Gold Rush-era mercury will flow during floods

The destructive environmental legacy of Gold Rush mining in the Sierra Nevada could last for thousands of years in the form of ongoing erosion of mercury-laced sediments, according to new research.
Mercury, a toxic heavy metal, was used in copious amounts in California's hydraulic gold mining operations in the mid- and late 1800s.

Miners blasted gold-bearing sediment out of vast, ancient gravel beds with water cannons. They then added liquid mercury to the slurry, allowing the gold-mercury amalgam to sink to the bottom of troughs.
http://www.appeal-democrat.com/news/article_fbd14150-4062-11e3-a043-0019bb30f31a.html