Lahars can be generated by hot volcanic flows that melt snow and ice or by landslides (debris avalanches) from weakened rock forming the steep upper flanks of the volcano. Lahars can sweep away or bury everything in their paths. Highways and structures close to river channels are at greatest risk of being destroyed by lahars. The degree of hazard decreases as height above a channel increases, but large lahars can affect areas more than 30 vertical m (100 vertical ft) above riverbeds.
Past lahars at Mount Hood completely buried valley floors in the Sandy and Hood River drainages all the way to the Columbia River and in the White River drainage all the way to the Deschutes River. Today, Interstate 84 and the Union Pacific Railroad line cross the Sandy River and Hood Rivers near their confluence with the Columbia River and could be affected by large lahars in the future. Likewise the Columbia River shipping channel could be affected by lahars or high sediment loads. These elements are key economic infrastructure to the region and their damage or uncertainty related to potential impacts could severely affect the region’s economy.