Mount Hood is an active volcano close to rapidly growing communities, recreation areas, and major transportation routes and therefore imposes heightened risk. Potential hazards include collapse of growing lava domes and generation of pyroclastic flows, which in turn melt snow and ice to form lahars that flow far down valleys; the long-term adjustment of river channels to the large quantities of volcanogenic sediment dumped into valleys that head on the volcano; and landslides of hydrothermally altered material from steep upper slopes of the volcano that spawn debris avalanches and related lahars. The most likely widespread and hazardous consequence of a future eruption would be for lahars to sweep down the entire length of the Sandy and White River valleys. Modest production of tephra would also pose chiefly non-life-threatening hazards to nearby communities.
Future eruptions of Mount Hood could seriously disrupt transportation (air, river, and highway) and hydroelectric power generation and transmission in northwest Oregon and southwest Washington. In addition some municipal water supplies are vulnerable to increased turbidity from falling tephra. The location and size of an affected area will depend on the location of the erupting vent or landslide, the volume of material involved, the snow and ice conditions around and down slope from the vent, and the character of an eruption, especially its explosivity. U.S. Highway 26, a major cross-Cascades route, and Oregon Highway 35, an important recreational road, could be severed by lahars and other hazards and would probably be closed during volcanic unrest and eruption. Depending on the character and impact of a future eruption, the highways could be lost for years or decades. Large loads of sediment delivered to the Columbia River by lahars or by the Sandy and other rivers transporting sediment from eruption-impacted watersheds would have a serious effect on the Columbia River shipping channel, which could last long after an eruptive period ends.
The Oregon Department of Geology and Mineral Industries recently completed a USGS American Recovery and Reinvestment Act funded multi–hazard risk and vulnerability study for Mount Hood for the two major valleys heading on the volcano. It showed that Mount Hood lahars could cause billions of dollars in damage to property and key infrastructure and affect the lives of thousands of residents.