Influence of extreme and annual floods on point-bar sedimentation: Inferences from Powder River, Montana, USA

Research Article| August 08, 2018 Influence of extreme and annual floods on point-bar sedimentation: Inferences from Powder River, Montana, USA M. Ghinassi; M. Ghinassi † 1Department of Geosciences, University of Padova, V. Gradenigo 6, 35131 Padova, Italy †massimiliano.ghinassi@unipd.it Search for other works by this author on: GSW Google Scholar J. Moody; J. Moody 2U.S. Geological Survey, 3215 Marine St., Suite E-127, Boulder, Colorado 80303, USA Search for other works by this author on: GSW Google Scholar D. Martin D. Martin 2U.S. Geological Survey, 3215 Marine St., Suite E-127, Boulder, Colorado 80303, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2019) 131 (1-2): 71–83. https://doi.org/10.1130/B31990.1 Article history received: 12 Dec 2017 rev-recd: 05 Apr 2018 accepted: 13 Jun 2018 first online: 08 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation M. Ghinassi, J. Moody, D. Martin; Influence of extreme and annual floods on point-bar sedimentation: Inferences from Powder River, Montana, USA. GSA Bulletin 2018;; 131 (1-2): 71–83. doi: https://doi.org/10.1130/B31990.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Effects of discharge variability on point-bar sedimentation are not well documented, although resulting changes in flow patterns are well known. This paper focuses on a meander of Powder River in Montana (USA). In May 1978, Powder River had a 50-year recurrence flood, which caused outer bank retreat of ∼70 m. This bank continued to retreat over ∼40 m in response to annual floods between 1979 and 2016. A trench, up to 2 m deep and 60 m long, was excavated in 2016 through the axial point-bar deposits accumulated during and since the 1978 flood. Deposits from the extreme 1978 flood consisted of stratified, coarsening-upward pebbles with subordinate sand, and show paleoflow toward the outer bank. Deposits accreted during subsequent annual floods consist of fining-upward, medium to fine sand with subordinate mud and gravel. These deposits contain sedimentary structures indicating transport from the channel up the bar. Field observations indicate that during extreme floods, the axial part of the bar was armored by coarsening-upward gravels and not affected by secondary helical circulation. In contrast, during annual floods, armoring was not observed and most of the flow was directed up the sloping point-bar surface, indicating secondary circulation near the bend apex. This paper shows that the area affected by the secondary helical circulation shifts downstream and upstream of the bend apex during extreme and annual floods, respectively. This causes significant changes in grain size and sedimentary facies distribution in point-bar deposits, which should be considered when analyzing meandering-river deposits in the ancient sedimentary record. You do not currently have access to this article.