Do the same river engineering works cause the same morphological adjustments? Context matters!

Abstract. Rivers in the Anthropocene have undergone significant geomorphological changes due to engineering interventions such as channelization and hydroelectric infrastructure. These modifications have disrupted hydrological and sedimentary processes, reducing hydrological connectivity and habitat diversity. Evaluating their impacts is crucial for river management and restoration. We studied four channelized and bypassed sections of the French Rhône River (>60 km; ~13 % of its length). We analyzed spatio-temporal trajectories of channel narrowing and margin terrestrialization using GIS-based analysis of historical maps, aerial photographs, and satellite imagery. Historical hydrosedimentary data, including longitudinal profiles and water-level records, were used to assess the contribution of successive development phases. Beyond qualitative observations, such as the simplification of the river pattern toward a single-thread channel, we quantified the variable contributions of channelization and flow diversion. Active-channel narrowing ranged from −43 % to −17 % following channelization and from −32 % to −17 % following flow diversion. Margin terrestrialization increased by 27–44 % and 41–66 %, respectively. Where channelization had limited effects on terrestrialization, flow diversion played a greater role via water-level changes and reshaping the alluvial mosaic dependent on hydrological gradients. Although bed incision is generally expected to be limited in bypassed reaches, it reached −4.1 m in the most affected section (−5 cm·y⁻¹ during the 20th century), largely due to gravel mining associated with dam construction. This four-site comparison displays that engineering pressures interacted with local hydrosedimentary conditions to produce contrasting, legacy-dependent trajectories. Engineering margins, as ecotonal zones, are key indicators of geomorphic adjustment, documenting the system’s response to successive development phases.