Radar signatures and structure of an avulsed channel: Rhône River, Aoste, France

Abstract

Near Aoste, France, an avulsion shifted the Rhone River from a southwesterly course through the valley of the Marais des Avenieres to its present northwesterly course in the Basses Terres. Ground penetrating radar (GPR) reflection paterns from the modern and avulsed channel are used to reconstruct and interpret the fluvial architecture of the avulsed channel deposits. A grid of GPR lines was run across and along the avulsed course of the Rhone. The radar reflections have impressive continuity and, for the most part, horizontal to gentle dips (1–2°); although, in places, the reflections have steeper dips (averaging 6°), which are often accompanied by mound-like reflection patterns. Draped over or stacked adjacent to the mounds are packages of dipping reflections which form an en echelon stacking pattern, which is interpreted as being produced by the downstream growth of gravel bars. The cross-valley profiles are composed of stacked concave reflections which are laterally continuous with dips that vary from 1° to a maximum of 8° along the flanks of the concavities. The complete assemblage of these multistoried concave reflection patterns is interpreted as a suite of complete or partial channel fills. The coarse-grained sediments of the pre-avulsion Rhone probably represents a transitional phase between a coarse-grained, chute-modified meandering depositional system and a wandering gravel-bed river depositional style. © 1997 by John Wiley & Sons, Ltd.