RECONSTRUCTED LATE PLEISTOCENE GLACIER EQUILIBRIUM LINE ALTITUDES PROVIDE EVIDENCE FOR LAKE-EFFECT SNOW FROM LAKE BONNEVILLE

Nineteen valley glaciers that existed in the northern Uinta Mountains during the late Pleistocene "Smiths Fork" Glaciation (local "Pinedale"/MIS Stage 2 equivalent) were reconstructed from geomorphic features that delineate their former margins, including terminal and lateral moraines, heads of outwash, ice-marginal channels, and cirque headwalls. Paleo-equilibrium line altitudes (ELAs) were determined for each of the 19 through a weighted average of independent estimates obtained from four methods: accumulation area ratio (AAR of 0.65), toe-headwall altitude ratio (THAR of 0.40), uppermost elevation of continuous lateral moraines, and elevation of the lowest northeast-facing cirque floor. The Smiths Fork-age ELAs average 3100 m above sea level (standard deviation of 100 m), and descend at 6 m/km from an elevation of 3230 m asl in the center of the range, to 2860 m asl at the far western end. Given the sensitivity of the ELA to changes in summer temperature (95 m/deg C) and winter precipitation (-0.3 m/mm) determined by Seltzer (1994), and assuming a summer adiabatic lapse rate equal to the modern value (6.7 deg C/km, determined through analysis of data gathered by active SNOTEL stations) the lowest western paleo-ELAs received up to 2000 mm more precipitation (as snow-water-equivalent, SWE) per winter than the central paleo-ELAs. Modern winter precipitation at these locations is essentially equal (450 to 500 mm SWE). An explanation for this discrepancy is that moisture derived from Lake Bonneville, which was located 100 km upwind of the western Uintas during the Smiths Fork Glaciation, produced tremendous lake-effect snow when westerly airflow was orographically lifted more than 1.5 km over the Wasatch Front and the western Uintas.