The discovery of gold and the ensuing stampede to the mines reduced California's Indian population (by far the majority in 1845) from around 150,000 to 30,000 in a single decade, due to disease, dislocation, starvation, and outright murder. (White crimes against native peoples were sometimes regretted, never punished.)
The economy of California transformed from cattle (for Mexican rancheros) and hunter/gathering (for many of the native peoples) to a full-blown mix of mining, agriculture, and transportation in the same time frame. California's industrial imports spurred economies in developed countries and drew off excess population from areas that were having a hard time (like Ireland, Canton, and European countries that suffered violent counter-revolutions after 1848).
In addition to Northerners and Southerners from the U.S., Chinese, Irish, Chilean, Sonoran, Australian, French, English, Italian, and German immigrants left their homelands for California. Thanks to the invention of the steamship (which made passage around the Horn relatively safe and affordable) and the discovery of gold, California became an instant cultural kaleidoscope, with over a quarter of a million people pouring in during the first 3 years of the Gold Rush. This was unprecedented in human history.
The inclusion of California as a (nominally) Free State was strictly attributable to the Gold Rush. California's entry wildly exacerbated tensions between the North and South and came within a hair of triggering a civil war in 1850. Secession conventions held during the crisis laid the political groundwork for the South's later withdrawal from the Union in '60 and '61. (Ironically, the state government of California fell into Southern hands right away and largely stayed there through the 1850's. John C. Calhoun, who gave his dying speech against California's entry, need not have bothered his frosty locks at all.)
The discovery of gold created California's modern economy, funded the credit system that ensured Union victory in the Civil War, and triggered the building of the transcontinental railroad in the 1860's. This latter rocketed U.S. expansion into the West forward by decades and spelled the utter ruin of the Californian Indians and the speeded the downfall of tribes along the emigration routes to the West.
The Gold Rush is the defining moment in California history. Everything that has happened subsequently has taken place in its shadow.
