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.

Images: (1) California Water Science Center; (2) Wikipedia

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In honor of Earth Day, we wanted to take a big look at a chronic environmental issue in the Bay Area, tracing it from its origins to the contemporary strategies to solve it. Mercury was the obvious choice: It's been flowing into the Bay since before California joined the union, and it continues to trickle in from not just the old culprits, like gold and mercury mines, but a modern crop of industries, like refineries and cement kilns. Even little things – like a broken mercury thermometer dumped into the sink – are part of the problem.

The key fact here is how incredibly potent mercury can be: Just one little globule from an old thermometer can poison all the fish in a 45-acre lake, making them unsafe for humans to eat. Mercury pollution is hardly unique to the Bay Area; what makes us interesting is that local officials are making real strides in trying to clean it up. Over the next 17 years or so, we'll spend $2.6 billion dollars on the project. Even then, we won’t have a clean bay for 120 years.

For a lot of people, mercury pollution in the Bay is largely theoretical, since few stores sell fish caught in the Bay, and relatively few residents fish for their food. But some still do – including many recent immigrants from fishing-intensive cultures like Laos. We’ll look at how mercury affects the health of local fishermen next week.

This piece marks our first-ever audio slide show, and what a difference it makes! We also hope you'll check out the mercury map above, where you can see how many pounds of mercury come from each of the Bay Area’s five refineries, plus other mercury sources and the bay's popular fishing spots.

Watch the audio slide show of "Mercury in the Bay" online, as well as find additional links and resources.

Amy Standen is a Reporter for QUEST and Radio News at KQED-FM.

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