Mount Rainier volcano (4,393 m) is potentially the most dangerous of the active volcanoes of the Cascade Range in the Pacific Northwest. The volcano towers more than 3 km above several river valleys that lead to populated river valleys, and it has a cover of snow and ice that is equal in volume to that at all the other Cascade Range volcanoes combined. The most recent eruptions (relatively small tephra-producing events) occurred in the 19th century.

Geologic study of the volcano's recent eruptive history indicates that Mount Rainier has repeatedly produced lahars that would be catastrophic today owing to intense development in several river valleys that spread from the volcano. In the past 6,000 years, at least 8 lahars have inundated one or more valleys all the way to Puget Sound more than 50 km downstream; scores of smaller lahars have inundated parts of these valleys. The largest lahars originated from collapse of major sectors of the volcano. The most recent landslide-triggered lahar about 500 years ago occurred from the volcano's west flank and swept more than 50 km down the Puyallup River valley to Puget Sound.