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Lahar Hazards at Three Sisters

Explosive eruptions or large debris avalanches on the Three Sisters can generate lahars of sufficient volume to travel tens of kilometers from source areas.

Mudline left on tree by May 18, 1980 lahars from Mount St. Helens n...
Mudline left on tree by May 18, 1980 lahars from Mount St. Helens near the Old Highway 99 bridge across Toutle River, geologist 6 ft tall.

The McKenzie and Deschutes River, as well as their tributaries that head at Three Sisters, may be affected, which could send lahars into communities such as McKenzie Bridge or Sisters or even those farther downstream. Small lahars were generated by rhyolite eruptions at South Sister about 2 ka, though much larger lahars swept down valleys during earlier eruptions. Eruptions that deposit large amounts of sediment in watersheds are typically followed by years to decades of unstable channels and increased flooding, even in areas beyond the reach of direct effects of the eruption. During the past century, at least five small lahars were generated by failure of glacial-moraine dams that impounded small lakes on all of the Three Sisters and Broken Top. Serious effects of these events were largely restricted to undeveloped areas within 10 km (6 mi) of their source.