Mt. Hood, one of several volcanic peaks in the Pacific Northwestern US.

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Agents of Natural Change in Watersheds

Natural Change Processes: Volcanic eruption
Volcanic eruptions can alter watersheds with tremendous force and over time periods as brief as hours or minutes. New lake basins are formed while old ones fill in. Changes in local topography create new routes for water that result in mountain streams where they had not existed before. Lava flows can cross stream channels either damming them to form lakes or diverting flow towards a new course. On Mt. St. Helens, the eruption of 1980 resulted in huge mudslides that formed a dam resulting in a much-enlarged Spirit Lake. Extensive areas of upland forest were incinerated and/or buried, and numerous nearby streams and rivers underwent fish kills due to heavy sediment loads in the water, primarily due to deposited ash. The gradual recolonization of the upland areas and water bodies has provided an excellent laboratory for observing recovery.

Volcanic events have a dramatic impact on upland forests, by destroying and modifying existing vegetation and creating new substrate. Primary impacts of volcanoes on forests are caused by the mechanical destruction of vegetation by lava flows or pyroclastic flows and associated fires, debris avalanches that occur with partial collapse of a volcano, mudflows which often follow existing stream channels and can transport debris, burial from ash deposits, and blowdowns from shockwaves. Secondary impacts can include soil changes and even climate changes. The ash from the Krakatoa eruption of 1883 encircled the globe and limited sunlight penetration which reduced growing seasons as far away as North America. The species of vegetation most affected and their recovery from a volcano are determined by their ability to survive the eruption, their distance from and characteristics of the eruption, and the time since the eruption occurred.