South Cascade Glacier is a large alpine glacier in Washington’s North Cascades, below Sentinel Peak in the Glacier Peak Wilderness. Its meltwater feeds South Cascade Lake, the South Fork Cascade River, and ultimately the Skagit River. The glacier is one of four benchmark glaciers monitored by the U.S. Geological Survey, with mass balance tracked since 1959, making it a key long-term site for studying mountain glacier response to climate conditions. That record has shown major retreat: its area declined from 2.71 square kilometers in 1958 to 1.8 in 2015, a loss of 34 percent, and it lost nearly half its volume between 1958 and 2009.