DEBRIS FLOW HAZARDS AT MT. HOOD, OREGON

Mt. Hood is a large glaciated Pleistocene volcano in the northern Oregon Cascade Range. Due to its location, size, eruptive history, and age, Mt. Hood has nearly ideal conditions for the generation of debris flows: steep slopes, confined channels, abundant loose material, and seasonal infusions of large amounts of water. In the last 10 years the frequency of large-scale debris flows at Mt. Hood has increased from previous years. These debris flows have destroyed or threaten state highways and bridges and Forest Service roads, bridges, culverts, campgrounds, trailheads, and trails. In some cases, structures that were destroyed by debris flows were replaced, only to be destroyed by another debris flow. Many of the large debris flows occur in the early fall, when three conditions are met: minimal snow cover on the mountain, freezing level above 8000 feet, and rainfall intensity exceeds 2 inches in 24 hours. Factors contributing to the increased frequency include glacial retreat, possibly a result of global warming. More large destructive debris flows are expected in the next several years.