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Glaciers at Mount Hood, Oregon

Glaciers and perennial snowfields on Mount Hood cover about 13.5 km2 (5 mi2) and contain more than 300 million cubic meters (nearly 400 million yd3) of ice and snow. The largest glaciers, Eliot and Coe on the north flank, are about 2.5-3 km (1.5 to 1.8 mi) long. Summer meltwater from the glaciers and seasonal snowpack provides irrigation water for the highly productive Hood River Valley fruit orchards and maintains flow in important fish habitats.

During the last ice age, glaciers radiated outward up to 15 km (9.3 mi) in all of the major Hood drainages and filled valleys with hundreds of meters of ice. Glaciers were even more extensive during several older ice ages. Many lava flows were erupted during times of extensive glacier cover, which strongly influenced their distribution. During the Polallie eruptive period, pyroclastic flows from collapsing lava domes mantled glaciers with debris, which was transported by the glacier and dumped in moraines that formed at the glacier snout.

Whether during times of relatively restricted glacier cover as now or during ice ages, glaciers and snow provided a ready source of water to mobilize long-traveled lahars by pyroclastic flows swiftly producing large volumes of meltwater.