Formation and evolution of valley-bottom and channel features, Lower Deschutes River, Oregon

AGU Water Science and Application Series 7- 5
By:  and 
Edited by: Jim E. O'Conner and Gordon E. Grant

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Abstract

Primary geologic and geomorphic processes that formed valley-bottom and channel features downstream from the Pelton-Round Butte dam complex are inferred from a canyon-long analysis of feature morphology, composition, location, and spatial distribution. Major controls on valley-bottom morphology are regional tectonics, large landslides, and outsized floods (floods with return periods greater than 1000 yrs), which include the late Holocene Outhouse Flood and several Quaternary landslide dam failures. Floods with a return period on the order of 100 yrs, including historical floods in 1996, 1964, and 1861, contribute to fan building and flood plain formation only within the resistant framework established by the major controls. Key processes in the formation of channel features, in particular the 153 islands and 23 large rapids, include long-term bedrock erosion, outsized floods, and century-scale floods. Historical analysis of channel conditions since 1911 indicates that the largest islands, which are cored by outsized-flood deposits, locally control channel location, although their margins are substantially modified during annual- to century-scale floods. Islands cored by bedrock have changed little. Islands formed by annual- to century-scale floods are more susceptible to dynamic interactions between tributary sediment inputs, mainstem flow hydraulics, and perhaps riparian vegetation. Temporal patterns of island change in response to the sequence of 20th century flooding indicate that many islands accreted sediment during annual- to decadal-scale floods, but eroded during larger century-scale floods. There is, however, no clear trend of long-term changes in patterns of island growth, movement, or erosion either spatially or temporally within the lower Deschutes River.

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Additional publication details

Publication type Book chapter
Publication Subtype Book Chapter
Title Formation and evolution of valley-bottom and channel features, Lower Deschutes River, Oregon
Series title AGU Water Science and Application Series
Series number 7
Chapter 5
DOI 10.1029/007WS08
Year Published 2003
Language English
Publisher American Geophysical Union
Publisher location Washington, D.C.
Contributing office(s) Alaska Science Center
Description 25 p.
Larger Work Type Book
Larger Work Subtype Monograph
Larger Work Title A peculiar river: Geology, geomorphology, and hydrology of the Deschutes River, Oregon
First page 95
Last page 119
Country United States
State Oregon
Other Geospatial Lower Deschutes River