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Enduring legacy of a toxic fan via episodic redistribution of California gold mining debris

Research output: Contribution to journalArticle

Author(s)

Michael Bliss Singer, Rolf Aalto, L. Allan James, Nina E. Kilham, John Lee Higson, Subhajit Ghoshal

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Abstract

The interrelationships between hydrologically driven evolution of legacy landscapes downstream of major mining districts and the contamination of lowland ecosystems are poorly understood over centennial time scales. Here, we demonstrate within piedmont valleys of California’s Sierra Nevada, through new and historical data supported by modeling, that anthropogenic fans produced by 19th century gold mining comprise an episodically persistent source of sediment-adsorbed Hg to lowlands. Within the enormous, iconic Yuba Fan, we highlight (i) an apparent shift in the relative processes of fan evolution from gradual vertical channel entrenchment to punctuated lateral erosion of fan terraces, thus enabling entrainment of large volumes of Hg-laden sediment during individual floods, and (ii) systematic intrafan redistribution and downstream progradation of fan sediment into the Central Valley, triggered by terrace erosion during increasingly long, 10-y flood events. Each major flood apparently erodes stored sediment and delivers to sensitive lowlands the equivalent of ∼10–30% of the entire postmining Sierran Hg mass so far conveyed to the San Francisco Bay-Delta (SFBD). This process of protracted but episodic erosion of legacy sediment and associated Hg is likely to persist for >104 y. It creates, within an immense swath of river corridor well upstream of the SFBD, new contaminated floodplain surfaces primed for Hg methylation and augments/replenishes potential Hg sources to the SFBD. Anticipation, prediction, and management of toxic sediment delivery, and corresponding risks to lowland ecology and human society globally, depend on the morphodynamic stage of anthropogenic fan evolution, synergistically coupled to changing frequency of and duration extreme floods.
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Details

Original languageEnglish
Pages (from-to)18436-18441
JournalProceedings of the National Academy of Sciences of the United States of America
Volume110
Issue number46
Early online date28 Oct 2013
DOIs
Publication statusPublished - 2013

    Research areas

  • Mercury, Lowland contamination, Flood frequency, Climate change, Morphodynamics

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