Dike fields as drivers and witnesses of twentieth-century hydrosedimentary changes in a highly engineered river (Rhône River, France)

Abstract

Hydraulic structures, such as groyne fields, were commonly used to channelize European and North American rivers, thus forming engineered margins on the edges of the active channel. On the Rhône River (France), which was corrected with dike fields (classical groyne fields and specific ones closed with a longitudinal submersible dike; i.e., closed fields) and equipped with numerous dams (mid-twentieth century), the engineered margins have mostly been filled with fine sediments and become terrestrial. On the 11 km bypassed studied reach (middle Rhône), 55.6 ha in 167 dike fields (i.e., 75 % of the cumulated surface) have been subject to terrestrialization (i.e., the transformation of aquatic areas into terrestrial ones). Our study aimed to understand the trajectory of dike fields that serve as both drivers (e.g., inducing in-dike fine sediment trapping) and witnesses (e.g., informing the diversion impact) of hydrosedimentary changes. We combined geohistorical analyses (aerial photographs, riverbed elevations, and water levels) and GIS modeling, as well as topographic and ground-penetrating radar surveys (GPR), to emphasize the terrestrialization patterns and hierarchize the drivers and processes involved. We obtained a classification of diachronic patterns (five types) that highlight local recurrences and specificities (inherited forms) in the terrestrialization trajectory. Topographic and GPR surveys complemented our data, shedding light on the main characteristics of sediment deposits (volumes, thicknesses) and structural units. We also determined that the contribution to terrestrialization of phase 1 (channelized state; 1900s–1970s) is lower than that of phase 2 (channelized and bypassed state; 1970s–2000s), at 42 % (23.4 ha) and 58 % (32.2 ha), respectively. In phase 1, fine sediment deposition leads to deposit construction, triggered by the dike field setting, which shapes spaces with reduced shear stresses. The riverbed incision (induced by channelization) is shallow on this reach, so its contribution (by lowering the water level) is considered negligible. In phase 2, terrestrialization is mainly provoked by the diversion-induced drop in water level that has caused the retraction of the active channel and promoted its abandoned edges as new terrestrial margins. Understanding the evolution of these ecotones made it possible to adjust our recommendations in terms of management and restoration. It also highlights the relevance of strategic dike field reconnections to support the river by recreating gradients of hydrological connectivity (which are favorable to habitat diversity).