River channel adjustments in contrasting environments from the analysis of historical maps and aerial images: the Karst Dinarides vs. the Pannonian Basin

Since the end of the 19th century, river channels across Europe have been noticeably modified by human interventions and climate change. In Croatia, rivers are most significantly altered by the construction of hydropower plants and flood management measures, which often involve hard engineering strategies such as river straightening and channelization. This study investigates changes in channel morphology on two Croatian rivers from contrasting environments (the Karst Dinarides and the Pannonian Basin) since the end of the 19th century, based on the analysis of historical topographic maps and aerial images in GIS.

The Cetina River, located in the Mediterranean basin, has a composite valley characterised by an interchange of narrow carbonate canyons and zones of lateral valley widening. Its channel morphology has been strongly affected by the construction of hydropower plants since the 1950s. Therefore, we analysed changes in active channel width and the occurrence of fluvial bars and islands in the pre-dam phase (1880s–1950/60s) and the post-dam phase (1950/60s–2010s). Partial channel narrowing and drying of marsh zones recorded in the pre-dam phase were attributed to the natural changes in humidity and discharge following the end of the Little Ice Age. Agricultural intensification most likely had an influence on the increase in sediment supply, as evidenced in particular by a considerable river delta progradation of 500 m by the 1960s. In the post-dam phase, severe reductions in discharge due to water diversion resulted in channel narrowing by about 50% along one third of the river length. Moreover, we observed a reduction in the bar area and an increase in the island number. These changes can also be related to land abandonment and natural reforestation in the catchment.

The meandering Orljava River, located in the Pannonian basin, is distinguished by its dynamic morphology due to frequent flash floods. The river has not been extensively channelized unlike most lowland Croatian rivers. Nevertheless, its morphodynamics has been under significant human impact due to removal of riparian vegetation, artificial cut-offs, and the construction of weirs. Preliminary results suggest that the acceleration of lateral channel migration after 2011 is strongly related to the combined effect of recent engineering works, particularly removal of riparian vegetation, and a major flood event in 2014, since higher discharges were not followed by such high channel migration rates in the previous decades. Recently increased erosion of river banks leads to the loss of adjacent agricultural land, demanding a shift in approach to river management.