Environmental history and geophysical investigation of the Nagy-sziget, Rácalmás, Hungary

The interplay of the hydrology of a great river and tectonics in floodplain setting is always a challenging research topic. The evolution of the river course, development of islands, oxbow lakes, changing channel sinuosity can be evaluated as signals of these two processes. In our study area, we seek to explain the evolutionary history of the Nagy-sziget, Rácalmás, Hungary by using a multidisciplinary approach. We integrate field observations, evaluation of historic maps, and results of our own geophysical measurements in order to reconstruct the environmental history of this island of the river Danube. The investigated island is located south of Budapest, in the southern corner of Csepel Island, where the Soroksári-Danube branch joins the main branch. The area is particularly interesting in geodynamic sense, as the region is crossed by the Mid-Hungarian Mobile Belt at the confluence of the Ráckeve (Soroksári) Danube branch close to the northern part of the island. During the field campaign we observed characteristic micro relief structures in the field that have been integrated with landforms indicated on historical maps. Furthermore, we wanted to know how these topographical patterns continue below the surface, therefore geoelectric measurements were carried out. We conclude that the area of the island started to grow very rapidly after the river regulations. Our geoelectrical tomography and electromagnetic measurements showed that, despite the very low relief, the subsurface geology is reflected in microtopography. Based on these results various scenarios for the formation of the island have been put forward; a final scenario will emerge from our planned measurements. We concluded that the evolution of the Nagy-sziget may have had tectonic, landsliding, sedimentary and flood-related components, which are specific to different regions. With the integrated results of the geoelectric and electromagnetic measurements and the study of historical maps, we managed to reconstruct the growth of the island in unprecedented detail over the past 250 years and we can formulate some implications to a certain extent for periods before written documents.