Downstream effects of dam-regulated flow on river-island morphology and lithofacies record in anabranching rivers

Hydropower regulation can substantially modify the morphodynamics and sedimentary architecture of river islands within anabranching rivers systems. In this study, we investigate a 6.5-km reach of the Dnipro River in central Ukraine, within the Kaniv Nature Reserve, at the transition between the Dnipro Upland and the Dnipro Lowland, downstream of the Kaniv Dam, by integrating lithofacies logging along proximal-distal island transects, grain-size statistical analyses, and diachronic planform mapping. Island deposits exhibit a consistent three-part stratigraphic succession comprising basal channel sands, pre-regulation overbank couplets, and a post-regulation overbank cap. The post-regulation unit is thicker, more homogeneous, and commonly displays upward coarsening, while containing fewer mud drapes. These characteristics indicate a shortened duration of low-energy settling conditions and an increased frequency of high-energy flow pulses associated with regulated discharges. Spatially, distal island surfaces increasingly resemble proximal zones in both lithofacies composition and inferred hydraulic conditions. Planform analyses reveal a persistent pattern of headward erosion coupled with down-island deposition, as well as progressive island coalescence-patterns consistent with sediment deficit and diminished lateral connectivity downstream of the dam. Present-day island length-to-width ratios (1.5-3.6) remain below typical equilibrium values, suggesting continued elongation and consolidation. Collectively, these geomorphic and sedimentological indicators constitute a facies-based stratigraphic signature of dam-induced alteration in the development and preservation of alluvial island bodies.