Lewis Glacier occupies the southeast slopes of South Sister in Oregon’s Cascade Range, generally above 9,000 feet on the flank of an inactive stratovolcano. Its former size remains visible in the terrain through moraines that mark both its last Glacial Maximum limit at about 1,965 meters and 6,445 feet and its Little Ice Age extent at about 2,375 meters and 7,790 feet. In recent history the glacier has generally retreated. Its terminus was reported in 2005 at roughly 2,680 meters and 8,790 feet, and imagery released by the Deschutes National Forest in 2024 appears to show it is no longer active.