Volcano and Earthquake Hazards in the Crater Lake Region, Oregon
Hazards of Lahars (Volcanic Debris Flows) and Their Runout Flows
Potential for Lahars at Crater Lake
Lahars could be generated as ejected lake water or melted snow mobilized the poorly-consolidated parts of the climactic eruption deposits in the drainages radiating from Mount Mazama. The mantle of climactic eruption deposits is up to ~100 m thick and forms an ample source of sediment for lahars. These deposits include (1) poorly-consolidated, sandy pyroclastic-flow veneer and coarse lithic breccia (rocky debris) on upland surfaces and in the heads of valleys, (2) pumiceous pyroclastic-flow deposits partially filling valleys below ~1,800 m (~6,000 feet) elevation, and (3) well-sorted pumice-fall deposits on hills east of the caldera. These materials are porous and permeable, and the unsaturated portions of such deposits probably would rapidly absorb substantial amounts of water emplaced upon them. If water is supplied faster than it can be absorbed, the excess will run off. Moreover, the deeper portions of these deposits commonly are water saturated, limiting their ability to absorb additional water.
There are many cases of volcanic melting of snow or ice, expulsion of water from a crater lake during a volcanic eruption, failure of a natural dam, or storm runoff causing lahars on the flanks of volcanoes in which volcanic deposits have been the source of sediment. Mount Pinatubo, Philippines, is a recent example where spectacular damage was wrought by such lahars on agricultural land, communities, and infrastructure. These granular (noncohesive) lahars begin as water flows over new pyroclastic deposits. The water entrains sediment until a debris flow is formed. Farther downstream, the flow wave is progressively diluted and transforms into hyperconcentrated flow (20 to 60 percent sediment by volume) and, finally, into normal streamflow with sediment concentrations below 20 percent. The granular nature of the flow reflects the entrainment sediment that has few particles in the clay (<0.004 mm) and silt (0.004-0.0625 mm) size fractions. These flows characteristically attenuate rapidly downstream, but the hyperconcentrated phase can persist for tens of kilometers.
