mass search for gold.
Rumors of the discovery of gold were confirmed in March 1848 by San Francisco newspaper publisher and merchant Samuel Brannan. Brannan hurriedly set up a store to sell gold prospecting supplies, and walked through the streets of San Francisco, holding aloft a vial of gold, shouting "Gold! Gold! Gold from the American River!"
On August 19, 1848, the New York Herald was the first major newspaper on the East Coast to report the discovery of gold. On December 5, 1848, US President James Polk confirmed the discovery of gold in an address to Congress. As a result, individuals seeking to benefit from the gold rush--later called the "forty-niners"--began moving to the Gold Country of California or "Mother Lode" from other countries and from other parts of the United States. As Sutter had feared, his business plans were ruined after his workers left in search of gold, and squatters took over his land and stole his crops and cattle.
San Francisco had been a tiny settlement before the rush began. When residents learned about the discovery, it at first became a ghost town of abandoned ships and businesses, but then boomed as merchants and new people arrived. The population of San Francisco increased quickly from about 1,000 in 1848 to 25,000 full-time residents by 1850. Miners lived in tents, wood shanties, or deck cabins removed from abandoned ships.
The Gold Rush propelled California from a sleepy, little-known backwater to a center of the global imagination and the destination of hundreds of thousands of people. The new immigrants often showed remarkable inventiveness and civic-mindedness. For example, in the midst of the Gold Rush, towns and cities were chartered, a state constitutional convention was convened, a state constitution was written, elections held, and representatives sent to Washington, D.C. to negotiate the admission of California as a state.
Large-scale agriculture (California's second "Gold Rush") began during this time. Roads, schools, churches, and civic organizations quickly came into existence. The vast majority of the immigrants were Americans. Pressure grew for better communications and political connections to the rest of the United States, leading to statehood for California on September 9, 1850, in the Compromise of 1850 as the 31st state of the United States.
Between 1847 and 1870, the population of San Francisco increased from 500 to 150,000. The Gold Rush wealth and population increase led to significantly improved transportation between California and the East Coast. The Panama Railway, spanning the Isthmus of Panama, was finished in 1855. Steamships, including those owned by the Pacific Mail Steamship Company, began regular service from San Francisco to Panama, where passengers, goods, and mail would take the train across the Isthmus and board steamships headed to the East Coast. One ill-fated journey, that of the S.S. Central America, ended in disaster as the ship sank in a hurricane off the coast of the Carolinas in 1857, with approximately three tons of California gold aboard.