Multi-thread rivers in the Polish Carpathians: Occurrence, decline and possibilities of restoration

Multi-thread rivers are typified by the presence of interconnected channels separated by mid-channel bars or the islands forming as a result of vegetation encroachment on mid-channel bars. High gradients of channels and valley floors, high discharge variability, relatively low content of fine, cohesive sediments in the alluvial plain, coarse bed material and high sediment supply are environmental conditions that favour the development of braided channel pattern. Under natural conditions of the Holocene, island-braided channel pattern was typical of the rivers in the foreland of the Tatra Mountains and most likely also those draining the parts of the flysch Outer Carpathians underlain by thick-bedded sandstones complexes. These braided channels formed in the rivers flowing through mostly non-cohesive floodplains and fed with coarse-grained material exceeding competence of the rivers. Progressive deforestation of mountain catchments with the increasing area of cultivated hillslopes resulted in higher flood peaks and substantially increased sediment supply to river channels. Coupled with greater humidity of the final stages of the Little Ice Age, this led to the transformation of the island-braided rivers into bar-braided ones. Throughout the nineteenth century, the braided channel morphology expanded downstream to the foreland river reaches. In turn, over the twentieth century the braided morphology nearly disappeared from Polish Carpathian rivers, mostly as a result of their channelization. The effectiveness of these works was, however, strengthened by a decrease in sediment supply to the rivers caused by land use changes and by the reduced availability of bed material for fluvial transport resulting from in-channel gravel mining and partitioning of rivers with dam reservoirs. Environmental changes which have since occurred in the river catchments and disruption of the continuity of sediment transport by dams limit the feasibility of the restoration of braided channel pattern in Polish Carpathian rivers to their reaches supporting such morphology under natural conditions of the Holocene.