South, Middle, and North Sister, as well as Broken Top are high, steep-sided peaks that could produce debris avalanches.
Avalanches of modest volume (less than about 10 million cubic meters – football stadium size) are the most probable and would affect areas primarily close to the volcanoes. Nevertheless, even modest-sized avalanches that contain sufficient water could transform into lahars that travel ten or more kilometers (six or more miles) down valleys. Similar to what occurred at Mount St. Helens in 1980, very large avalanches, those involving hundreds of millions of cubic meters of rock debris, would likely be preceded by pronounced changes in volcanic activity such as increased earthquake frequency, deformation of the volcano, and thus would give advanced warning.