Suspended Sediment Concentration in the San Francisco Bay, USGS. Click here for a larger version of the image.
Much of the gold extracted from the Sierra foothills during the Gold Rush was in placer deposits. That is, it was mixed with the rest of the sediment naturally eroding from the mountainside. Flecks of gold have a greater density than almost all the other particles and, thus, can be concentrated through natural water movement. A similar process is seen when you go to the beach. When the mixture of minerals and waves are just right you might notice darker grains of sand creating streaks or patches in the wet sand.
Miners had to devise ways to extract the gold because it was still a minor component even in rich placer deposits. Methods like panning and simple equipment like sluice boxes were used with moving water to enhance the natural mineral separation process.
When all this relatively easy-to-get gold was extracted from the streams and rivers prospectors turned to hydraulic mining to obtain the riches. Hydraulic mining was the process of using high-powered water canons to artificially erode gold-bearing hills made of sedimentary deposits. These sedimentary deposits were ancient stream beds that contained gold in placer deposits much like the modern streams did. Essentially, hydraulic mining eroded ancient river sediment from the hillside and diverted the material into the modern river where miners then extracted the gold.
Unsurprisingly, the activity of hydraulic mining devastated the local environment. The landscape was scarred and the mountain streams choked with gravel and sediment. And the effects weren’t just local. These rivers and streams flowed into the San Joaquin River and Sacramento River and deposited some of this sediment in the Central Valley causing flooding and navigation problems. Some of the finer sediment was transported even further, to the San Francisco Bay.
The effects of hydraulic mining practices are still measurable in the Bay today. Geologists from the USGS are studying the amount of sediment the Sacramento-San Joaquin Delta delivers to the Bay and are finding that the Gold Rush-induced sediment levels might be diminishing:
“[USGS geologist David Schoellhamer] says all the extra sediment has finally worked its way past the Golden Gate. The bay’s water is about 30 percent clearer than it was 10 years ago.”
It is taken many decades for this complex sediment delivery system to reach a new equilibrium. However, the readjustment of the estuary to these ‘new’ conditions might create new problems:
“Less sediment in the bay could spell trouble if scientists’ predictions about rising sea levels come to pass. These delicate tidal marshes could be inundated over the next century.”
What I find fascinating, yet also extremely challenging, is how the choices we’ve made as a civilization over the decades and centuries combine and sum to create the issues we face right now. There are no simple answers. Regardless of how well-intentioned some environmental programs may be there will always be some uncertainty about how natural systems respond. Continuing scientific research of these systems will reduce that uncertainty and inform policy decisions of the future.