GEOMORPHIC RESPONSE TO LATE HOLOCENE CLIMATE CHANGE IN MOUNTAIN WATERSHEDS IN CENTRAL NEVADA

During the past century, mountain streams in the Central Great Basin have been experiencing episodes of rapid incision. Channel incision has exposed Late Holocene stratigraphy dating back to the Late Mid-Holocene Neoglacial (≈4,000 YBP). Climate reconstructions are based on woodrat midden composition and palynology. Stratigraphic analysis of sediments exposed in mountain streams in the Toiyabe, Toquima, and Monitor Ranges reveals a consistent regional picture of landscape response to Late Holocene climate change. The oldest alluvial unit (Qa1) observed in the valley fills ranges from 4,180 +/- 100 to 3,450 +/- 160 YBP, and corresponds to the Neoglacial, a cool wet period characterized by a high number of plant taxa. A fairly well defined paleosol in Qa1 suggests a period of landscape stability and non-deposition in the valley floor. Qa1 is buried by alluvium (Qa2) that interfingers with ‘side-valley' alluvial fan sediments (Qf2), indicating that fan building and valley aggradation were synchronous. Qa2 and Qf2 deposition dates to 1,960 +/- 50 – 2,580 +/- 7 YBP, which correlates to the onset of the Post-Neoglacial drought when the number of plant taxa declined by over 50%. Qa2 and Qf2 sediments are also rich in charcoal stringers and charcoal-rich sediment, which suggests that this period experienced frequent wildfires. The decrease in vegetation cover and increase in fire frequency led to increased sediment yield from the uplands and significant deposition on side-valley fans and the axial valley. A terrace (Qa3) is inset into Qa2 and is composed of sediments ranging in age from 1,310 +/- 50 to 1,250 +/- 50 YBP. These deposits correlate with the onset of the Medieval Warm Period and an increase in precipitation. The youngest valley fill unit, Qa4 is limited to paleochannel deposits located upstream of valley constrictions caused by side-valley alluvial fans. Qa4 dates from approximately 290 YBP and extends to the present, suggesting that the most recent phase of channel incision began prior to Anglo-American settlement. Small terraces inset below the valley floor have been dated using dendrochronology and correspond to years of unusually large amounts of runoff including the early 1970's, 1983, and 1995. The geomorphic events described above are primarily related to the dynamic interplay between vegetation and hillslope erosion associated with Late Holocene climate change.