DESCRIPTION:
Volcano-Related Floods

Lava flows can melt snow and ice and cause pyroclastic flows, lahars, and floods

Lava flows extruded on snow or ice-covered terrain can generate sufficient meltwater to cause small lahars and floods. More dangerous, however, is extrusion on terrain so steep or icy that the lava flow breaks apart and produces avalanches of hot lava fragments. Such avalanches from near the summit of Mount Adams can form pyroclastic flows that may travel downslope as far as 15 kilometers (9 miles). Pyroclastic flows can erode and melt large quantities of ice and snow, transform into lahars, or produce water floods. These lahars or floods can then travel tens of kilometers farther down valleys.

Floods related to volcanism can be produced by melting of snow and ice during eruptions of ice-clad volcanoes like Mount Shasta and by heavy rains that may accompany eruptions. Floods carrying unusually large amounts of rock debris can leave thick deposits of sand and gravel at and beyond the mouths of canyons and on valley floors leading away from volcanoes. Eruption-caused floods can occur suddenly and can be of large volume; if rivers are already high because of heavy rainfall or snowmelt, such floods can be far larger than normal. Danger from them is similar to that from floods having other origins, but floods caused by eruptions may be more damaging because of an unusually high content of sediment. Floods can also be generated by eruption-caused seiches (waves) that could overtop dams or move down outlet streams from lakes.

Floods related to volcanism can be produced by melting of snow and ice during eruptions of ice-clad volcanoes, by heavy rains that may accompany eruptions, and by transformation of lahars to stream flow. Floods carrying unusually large amounts of rock debris can leave thick deposits at and beyond the mouths of canyons and on valley floors leading away from volcanoes. Eruption-caused floods can occur suddenly and can be of large volume; if rivers are already high because of heavy rainfall or snow melt, such floods can be far larger than normal.

Danger from eruption-caused floods is similar to that from floods having other origins, but floods caused by eruptions may be more damaging because of an unusually high content of sediment. The hydrology of river systems may be altered for decades following the rapid accumulation of great quantities of sediment (e.g., U.S. Army Corps of Engineers, 1984). Subsequent reworking of this sediment may lead to further channel aggradation, and aggravate overbank flooding during high river stages. Floods can also be generated by waves in lakes that overtop or destroy natural or man-made dams; such waves can be produced by large masses of volcanic material moving into the lake suddenly as a debris avalanche, lahar, or pyroclastic flow.

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