The Convex Bar: Member of the Alluvial Channel Side-Bar Continuum

Abstract

ABSTRACT Bars which form on the convex side of bends in Antelope Creek in eastern Wyoming superficially resemble point bars but have neither the genesis nor internal structure of true point bars in meandering perennial streams. Our observations of sediments in this shallow, sandy, ephemeral stream suggest that these bars are a function of flashy flow and large width-depth ratios which prohibit strong lateral flows related to helical cells. Apparently, sediment is transported parallel to downstream velocity vectors. During high flow, a large sediment mass is emplaced in the channel bend where flow expansion causes deposition. Further deposition takes place by vertical accretion as flows continue to overtop the bar. Subsequent low flows cause erosion of the bar by thalweg meandering and chute hannel development, and deposition of fine material takes place in chute channels and on the bar margin. These processes result in the creation of a side bar with internal structure consisting of a main flood fill sediment body and vertically accreted topset beds with minor marginal foreset strata. A thin veneer of low-water fine sediment may occur on the outer bar margin and in chute channels. We suggest that these bars should be distinguished from true point bars and other types of side bars and herein propose the descriptive term convex bar.