LEWIS GLACIER, SOUTH SISTER MOUNTAIN, OREGON: FAREWELL OLD FRIEND?

Lewis glacier is a small alpine glacier on the south face of South Sister Mountain, Oregon. South Sister itself is a large composite volcano, and although it has been volcanically active during the post-glacial period, the Pleistocene and Holocene glacial landforms in the vicinity of Lewis Glacier are well preserved. Glacial landforms record the maximum extent and pattern of retreat of the Lewis Glacier. Reconstructions of Lewis Glacier for the Last Glacial Maximum (LGM) and the Little Ice Age (LIA) can be used along with inverse modeling to determine the change in overall mass balance. The LGM ice limit is marked by a prominent moraine at an elevation of 1965 m. Few landforms record the retreat of the ice and Holocene-age lava flows cover much of the terrain. The Little Ice Age limit is marked by a moraine at an elevation of 2375 m, and the volume of sediment suggests that Lewis Glacier was aggressively eroding its bed and transporting sediment to the ice margin. Retreat from the LIA maximum was apparently well underway by 1920. Currently, the margin of Lewis Glacier sits at 2680 m and the ice appears to be thinning rapidly. Much of the ice at the head of the cirque has collapsed into the Bergshrund and bedrock highs are beginning to protrude above the ice in the middle of the glacier. Analysis of climate data suggests that Lewis glacier sits on the local glaciation threshold. Mean annual temperature midway along the profile is estimated at -2 degrees C, which along with the annual precipitation, places Lewis glacier at or outside the limits of sustainability. Farewell old friend.