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Eruption-related lahars and sedimentation response downstream of Mount Hood; field guide to volcaniclastic deposits along the Sandy River, Oregon

Thomas C. Pierson, William E. Scott, James W. Vallance and Patrick T. Pringle
Eruption-related lahars and sedimentation response downstream of Mount Hood; field guide to volcaniclastic deposits along the Sandy River, Oregon (in Volcanoes to vineyards; geologic field trips through the dynamic landscape of the Pacific Northwest, Jim E. O'Connor (editor), Rebecca J. Dorsey (editor) and Ian P. Madin (editor))
Field Guide (Geological Society of America) (December 2009) 15: 221-236

Abstract

Late Holocene dome-building eruptions at Mount Hood during the Timberline and Old Maid eruptive periods resulted in numerous dome-collapse pyroclastic flows and lahars that moved large volumes of volcaniclastic sediment into temporary storage in headwater canyons of the Sandy River, During each eruptive period, accelerated sediment loading to the river through erosion and remobilization of volcanic fragmental debris resulted in very high sediment-transport rates in the Sandy River during rain- and snowmelt-induced floods. Large sediment loads in excess of the river's transport capacity led to channel aggradation, channel widening, and change to a braided channel form in the lowermost reach of the river, between 61 and 87 km downstream from the volcano. The post-eruption sediment load moved as a broad bed-material wave, which in the case of the Old Maid eruption took approximately 2 decades to crest 83 km downstream. Maximum post-eruption aggradation levels of at least 28 and 23 m were achieved in response to Timberline and Old Maid eruptions. In each case, downstream aggradation cycles were initiated by lahars, but the bulk of the aggradation was achieved by fluvial sediment transport and deposition. When the high rates of sediment supply began to diminish, the river degraded, incising the channel fills and forming progressively lower sets of degradational terraces. A variety of debris-flow, hyperconcentrated-flow, and fluvial (upper and lower flow regime) deposits record the downstream passage of the sediment waves that were initiated by these eruptions. The deposits also presage a hazard that may be faced by communities along the Sandy River when volcanic activity at Mount Hood resume.


ISSN: 2333-0937
EISSN: 2333-0945
Serial Title: Field Guide (Geological Society of America)
Serial Volume: 15
Title: Eruption-related lahars and sedimentation response downstream of Mount Hood; field guide to volcaniclastic deposits along the Sandy River, Oregon
Title: Volcanoes to vineyards; geologic field trips through the dynamic landscape of the Pacific Northwest
Author(s): Pierson, Thomas C.Scott, William E.Vallance, James W.Pringle, Patrick T.
Author(s): O'Connor, Jim E.editor
Author(s): Dorsey, Rebecca J.editor
Author(s): Madin, Ian P.editor
Affiliation: U. S. Geological Survey, Vancouver, WA, United States
Affiliation: U. S. Geological Survey, Portland, OR, United States
Pages: 221-236
Published: 200912
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 43
Accession Number: 2010-060161
Categories: HydrogeologySedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., sketch maps
N45°15'00" - N45°30'00", W121°49'60" - W121°40'00"
Secondary Affiliation: University of Oregon, USA, United StatesOregon Department of Geology and Mineral Industries, USA, United StatesCentralia College, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201033
Program Name: USGSOPNon-USGS publications with USGS authors
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