DISRUPTION OF DRIFT GLACIER AND ORIGIN OF FLOODS DURING THE 1989-1990ERUPTIONS OF REDOUBT VOLCANO, ALASKA

Melting of snow and glacier ice during the 1989-1990 eruption of Redou bt Volcano caused winter flooding of the Drift River. Drift glacier wa s beheaded when 113 to 121 X 10(6) m(3) of perennial snow and ice were mechanically entrained in hot-rock avalanches and pyroclastic flows i nitiated by the four largest eruptions between 14 December 1989 and 14 March 1990. The disruption of Drift glacier was dominated by mechanic al disaggregation and entrainment of snow and glacier ice. Hot-rock av alanches, debris flows, and pyroclastic flows incised deep canyons in the glacier ice thereby maintaining a large ice-surface area available for scour by subsequent flows. Downvalley flow theologies were transf ormed by the melting of snow and ice entrained along the upper and mid dle reaches of the glacier and by seasonal snowpack incorporated from the surface of the lower glacier and from the river valley. The season al snowpack in the Drift River valley contributed to lahars and floods a cumulative volume equivalent to about 35 X 10(6) m(3) of water, whi ch amounts to nearly 30% of the cumulative flow volume 22 km downstrea m from the volcano. The absence of high-water marks in depressions and of ice-collapse features in the glacier indicated that no large quant ities of meltwater that could potentially generate lahars were stored on or under the glacier; the water that generated the lahars that swep t Drift River valley was produced from the proximal, eruption-induced volcaniclastic flows by melting of snow and ice.