Lahars and Debris Avalanches at Mount Hood, Oregon
Both eruptive and noneruptive processes at Mount Hood generate several types of landslides and water-mobilized flows that sweep the volcano’s flanks and surge down river valleys.
Volcanic mudflows can be triggered by an eruption, can develop during a landslide, or can occur during periods of heavy precipitation and high runoff. These lahars sweep rapidly down valleys picking up additional debris while eroding the channels in which they travel. As the flows run out, they deposit the mass of material that was gathered along the way.
Lava-dome eruptions are characterized by frequent rockfalls and collapses of the growing dome. Collapses typically generate pyroclastic flows of a variety of sizes, all of which can swiftly melt snow and ice. Pyroclastic flows are confined to the volcano's flanks, but melt water can mix with rock debris to form lahars, the largest of which can flow far downstream.
Intense rainfall and sudden release of water from glaciers also cause small versions of lahars called debris flows. A rainstorm on Christmas day, 1980, triggered a landslide at the steep head of Polallie Creek on the east flank. The landslide transformed into a debris flow that scoured sediment from the creek’s channel banks and entered the East Fork Hood River, carrying a volume 20 times greater than the initial landslide. The debris flow temporarily dammed the East Fork. About 12 minutes later, the dam was breached and a flood surged down the East Fork, destroying about 10 km (6 mi) of Oregon Highway 35—a total of $13million in damage in 1980 dollars). One person was killed. Similar events have swept down most of the valleys on Mount Hood during the past century, but flows on White River, Newton Creek, Eliot Branch, and Ladd Creek have done the most damage to roads and bridges.
Rapidly moving landslides, called debris avalanches, occurred numerous times in the past when the steep upper parts of Mount Hood collapsed under the force of gravity. Small ones tend to be restricted to the upper flanks of the volcano, and large ones typically contain sufficient water to rapidly transform into far-traveling lahars.
About 1,500 years ago, a debris avalanche originating on the upper southwest flank of Mount Hood produced a lahar that flowed down the Zigzag and Sandy River valleys. It swept over the entire valley floor in the Zigzag-Wemme-Wildwood area and inundated a broad area near Troutdale, where the Sandy flows into the Columbia River —a total distance of about 90 km (55 mi). The debris avalanche created the breached summit crater that has since caused most eruptive products to flow into the Sandy River basin while the Hood River basin remains sheltered.
More than 100,000 years ago, a much larger debris avalanche and related lahar flowed down the Hood River, crossed the Columbia River, and flowed several kilometers up the White Salmon River on the Washington side. Its deposits must have dammed the Columbia River at least temporarily.