Bedload infilling and depositional patterns in chute cutoffs channels of a gravel‐bed river: The Ain River, France

Abstract

AbstractAlthough abandoned channels are common and identifiable features in alluvial plains, their detailed internal architecture remains overlooked, particularly their coarse permeable compartment, with implications for underground water flow. The actively shifting gravel‐bed Ain River (France) provides an opportunity to study the geometry and architecture of bedload deposits (plug) during channel disconnection, in relation to river discharge and planform evolution. In this study, combined geomorphic and grain‐size surveys were conducted on the bedload deposits associated with the closure phase of four cutoff channels. We find that bedload accumulates mainly through (i) the initiation of a bar at the mouth of the abandoned channel in the flow separation zone, which reduces flow connectivity; (ii) lateral accretion of coarse‐grained bars, resulting in channel narrowing downstream of the initial upstream bar; and (iii) merging of coarse‐grained longitudinal bars—anchored to the initial plug bar. The channel plug progrades downstream until occupying ca. 0.5–0.7 times the channel surface and then starts thickening. We also find that channel plug volume is controlled by sediment supply and channel inherited topography, which are partially controlled by the dynamics of neighbouring channel bends.