Gold mining in California in the 19th century was a boon for the state's economy but not so much for the environment. Mining left a protracted legacy that impacts the natural landscape even today. Mercury, used in the gold extraction process, has been detected throughout the Lower Yuba/Feather River system in the state's Central Valley, and its presence could prove dangerous to local wildlife.
That mercury, which will remain in dry river sediment for thousands of years, generally poses a problem only when exposed to extreme water conditions. Flooding triggers a process called methylation, which causes a portion of the mercury to become toxic. When ingested by wildlife, this so-called methylmercury can negatively affect cardiovascular and central nervous systems.
http://www.sott.net/article/314725-Toxic-legacy-of-gold-mining-Flooding-causes-mercury-in-dry-river-sediment-to-become-toxic
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