Multi-decadal hydroengineering works as a driver of channel pattern changes in mountain rivers

This study examines the long-term effects of hydroengineering projects on river channel patterns in the Polish Carpathians from 1864 to 2019. It utilises a comprehensive set of historical maps, a GIS-based classification of multi-thread channels, and archival sources to analyse the spatial and temporal transformation of anabranching and braided reaches across seven rivers. Spatial analyses were performed on a set of vector data that included riverbanks, mid-channel forms, and the locations of regulatory structures. The results indicate that multi-thread channels, particularly anabranching types, were prevalent prior to the implementation of systematic river regulation, especially in western catchments with high relief energy and sediment supply. Large-scale river training was implemented in the early 20th century primarily to improve navigation, followed by an emphasis on flood and erosion control. This led to a significant transition towards incised, single-thread channels. This process was further exacerbated by the tributary regulation, gravel mining, and dam construction. Notably, self-restoration processes observed in the 21st century have resulted in limited reestablishment of multi-thread reaches, though legacy effects such as deep incision and riprap structures constrain these processes. This study shows that river regulation was the main factor influencing channel pattern change at the regional scale, while natural variables, such as slope or land use, had a smaller impact. Anabranching reaches have demonstrated greater resilience than braided types, persisting or re-emerging in regions characterised by specific geomorphic or hydrological conditions. These findings reposition Carpathian rivers within the broader context of European river transformations and highlight the lasting geomorphological impact of historic hydroengineering in mountain river valleys.