Effects of the 1997 flood on the transport and storage of sediment and mercury within the Carson River valley, west-central Nevada

Intense, warm rains falling on a heavy snowpack in the Sierra Nevada at the end of December 1996 produced some of the largest floods on record in west -central Nevada. Within the Carson River basin, a peak discharge of 632 cm was recorded at the Fort Churchill gaging station on January 3, 1997, a flo w exceeding the 100-yr event. Geomorphic impacts of the event, and the redi stribution of mercury (Hg) released to the Carson River valley by Comstock mining operations during the mid- to late-1800s, were assessed by combining field data with the interpretation of aerial photographs. Geomorphic. impa cts included significant increases in channel width, measuring up to 280% o f preflood conditions, and large-scale shifts in channel position, ranging from <10 to 110 m. Both changes in channel width and position vary as a fun ction of valley morphometry (width and slope) and differ from the long-term trends measured from 1965 to 1991. The 1997 flood also produced widespread overbank deposits that vary morphologically and sedimentologically accordi ng to distance from the channel and the nature of the vegetation on the val ley floor. Within the overbank deposits, Hg is primarily associated with th e fine-grained (<63 mu m) sediment fraction, which makes up a larger percen tage of the deposits immediately adjacent to the channel and at the extremi ties of overbank deposition. Mass balance calculations demonstrate that, al ong reaches with narrow valleys (<450 m), approximately 10%-65% of the sedi ment eroded from the channel banks was stored in overbank deposits, whereas more than 90% of the sediment eroded along reaches with wider valleys was stored on the valley floor. Locally, however, storage exceeded 650% where m eander cutoff was extensive. The above data indicate that the erosion, rede position, and storage of sediment and sediment-bound Hg were greater along reaches characterized by low gradients and wide valley floors. Downstream t rends in Hg concentration within the channel bed did not change following t he 1997 flood and are presumably controlled by the overall structure of the system, including valley morphometry, the location of tributaries that del iver "clean" sediment to the channel, and the distribution of Hg within the valley fill.