Hydraulic mining, or just "hydraulicking," was a method of gold mining originally invented by the Romans in Spain. Rather than using human labor to dig gold out of the hills, powerful jets of water were sent out from holding tanks as much as 800 feet above the mining site. The jets broke up the ground and exposed the gold-bearing rocks a lot more quickly than digging did. Unfortunately, it also washed away a lot of soil and rock, decreasing available farmland and silting rivers.

The method was used intermittently in Europe over centuries (as you can imagine, mostly in less-settled areas). But it made a major reappearance in the American West after the 1849 California Gold Rush. The individual miners became teams, and eventually companies took over to get to gold that was not easily accessible from the surface. Again, gravity was used to channel jets of water through pipes that were essentially water cannons, blasting mountains into rubble. The resulting gravel was washed through sluices so that the heavier gold could be separated from the lighter rocks and soil, which ended up in the nearest convenient river valley. Some estimates say the amount of earth moved in California gold mining was eight times that excavated for the Panama Canal.

However, the farmers and settlements downstream were not happy. Some rivers were so clogged that ships could no longer navigate them, and not surprisingly, the fish and other wildlife suffered a lot. The political power of the mining lobby kept hydraulic mining going in California until 1884 when federal circuit judge Lorenzo Sawyer ruled that debris could not be dumped where it could reach farmland or navigable rivers. This environmentalist step was a beginning, though some few mines that could find alternate dumping sites did continue operating. Many of the water supplies originally used for mining were converted to transport water for irrigation or hydropower. But the minerals leaching out of the debris affected the water quality of parts of Northern California for a long time.

Sources:
Bernstein, Peter L. The Power of Gold: The History of an Obsession. New York: John Wiley & Sons, Inc., 2000.
http://www.museumca.org/goldrush/fever19-hy.html
http://cprr.org/Museum/Hydraulic_Mining/hydraulic_mining_1of1.html
http://water.wr.usgs.gov/mining/
http://memory.loc.gov/ammem/award97/codhtml/hawpSubjects156.html

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