<P> In the most complex placer mining, groups of prospectors would divert the water from an entire river into a sluice alongside the river, and then dig for gold in the newly exposed river bottom . Modern estimates by the U.S. Geological Survey are that some 12 million ounces (370 t) of gold were removed in the first five years of the Gold Rush (worth over US $16 billion at December 2010 prices). </P> <P> In the next stage, by 1853, hydraulic mining was used on ancient gold - bearing gravel beds on hillsides and bluffs in the goldfields . In a modern style of hydraulic mining first developed in California, and later used around the world, a high - pressure hose directed a powerful stream or jet of water at gold - bearing gravel beds . The loosened gravel and gold would then pass over sluices, with the gold settling to the bottom where it was collected . By the mid-1880s, it is estimated that 11 million ounces (340 t) of gold (worth approximately US $15 billion at December 2010 prices) had been recovered by "hydraulicking". </P> <P> A byproduct of these extraction methods was that large amounts of gravel, silt, heavy metals, and other pollutants went into streams and rivers . As of 1999 many areas still bear the scars of hydraulic mining, since the resulting exposed earth and downstream gravel deposits do not support plant life . </P> <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>

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