How Many Real Islands Have Existed along a Mid-Danube Section during the Past 250 Years?

Abstract

The consequences of river regulation work and other human interventions from the past can still be followed and cause various effects on the Danube river, especially on the formation and state of the islands. We aimed to compile an inventory of the islands along the Hungarian Danube stretch between Vének (1797 rkm) and Budapest (1647 rkm) during the past 250 years, focusing on their landscape history. Real islands were defined as gravel bars that are permanently covered with pioneer woody vegetation, constantly surrounded by water (side-branch is not closed), and their surface remains unflooded at an average water level. We identified 123 real islands that existed on this 150 km long river stretch in the last centuries. These were real islands for a shorter or longer period between the 18th and the 21st century. A total of 66 of them existed at the same time about 200 years ago, i.e., before the large-scale water regulation works, while today this number is only 18, meaning that 73% of current potential Danube islands are at a stage of side-branch succession. Before the river regulation works, the natural successional changes of a real island occurred over hundreds of years, but today they happen rapidly. The formation of new islands became very limited compared to the past due to the lack of sediments and the altered river dynamics. In order to conserve this unique ecological corridor and green infrastructure element in the long run, and restore its damaged floodplain habitats, no more hard-tech interventions should be allowed in the fluvial system. These aspects need to be taken into consideration in decision-making processes with an integrated approach.