CONSTRAINING THE TIMING, MAGNITUDE, AND SPATIAL DISTRIBUTION OF HOLOCENE ALLUVIAL FAN ACTIVITY AT WALKER LAKE, NV USING DATED SHORELINES

The Holocene lake-level record of Walker Lake was used to decipher the timing and spatial occurrence of surrounding alluvial fan activity to better understand hydrologic and landscape responses to relatively minor climate changes in the western Great Basin. These records are highly complementary because changes in lake level reflect the hydrologic balance of the basin over decadal and longer timescales, while alluvial fan activity is dictated by the frequency and seasonality of high-intensity rainfall events.

Dated Holocene shorelines at Walker Lake provide basin-wide horizontal chronostratigraphic timelines that were used to constrain the timing and distribution of alluvial fan activity around the basin through detailed mapping (1:5000-scale). Several late Holocene shorelines (< 3600 cal yr BP) occur in a band from 1253 – 1262 masl, or just above the 1868 CE highstand. Assuming a total piedmont area surrounding Walker Lake of about 564 km², historical alluvial fan deposits (Qf5) cover about 1.5 % (8.6 km²), Qf4 fans (2700 – 82 cal yr BP) cover 1.2 % (6.5 km²), Qf3 fans (3600 – 2700 cal yr BP) cover 12.8 % (72.3 km²), Qf2 fans (15,500 – 3600 cal yr BP) cover 11.4 % (64.3 km²), and Qf1 fans (>15,500 cal yr BP) cover about 4 % (22.7 km²) of the piedmont. It is notable that the largest cumulative area of fan deposition (Qf3) occurred over a relatively brief period when lake level was relatively high.

Most of the young fans, including historical ones, are composed of coarse bouldery debris flow deposits but sheetflood and hyperconcentrated (HC) flow textures are also present. According to radar data, a HC flow emanated from Copper Canyon on the west side of Walker after about 40 – 50 mm of rain fell in about 1.5 hours during August 2014. At Winnemucca Lake to the northwest, 100 – 125 mm of rain fell in a period of about 4 hours in June 2013, causing abundant coarse debris flows and alluvial fan expansion. These are two examples of the types of events necessary to cause alluvial fan sedimentation in the western Great Basin.

Similar shoreline-fan mapping activities are underway at Pyramid, Winnemucca, and Owens lakes, which will provide a broader perspective on the hydrologic effects of relatively minor climate changes and help define the natural hydrometeorlogic variability that is possible under current climate boundary conditions.