9714150 Miller Mining of the Comstock Lode (1860-1890) resulted in the release of an estimated 7,500 tons of mercury to the Carson River system including Lahontan Reservoir. Much of the polluted sediment is associated with filled paleochannels which are thought to have been produced by the filling of abandoned meanders previously cutoff from the main channel. Given that a majority of the present day mercury load in the Carson River is derived from bank erosion, it follows that during floods, much of the eroded sediment is returned to storage by the deposition of sediment within the channels of the abandoned meanders. Thus, there may be a cycling of contaminated debris between the channel and the valley fill during flood events which inundated the valley floor. In early January, 1997 flood flows, with a magnitude which has only been exceeded once in the past 57 years of record, inundated large regions of the Carson River valley. Field observations made during and following the event revealed that numerous reaches along the channel suffered extensive erosion, while thick sedimentary sequences were deposited over other regions of the valley floor. Based on these field observations, we believe that this low frequency, high magnitude event had a significant affect on the transport and storage of mercury contaminated particles along the Carson River valley. The primary objectives of this investigation are to (1) document the patterns of erosion and deposition that occurred along the Carson River valley from Carson City to Lahontan Reservoir during the January, 1997 flood, (2)determine the spatial variations in mercury concentration that occur within the newly formed flood deposits, and (3) assess the primary controls on both the deposition of contaminated sediments and the distribution of mercury within those deposits. By combining the conclusions reached in this investigation with previous studies of the Carson River, we will be able to assess the transport and fate of mercury by phys ical processes over time periods ranging from individual flood events to decades. In addition, a comparison of the geographic trends measured prior to and following the storm will lead to significant insights into the importance of rare, high magnitude floods on the dispersal of heavy metals in alluvial deposits.
