Volcano and Earthquake Hazards in the Crater Lake Region, Oregon
Hazards of Lahars (Volcanic Debris Flows) and Their Runout Flows
Lahars are rapidly flowing mixtures of water and rock debris that originate from volcanoes. They can range from dense, viscous slurries resembling wet concrete (containing about two thirds sediment and one third water by volume) to turbulent muddy floods that carry relatively little sediment. Lahars can develop from (1) water-saturated debris avalanches or (2) originate from erosion and incorporation of sediment or rock debris by large, rapidly released volumes of water. Although a major concern at Cascade volcanoes such as Mount Rainier, the first type of lahar is not considered possible at Crater Lake because the main volcanic edifice of Mount Mazama, which would have been the potential source for a debris avalanche, was engulfed 7,700 years ago by collapse of Crater Lake caldera. The second type of lahar would be possible if lake water were rapidly ejected from the caldera during a volcanic eruption or if hot volcanic deposits melted a large amount of snow.
Lahars are channeled into valleys as they move downhill under the force of gravity. They can get bigger as they move downstream by incorporating additional sediment and water en route (called bulking), commonly increasing in volume by a factor of 3 to 5. The amount of water available limits the potential size of a lahar. Lahars travel faster than water in channels of similar depth and slope. Their velocities may be as great as 20 m/s (45 mph) in steep channels close to a volcano but diminish to 5 to 10 m/s (about 10 to 20 mph) in the broader, more gently inclined channels farther away. In relatively narrow canyons, lahars may be many tens of meters deep. As they get farther from a volcano, lahars spread out in the wider, flatter river valleys, often burying roads, bridges, and buildings with their deposits. Lahars commonly travel tens of kilometers (tens of miles), and the largest have traveled 100 km (60 miles) or more from Cascade Range volcanoes.