Geomorphic diversity in the braided-wandering Belá River, Slovak Carpathians, as a response to flood variability and environmental changes

• We assess the effect of different flood periods on river evolution during six last decades.
• The post-flood period serial geomorphic analysis (POPSEGA) was used.
• Decrease in active zone area and geomorphic diversity of the river was identified.
• Increase in island area and reduction in river bank shift are related to changes in floods during last decades.

Changes in the morphological pattern of the gravel-bed Belá River, Slovak Carpathians, were investigated to infer about the impact of flood events and environmental changes on the evolutionary trend of the river since the mid-twentieth century. The post-flood period serial geomorphic analysis (POPSEGA) approach coupled with GIS analysis was used to identify morphological changes that occurred during particular flood periods. The study focused on assessing how six flood periods (FP) with differing flood characteristics have influenced the evolutionary trend of the river. Several sets of aerial photos (1949, 1961, 1973, 1986, 1992, 2003, 2009) taken after the periods with the occurrence or lack of major floods provided data about channel landform changes in 227 river segments of 100-m length. A decreasing trend of the area of the river's active zone (from 2.5 km2 in 1949 to 1.4 km2 in 2009) was identified. The first FP (1949–1961), with a very high-magnitude flood in 1958 (RI50), was characterized by increasing total channel length, island number/area, and the number/density of confluence–diffluence pairs (nodes), and by decreasing bar area. During this FP the frequency of cores of channel dynamics as well as the Shannon's index of landforms diversity reached the highest values. The following FP (1962–1973) with four 5- to 10-year floods exhibited the inverse pattern of changes in channel form parameters, although the core frequency of channel dynamics and Shannon's diversity index were still high. The river system seems to have been relatively stable between 1974 and 1986 (third FP) with some decrease in the area of channel forms and in pattern metrics values. During the fourth FP (1987–1992) several 1.5- to 3-year floods occurred and were associated with a decrease in some channel forms and a further decrease of the pattern metrics values. An increase in all analysed channel parameters, especially bar area and pattern metrics values, occurred in the years 1993–2002 (fifth FP) as a consequence of two floods of moderate magnitude in 1997 (RI7) and 2001 (RI3.5). The lowest values of bar area and pattern metrics during the whole study period were recorded during the last FP (2002–2008), whereas island area increased at that time. The study shows that the occurrence of floods during the last six decades has not returned the Belá to its former state because of the overall decrease in flood magnitudes, changes in catchment sediment supply induced by the increasing forest cover in the catchment (from 22.7% in 1957 to 34.8% in 2008), and changes in channel boundary conditions (channelization, channel incision, small hydropower plant constructions).