Simple panning and sluce mining was far to slow to suit the rich mining companies, and they had a captive job force of miners that couldn't find enough gold on their own, so they came up with a high-tech improvement in the process. The pictured monitor would be fed water from a high lake, and would use the water to blast away hillsides. The runoff would be channeled through a large sluice where mercury was used to help catch the gold. This runoff was then channeled into the Sacramento river were it found its way to San Francisco Bay. The state eventually woke up and made the practice illegal. But even today, 100 years later, the Bay is still heavily silted with this runoff and what fish remain have unsafe levels of mercury for human consumption.

Think about this next time you want to call environmentalist stupid tree-hugging hippy freaks.