LAKE-LEVEL, LAKE-SURFACE AREA, AND LAKE-VOLUME CHANGES AT PYRAMID LAKE, NV OVER THE LAST 16 CAL KA

Pyramid and Winnemucca lakes (PWL) lie at the terminus of the Truckee River that drains about 7050 km² of the northern Sierra Nevada, including Lake Tahoe. These two terminal basins are integrated through a low sill (Mud Lake Slough) at about 1177 m, so have shared a common history multiple times over the last 16 cal ka when lake levels were above this elevation. Changes in lake level, lake-surface area, and lake volume through time primarily reflect changes in the balance of Truckee River inflow vs. the volume of evaporation, with fluctuations in inflow being the predominant driver of changes in lake state. During the last 100 years annual discharge of the Truckee River (at Farad, CA) ranged between 0.1 to 1.2 km³/yr, with a mean of ~ 0.48 km³/yr. A new lake-level curve for PWL, combined with the hypsometry of the basins, allows quantification of lake level, surface area, and volume changes since the late Pleistocene.

Shortly after the Lake Lahontan highstand (~ 1338 m; ~ 15.7 cal ka), lake levels fell dramatically to or below the elevation of Emerson Pass (~ 1200 m), which serves as a sill connecting PWL to the downstream Smoke Creek-Black Rock Desert (SCBR) subbasin of the Lahontan system. During the Younger Dryas period, lake levels rose to ~ 1230 m, flooding Emerson Pass and integrating PWL with SCBR and the Honey Lake subbasin, corresponding to a lake-surface area of ~ 5560 km² and volume of ~ 210 km³. Lake levels then rapidly fell to below 1154 m at around 11 cal ka. Lake levels again rose to Emerson Pass at around 7.7 cal ka (~ 930 km² and ~ 61 km³) before descending to relatively low levels through the mid-Holocene. During the late Holocene and into the historical period, lake levels have fluctuated between ~ 1195 m (~900 km² and ~56 km³) and 1155 m (~ 440 km² and ~ 25 km³). The paleohydrologic variability expressed over the last 5000 years corresponds to surface area changes of + 10 % to – 46 % relative to the surface area (~ 817 km²) associated with the common early historical elevation of ~ 1178 m, while volumetric changes ranged between + 37 % and – 39 % relative to the lake volume of the early historical period (~ 41 km³). This range of variability reflects long-term (decadal to centennial) changes in water balance and probably encapsulates what is possible in this system under modern climate boundary conditions.