The formation of low-energy meanders in loess landscapes (Transdanubia, central Europe)

Abstract Despite many models describing the evolution of meandering rivers, little is known about low-energy rivers formed in loess landscapes. This is the first study that focuses on a spatial and temporal evolution of meandering river planforms in Transdanubia (south Hungary). Field research was conducted in the Kapos and Koppany valleys. Geological and geophysical surveys, supported by AMS radiocarbon dating and historical maps, allowed for reconstructing the evolution of a unique low-energy meandering system in the Koppany valley characterized by elongated bends with circular pools near apexes. Silts constituting the river bed, oblique accretion within the inner banks, cutoffs formation between 14,000 and 11,100 cal. BP, and flow discontinuation after 3900 cal. BP, after which the river was turned into wetlands, were specific features of this river course. A spatial transition from compound to large-scale meanders that may have been caused by neotectonic activity, was noted in the Kapos valley. The river floodplains were transformed by canals constructions during the last 250 years. We show the identified fluvial forms and processes against a background of a conceptual model of meander bends evolution for both high- and low-energy rivers evolving in various geomorphic settings. Large-scale meanders are formed by high discharges with a significant content of bed-load in convective regimes characterized by perturbations propagating downstream. However, large meanders can also be formed in low-energy rivers e.g. owing to the formation of faults. Bends with cutoffs and elongate meanders occur both in low- and high-energy rivers. They evolve in absolute conditions when perturbations propagate both downstream and upstream. The occurrence of circular pools allows for distinguishing low- from high-energy courses. In the loess landscape of Transdanubia, the absolute conditions were induced by the downstream base-level rise that, in turn, limited sediment delivery to fines from loess hills. As a result, the low-energy river evolved over a coarser layer inherited from the activity of a high-energy braided river system.