Fluvial development of a floodplain-alluvial system in the Great Hungarian Plain - Is the present river-network representative to archaeological periods?

On the Great Hungarian Plain during the Pleistocene and Holocene one of the most complex fluvial systems of Europe developed, influenced by tectonic, climatic factors and vegetation change. The aim of the study is to summarize these controlling factors, and to describe the evolution of the SE part of the Great Hungarian Plain. Special attention is paid to the latest results on Late Holocene development of the Tisza River, and its interaction with its largest tributary, the Maros River.

The main controlling factor on river course changes is elevation change, which could be extrinsic as tectonic activity (subsidence and uplift), or the elevation changes of the local base level (Danube in case of the Tisza; and Tisza in case of the Maros,), whilst intrinsic factor is the development of convex (elevated) floodplain. These factors resulted in avulsions, which fundamentally altered the landscape development both in areas where they arrived (usually the slope of the rivers increased leading to incision and the development of floodplains and terraces), and also on the regions which they abandoned (development of misfit channels and dry surfaces, where aeolian activity could begin).

Simultaneously, there is climatic and vegetational control on rivers, which could result in channel pattern change (e.g. Maros) or incision (e.g. Tisza).

The development history of the Tisza–Maros floodplains and alluvial fan is even more complex, if we consider that the different factors resulted in spatially and temporally varied erosional and accumulational fluvial responses. The time-lag (and decreasing magnitude) between the disturbance and the response along the river network makes the paleo-hydrological reconstructions even more complex.

The larger rivers (e.g. Tisza River) in the centre of the Great Hungarian Plain had meandering pattern throughout the Late Pleistocene and Holocene, though the tributaries (e.g. Maros) frequently changed their channel patterns on their alluvial fans, very often independently of climate change.

The surface of the extensive Maros Alluvial Fan is densely (0.78 km/km²) covered by distinct paleo-channels of meandering, braided, anastomosing or misfit pattern. The existence of these channels indicates that the main process of the fan development was avulsion rather than lateral channel shift, and the overbank floodplain accumulation played minor role, as the paleo-channels were not buried. (These facts have consequences on settlement structure and also on archeological possibilities!) During the Holocene the Maros built the southern lobe of its alluvial fan. Considerable channel shifts occurred 8.5±0.9 – 7.1±1.0 ka ago, 6.1±1.1 – 3.5±0.4 ka, and 1.9±0.3 – 1.6±0.3 ka ago, and it seems that its present course developed just relatively recently.

This paleo-hydrological reconstruction is further refined, if one considers the upstream effects of the Tisza River ongoing incision, the development of terrace levels and paleo-discharge changes.