There are 26 major glaciers on Mount Rainier and numerous unnamed snow or ice patches. The Emmons Glacier has the largest area (4.3 square miles) and Carbon Glacier has the lowest terminus altitude (3,600 feet) of all glaciers in the contiguous 48 states. The Nisqually Glacier has shown dramatic changes in dimension within the last century (Heliker, Johnson and Hodge 1983). Mount Rainier's glaciers are important indicators of climatic change, major visitor interpretive objects, sources of water for park aquatic systems, and hydroelectric and recreation pursuits outside of the park.
Historically, glacial outburst floods, torrential rains, and stream capture have caused small to moderate size debris flows. Most occur in drainages with large glaciers. Less common are the debris flows triggered by a drainage diversion in an unglaciated drainage basin. For example, the diversion of the Kautz Glacier meltwater into the Van Trump Basin triggered debris flows on the south side of the mountain in 2001 and 2003.