Massive Debris Flow Event on Pacific Northwest Volcanoes, USA, November 2006: Causes, Effects and Relationship to Climate Change

Abstract

During the second week of November, 2006 a warm, wet air mass, locally called the “Pineapple Express”, hit Oregon and Washington and dumped over 50 cm rain onto the volcanoes within 36 h before there was snow cover. It mobilized the abundant sediment on the slopes of the volcanoes and created many devastating debris flows. The amount of available sediment on the slopes has increased in recent years as glaciers have retreated, exposing the inside of lateral moraines. Mt. Hood was the most devastated with 7 of 11 major drainages producing significant debris flows. Eliot Creek and White River produced extensive debris flows that measured well over 2 million cubic meters each and closed major highways in the region for weeks. A large delta formed in the Columbia River at Hood River resulting from the debris flows. At Mt. Rainier the southwest drainages were hit hard. Kautz Creek rerouted. The Nisqually River enlarged greatly in size and almost undercut the Longmire emergency response headquarters of the national park. The Sunshine Campground disappeared. At Mt. St. Helens, the road to Lava Canyon was washed out in two places. Milk Creek was the major debris flow on Mt. Jefferson. On Mt. Adams, many debris flows were generated in the drainages of Salt Creek, Adams Creek and the Big Muddy. The largest debris flow collapsed the moraine at Crofton Ridge. As the climate continues to change, we will see more large debris flows as these air masses arrive before snow cover can shield their impact on the volcanoes.