<P> After the Gold Rush had concluded, gold recovery operations continued . The final stage to recover loose gold was to prospect for gold that had slowly washed down into the flat river bottoms and sandbars of California's Central Valley and other gold - bearing areas of California (such as Scott Valley in Siskiyou County). By the late 1890s, dredging technology (also invented in California) had become economical, and it is estimated that more than 20 million ounces (620 t) were recovered by dredging (worth approximately US $28 billion at December 2010 prices). </P> <P> Both during the Gold Rush and in the decades that followed, gold - seekers also engaged in "hard - rock" mining, that is, extracting the gold directly from the rock that contained it (typically quartz), usually by digging and blasting to follow and remove veins of the gold - bearing quartz . By 1851, quartz mining had become the major industry of Coloma . Once the gold - bearing rocks were brought to surface, the rocks were crushed and the gold separated, either using separation in water, using its density difference from quartz sand, or by washing the sand over copper plates coated with mercury (with which gold forms an amalgam). Loss of mercury in the amalgamation process was a source of environmental contamination . Eventually, hard - rock mining wound up becoming the single largest source of gold produced in the Gold Country . The total production of gold in California from then till now is estimated at 118 million ounces (3700 t). </P> <Ul> <Li> <P> Forty - niner panning for gold </P> </Li> </Ul> <Li> <P> Forty - niner panning for gold </P> </Li>

How did the gold rush affect the population of california