Geological field trips in southern Idaho, eastern Oregon, and northern Nevada

Abstract

We reconstructed equilibrium-line altitudes for late-Pleistocene glaciers in eastern Oregon, central and northern Idaho, and western Montana. Over 500 cirque to small valley glaciers were mapped where moraines and other evidence for ice margins could be confidently interpreted on digital topographic maps. Equilibrium-line altitudes (ELAs) were estimated using the accumulation-area ratio method. Spatial patterns of ELAs show a strong correspondence to present-day precipitation patterns. Modern dry regions have relatively high ELAs (e.g., 2,600-2,900 m at about lat 44.5°N. in the Lost River and Lemhi Ranges south-central Idaho), whereas wetter regions at similar latitudes have considerably lower ELAs (e.g., 2000-2200 m in mountains southwest of McCall, Idaho). Steep eastward increases in ELAs across larger massifs such as the Wallowa, Sawtooth, and central Bitterroot Mountains reflect 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., <1,800 m around lat 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 atmospheric moisture. Overall, the ELA data point strongly toward a moisture source in the north Pacific Ocean. General circulation climate model results indicate that at the last continental glacial maximum, an anticyclone centered over the continental ice sheets and southward deflection of the jet stream should produce dry conditions in the interior northwestern United States. Our results suggest that the anticyclone is weaker than in some previous simulations, and easterly winds are not clearly indicated across the study region. By 15 ka, northward retreat and decline in continental ice-sheet elevation caused contraction of the anticyclone, and winter westerlies from the north Pacific continued 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-late glacial maximum period, before the dramatic warming into the early Holocene. Similar positions and topography of continental ice sheets during buildup prior to the late glacial maximum also might promote glacial advances by focusing strong westerly flow on mountain ranges of the interior northwest. Further dating of mountain glacier advances is necessary to test these hypotheses.