Sediment-dynamics indicators for assessing fluvial-memory controls on suspended-sediment routing and deposition in a valley-type reservoir

Abstract. River impoundment fundamentally alters hydraulic conditions and sediment-transfer processes, but the extent to which pre-impoundment fluvial morphology continues to structure sediment dynamics remains insufficiently quantified. In valley-type reservoirs, submerged channels, islands, bars and floodplain surfaces may preserve an inherited geomorphological template that influences contemporary flow pathways and depositional patterns. This study examines the persistence and sedimentary significance of this fluvial memory in the Włocławek Reservoir, a lowland through-flow reservoir on the Vistula River, Poland, more than five decades after impoundment. Reconstructed pre-dam morphology, measured bathymetry and suspended-sediment grain-size distributions were combined with two-dimensional hydrodynamic modelling (AdH) and Lagrangian particle tracking (PTM) under low-, medium- and high-flow conditions. Three dimensionless indicators—Relative Sedimentation Efficiency (RES), Sediment-deposition Stability Index (SDI) and Redeposition Index (RI)—were developed to quantify spatial contrasts in sediment capture, primary-deposit stability and secondary redistribution among inherited morphodynamic zones. Simulated sediment retention increased from 66 % during high flow to 73 % during medium flow and 94 % during low flow. Former thalweg and island/bar zones exhibited disproportionately high sediment capture, with maximum RES values of 4.10 and 2.63, respectively, whereas former floodplain surfaces showed relatively low sedimentation efficiency. Higher flows reduced the stability of primary deposits and enhanced resuspension–redeposition processes. These findings indicate that inherited channel–island–floodplain morphology remains an important spatial organizer of sediment routing and deposition long after the transition from riverine to reservoir conditions. The proposed indicators offer a quantitative framework for investigating the sedimentary expression of geomorphological inheritance in valley-type reservoirs and for distinguishing depositional efficiency from sediment-storage stability.