The first techniques of placer mining—essentially using the natural hydraulic energy of moving streams to sift gravel through pans and sluices—were not by themselves especially destructive. But the mere arrival of hundreds of thousands of prospectors in the previously sparsely inhabited Sierra foothills quickly resulted in large-scale deforestation and the dispersal of all wildlife.
Later, after the easy gold had been collected and mining required more intensive methods, the environmental costs rose steeply. Entire streams were diverted into massive sluices in order to filter ore, while larger rivers were dammed to store water for sluicing in the dry summer months.
When even these techniques proved ineffectual, miners developed a new technology: hydraulic mining, in which miners blasted water from high-pressure hoses against ore-bearing hillsides, eroding away massive amounts of earth to uncover the gold hiding within. This technique proved disastrous in years of heavy snowpack, when the spring runoff would carry vast quantities of silt and sediment downstream, where it buried the fertile farmland in the valleys below. The farmers called the muck unloosened by hydraulic mining "the slickens," and on at least one occasion a veritable flood of the stuff completely buried a farmhouse—eighteen feet deep! Despite the environmental chaos being wrought by the slickens, hydraulic mining remained the technology of choice in the Sierra foothills for nearly twenty years, irreparably destroying countless mountain watersheds and thousands of acres of farmland in the Sacramento valley. Even today, more than 150 years later, the serpentine gray sediment left behind by the slickens in California's Yuba River Valley remains visible from space.