Long-term evolution of mid-channel forms in gravel-bed rivers of the Polish Carpathians

Mid-channel forms are key components of fluvial morphology, particularly in gravel-bed rivers where their development reflects sediment transport dynamics, vegetation establishment, and human impacts. This study investigates the long-term changes in mid-channel forms, including bars, islands, and inter-channel areas (ICAs) in mountain and foreland rivers of the Polish Carpathians over the past 150 years. Based on five historical and contemporary maps from 1869 to 1887, 1906–1913, 1924–1939, 1976–1989, and 2019, a spatial database of nearly 3,000 forms was developed and analyzed using morphometric methods and multivariate statistics, specifically Multivariate Analysis of Variance (MANOVA) and Linear Discriminant Analysis (LDA). The classification of forms into bars, islands, and ICAs was based on land cover and geometric relationships to channel width. Statistical analyses confirmed that ICAs and islands represent two morphometrically distinct types of persistent mid-channel features. A decline in the surface area and shape compactness of these forms was observed over time, followed by a recent resurgence in their number, especially in reaches where river training structures have deteriorated. The findings highlight the dynamic nature of mid-channel forms under long-term anthropogenic and hydromorphological pressures. By offering a standardized morphometric approach and a publicly available vector dataset, this study contributes to the broader understanding of channel pattern evolution in regulated gravel-bed rivers and supports future comparative analyses in fluvial geomorphology and river management.