LATE-PLEISTOCENE EQUILIBRIUM-LINE ALTITUDES, ATMOSPHERIC CIRCULATION, AND TIMING OF MOUNTAIN GLACIER ADVANCES IN THE INTERIOR NORTHWESTERN UNITED STATES

We present equilibrium-line altitude (ELA) data for late-Pleistocene glaciers in eastern Oregon, central and northern Idaho, and western Montana, U.S.A. Over 500 cirque to small valley glaciers were reconstructed where moraines and other evidence for ice margins could be confidently interpreted on digital topographic maps. ELAs were estimated using the accumulation-area ratio method. Spatial patterns of ELAs show strong correspondence to present-day precipitation patterns, where modern dry regions have relatively high ELAs (e.g., 2600-2900 m at about 44.5°N in the Lost River and Lemhi Ranges, south-central Idaho). Wetter regions at similar latitudes have considerably lower ELAs (e.g., 2000-2200 m in mountains southwest of McCall, Idaho). Abrupt eastward increases in ELA across larger massifs such as the Wallowa, Sawtooth, and central Bitterroot Mountains imply orographic effects on westerly flow. The Columbia River basin of eastern Washington and Oregon provided a lowland corridor for moist, eastward-moving Pacific airmasses, producing anomalously low ELAs in bordering ranges, e.g., < 1800 m around 46.5°N in the Clearwater River drainage of northern Idaho, currently the wettest region of the study area. Smaller-scale features such as the Salmon and Payette River canyons also appear to have acted as conduits for moisture. GCM climate model results indicate that at the LGM, an anticyclone centered over the continental ice sheets and southward deflection of the jet should produce dry conditions in the interior northwestern U.S., although the anticyclone and associated easterlies across our ELA study area are weaker than in some previous simulations, and the ELA data point strongly toward a moisture source in the north Pacific Ocean. By 15 ka, northward retreat and decline in ice-sheet elevation cause contraction of the anticyclone, and winter westerlies from the north Pacific continue to strengthen across the study area until 12 ka. An associated increase in snowfall may have allowed more precipitation-sensitive mountain glaciers to remain near their maxima or expand during the post-LGM period, before the dramatic warming into the early Holocene. Similar positions and topography of continental ice sheets during buildup prior to the LGM might also promote glacial advances in mountain ranges of the interior northwestern U.S. by allowing strong westerly flow.