Swis Nature

Fertö-Hanság National Park

Szigetköz is the child of the Danube

Delimited by the two branches of the river Danube, Öreg-Danube and Moson-Danube, Szigetköz stretches across Europe’s largest alluvial fan, an area of 375 square kilometres.

The Little Hungarian Plain (Kisalföld) was covered by the Pannonian inland sea back in the Tertiary. Towards the end of this period, the sea began to shrink and eventually a system of freshwater lakes formed in its place. This was a time that saw numerous rivers, flowing in from the Alps and the Carpathians deposit their sediment in the basin. Some 2-2.5 million years ago the ancient Danube passed the Alps at what is today known as the town of Királyhida (Bruck an der Leitha) and continued to flow southwards. Later on, in the Quaternary, the rivers took a different direction; the Danube entered the basin between the Hainburg hills and the Little Carpathians, shaping anew its riverbed eastwards. Exiting the hilly regions, the gradient of the river became less steep and the water deposited its sediment, forming and washing away sandbanks constantly. As these banks were gradually occupied by permanent vegetation, they turned into small islands, thereby leaving the river-beds broken up into branches. River-beds could be further shaped by a combination of various factors such as trees falling into the river, accumulated silt, overflowed or collapsed riverbanks, drift ice and floods.

The obstacles created by the river itself forced this huge body of water to carve out new beds; as a result, a complex and an ever-expanding system of branches, spreading out like a fan and then converging, started to take shape.

The deposited sediment was redistributed and washed out several times while the fine-grained silt drifted further away. As a result of these processes, a thick alluvial cone, comprising mainly of gravel, was formed, reaching a width of as much as 410 metres. As severe inundations followed each other in a seemingly endless succession, the Danube scattered its fluvial drift unevenly above the aforementioned layer of gravel. Constant flooding ensured that no vegetation could leave a permanent imprint on the soil - with this in mind, the present low humus levels of the area pose no surprise. These factors, combined with the shallow depth of ground-water, create the conditions for the formation of alluvial meadow soils.