Evolution of a reservoir shore zone: Geomorphological and ecological responses to water-level fluctuations

• Shore zone change depends on reservoir regime and slope morphometry. • >1 m water-level change drives decades-long shoreline evolution in reservoirs. • Eulittoral zone expands with age but is limited by fluctuation range. • Vegetation succession in shore zones is limited by shore processes, which weaken with age, and by water-level fluctuations. The article presents how shorelines in lowland reservoirs can be transformed. Particular focus is placed on defining the environmental conditions that fundamentally shape the littoral zone, which we here call the “aquatic–terrestrial transitional zone” (ATTZ). The study is based on a multi-decadal analysis combining field-based geomorphological mapping, multitemporal aerial imagery, terrestrial and airborne laser scanning (LiDAR), and geospatial analyses. These methods enabled the identification and classification of shoreline types and their transitions over time, as well as the expansion patterns of vegetation within the ATTZ. Based on this approach, we developed an indicator of ecological changes in artificial water bodies. The test area selected for the study is a large, lowland reservoir with significant annual water-level fluctuations of 5.5 m. By covering the entire period of the reservoir’s operation from its creation and extending for over 30 years, we established that, in the first decade of the reservoir’s existence, active (i.e., erosional or depositional) shore fragments constituted 60.2 % of the shoreline’s length. As the reservoir aged, the share of active shoreline decreased to about 40 % by the end of the second decade of its operation and has remained stable since then. However, the relative proportions of shore types changed: the lengths of erosional and neutral shore decreased, while the share of depositional shore increased. In addition, there appeared a previously unidentified “biogenic” shore type dominated by perennial vegetation. The study indicates that shore zone transformation varies depending on the initial slope inclination or morphometry of the slopes being transformed. Transformation began later and was less dynamic in shallow shore zones and sections immediately neighbouring shallows than in deep-water shore zones, where changes occurred more quickly and were longer-lasting. In turn, large water-level fluctuations led to the creation of a “moving” zone between the aquatic and terrestrial environment, and the expansion of coastal vegetation; as the reservoir has aged, vegetation has occupied new areas, but its development is limited by the persistent water-level fluctuations. These findings highlight how reservoir management and water-level regimes drive shoreline dynamics and long-term ecological patterns in regulated aquatic systems.