Influence of extreme and annual floods on point-bar sedimentation: Inferences from Powder River, Montana, USA
Effects of discharge variability on pointbar 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 50year 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 paleo flow 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 pointbar 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 meander ing-river deposits in the ancient sedimentary record. INTRODUCTION Point bars are common sedimentary features of meandering rivers in ancient and recent sedimentary records. Secondary helical flow circulation developed in meander bends has been widely invoked to explain mechanisms of sediment accumulation on point bars (e.g., Labrecque et al., 2011; Smith et al., 2011). Although this sec ondary helical circulation has received considerable attention from fluvial sedimentologists, the hydrodynamic structure of fluvial meanders is more complex and varies from one part of the bend to another (Kasvi et al., 2013; 2017; Lotsari et al., 2014). In general, flow in the upstream portion of the bend is outward from the point bar, and impinges along the outer bank creating a superelevation of the water surface. This superelevation induces a cross-stream pressure gradient causing return flow farther downstream along the bed, toward and up the sloping point-bar surface (Bathurst et al., 1977). Its magnitude is determined by the rate that curvature decreases around the bend, which introduces a corresponding increasing centrifugal acceleration, and by the change in bed topography, which introduces convective accelerations (Leopold and Wolman, 1960; Dietrich and Smith, 1983; Furbish, 1991; Frothingham and Rhoads, 2003; Kasvi et al., 2013). The combined centrifugal and convective accelerations of the sec ondary helical flow around the point bar, increase the boundary shear stress (Dietrich and Smith, 1983; Willis, 1989), and thus, the transport of sediment particles along the bar. Farther downstream, coarse-grained particles accumulate in the thalweg zone, whereas finer-grained particles are transported toward and farther up the sloping point-bar surface, as the flow-drag force exceeds the gravitational force (Dietrich et al., 1979; Parker and Andrews, 1986), creating a distribution of grain size that decreases with distance up the point bar. These fining-upward grain size trends and transport up the slope point-bar surface are considered to be distinctive features of point-bar deposits, and are commonly shown by classical facies models in cross sections parallel to bar axis (e.g., Allen, 1982). Secondary flow structure along a meander bend varies with bend geometry, which affects the sediment transport. For example, the helical flow can vanish before the crossover (i.e., inflection zone) in sharp bends (Frothingham and Rhoads, 2003), but can maintain some degree of strength across the riffle zone in gently curved bends (Thorne and Hey, 1979). Flow structure also varies within the same meander bend at different discharges (Kasvi et al., 2013, 2017). For example, at high-flood discharge, the surface flow crosses over the point bar rather than following the deeper flow in the channel around the point bar, and thus, the zone of maximum velocity shifts over the bar ( Dietrich and Smith, 1983; Furbish, 1988; Ferguson et al., 2003). This causes a corresponding downstream shift of the zone of maximum erosion of the outer bank ( Dietrich and Smith, 1983; Furbish, 1988) by the surface flow and by the helical secondary flow, which evacuates sediment from the base of the outer bank (Furbish, 1988). The effects of variable discharge on bank erosion are well known (Nanson and Hickin, 1986; Odgaard, 1987; Papanicolaou et al., 2007; Visconti et al., 2010; Kasvi et al., 2013, 2017), but the effects of variable discharge on meanderbend sedimentation are less documented and are commonly associated with development of internal truncations (Miall, 1985; Jackson, 1976; Ghazi and Mountney, 2009; Durkin et al., 2015), or with formation of poorly developed macroforms (Plink-Björklund, 2015). This study aims to understand the role of variable discharge on point-bar sedimentation. It focuses on a point bar in a study reach of the meandering Powder River (Montana, USA). The study reach has been surveyed frequently from 1975 through 2016 and the geomorphic effects of extreme and annual floods have been documented (Moody and Meade, 2017, 2018). This long-term data set (40+ years) represented a unique opportunity to compare sedimentary structures formed during an extreme flood with GSA Bulletin; January/February 2019; v. 131; no. 1/2; p. 71–83; https:// doi .org /10 .1130 /B31990 .1; 9 figures ; published online 8 August 2018. massimiliano .ghinassi@ unipd.it For permission to copy, contact editing@geosociety.org © 2018 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/1-2/71/4604613/71.pdf by Utah State University Libraries user on 17 September 2019