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Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah

Alexander E. Walker, Johnnie N. Moore, Paul E. Grams, David J. Dean and John C. Schmidt
Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah
Geological Society of America Bulletin (March 2020) Pre-Issue Publication


The lower Green River episodically narrowed between the mid-1930s and present day through deposition of new floodplains within a wider channel that had been established and/or maintained during the early twentieth century pluvial period. Comparison of air photos spanning a 74-yr period (1940-2014) and covering a 61 km study area shows that the channel narrowed by 12% from 138+ or -3.4 m to 122+ or -2.1 m. Stratigraphic and sedimentologic analysis and tree ring dating of a floodplain trench corroborates the air photo analysis and suggests that the initial phase of floodplain formation began by the mid-1930s, approximately the same time that the flow regime decreased in total annual and peak annual flow. Tamarisk, a nonnative shrub, began to establish in the 1930s as well. Narrowing from the 1940s to the mid-1980s was insignificant, because floodplain formation was approximately matched by bank erosion. Air photo analysis demonstrates that the most significant episode of narrowing was underway by the late 1980s, and analysis of the trench shows that floodplain formation had begun in the mid-1980s during a multi-year period of low peak annual flow. Air photo analysis shows that mean channel width decreased by approximately 7% between 1993 and 2009. A new phase of narrowing may have begun in 2003, based on evidence in the trench. Comparison of field surveys made in 1998 and 2015 in an 8.5 km reach near Fort Bottom suggests that narrowing continues and demonstrates that new floodplain formation has been a very small proportion of the total annual fine sediment flux of the Green River. Vertical accretion of new floodplains near Fort Bottom averaged 2.4 m between 1998 and 2015 but only accounted for approximately 1.5% of the estimated fine sediment flux during that period. Flood control by Flaming Gorge Dam after 1962 significantly influenced flow regime, reducing the magnitude of the annual snowmelt flood and increasing the magnitude of base flows. Though narrowing was initiated by changes in flow regime, native and nonnative riparian vegetation promoted floodplain formation and channel narrowing especially through establishment on channel bars and incipient floodplains during years of small annual floods.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: Pre-Issue Publication
Title: Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah
Affiliation: Utah State University, Department of Watershed Sciences, Logan, UT, United States
Published: 20200327
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 81
Accession Number: 2020-030440
Categories: GeomorphologyApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2020147
Illustration Description: illus. incl. sects., 5 tables, sketch map
N38°30'00" - N38°30'00", W110°00'00" - W110°00'00"
Secondary Affiliation: University of Montana, USA, United StatesU. S. Geological Survey, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 2020
Program Name: USGSOPNon-USGS publications with USGS authors
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