FLUVIAL STYLE CHANGES DURING THE LAST 35,000 YEARS IN THE TISZA VALLEY (HUNGARY)

In the study of rivers’ responses to climatic changes a detailed analysis of regional differentiation is a valuable tool by distinguishing local from regional impacts. The Tisza river system was chosen as a central European equivalent of NW European lowland river systems. It shows a series of drastic morphological changes over the past 35,000 years. During the Weichselian Middle Pleniglacial (before c. 27 kyr BP) the Tisza was meandering or anastomosing. This was not commonly the case in other parts of Europe: for instance the river pattern of most British, some Dutch and east German rivers was braided during that cool, not extremely cold, period. The shift to a multichannel, low-sinuosity pattern during the Last Glacial Maximum is more general. Also the successive change to the formation of large meanders at the beginning of the Weichselian Lateglacial is very common. These meanders persisted during the cooling of the Younger Dryas in the Tisza valley, which is in contrast to some Dutch and British rivers more to the north. Finally, also the evolution to considerably smaller meanders is typical for most European rivers. While the general climatic conditions were slightly warmer and more continental than in NW Europe, especially the very low longitudinal gradient (due to the position of the Tisza in the subsiding Pannonian Basin) is characteristic. In addition, this river has a very fine-grained sediment load, partly induced by the vegetation cover that was denser than in the north. The combination of these properties resulted in a general low-energy level of the Tisza river that provoked river types and processes that were different from the more northern ones at certain periods.