22 Catastrophic Flood or Lahar From Drainage of Crater Lake

Volcano and Earthquake Hazards in the Crater Lake Region, Oregon 

Events of High Consequence but Low Probability

Catastrophic Flood or Lahar From Drainage of Crater Lake

Crater Lake contains 17 km3 of water (Phillips and Van Denburgh, 1968). Should the caldera wall fail and allow the lake to drain, the ensuing flood of water, rock, and remobilized pyroclastic debris would be devastating. In order for Crater Lake to breach its walls the water level would have to rise dramatically or the wall would have to fail. At its lowest elevations at Kerr Notch, Wineglass, and northwest of Round Top, the caldera rim is ~165 m above the lake. Nothing short of major volcanic activity or drastic change in climate is likely to cause such a rise in lake level.

The amount of rock that would have to be removed by wall failure in order for the lake to overflow into one of the valleys on the flanks of Mount Mazama, assuming a minimum width of 500 m, is on the order of 0.1 km3. Even if the lake should overflow, whether outflow becomes catastrophic would depend on the rate of downcutting and breach enlargement. A range of possible maximum discharges varying by two orders of magnitude (1.3 x 105 to 3.9 x 107 m3/s) can be estimated as theoretically possible by analogy with catastrophic drainage of a prehistoric lake in Aniakchak caldera, Alaska (Waythomas and others, 1996). However, there does not appear to be a mechanism by which breach of the walls of Crater Lake caldera could be accomplished. An extremely low level of risk from catastrophic lake drainage is understood to exist throughout all downstream lowlands around Mount Mazama.

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