Hydrological indicators of the riverbed incision along the free-flowing Danube River reach from Budapest to Slankamen relevant for the lateral connectivity between the river channel and floodplains

Abstract

Hydrological regime plays a primary role in the sustainable management of floodplains, as floodplain ecosystem dynamics and balance are largely based on the dynamics of the flow regime of rivers. In the 1980’s a drying process of the floodplains on the lower Danube reach in Hungary became evident. The first statistical analyses were made for the Hungarian reach in 1992, because foresters noticed an alarming drying process of the floodplain forests. Analyses indicated the lowering of the riverbed of the Danube River. It is considered that the river training works were the main reason for the incision of the riverbed.

To the authors’ knowledge, no comprehensive analysis of the changes in the water regime for the whole alluvial reach of the middle Danube has been made to date. To fill this gap, authors analyse available time series of the water level and discharge data for the free-flowing alluvial reach of the Danube River, downstream of Budapest, Hungary to the confluence of the Tisa River, in Slankamen, Serbia. This analysis aims at: 1) recognising hydrological indicators of the incision and/or aggradation of the riverbed, 2) estimating the extent of the two processes and the rate of change of the riverbed in time based on these indicators, as well as 3) estimating possible consequent changes in the frequency of extremities (low flows and high flows). According to the Water Framework Directive (WFD) the „good status” of the Danube River reach must be achieved. As the river and its floodplains constitute a complex ecological system, this can only be done through the harmonisation of the nature protection aspect reconstruction projects, WFD programmes of measures, flood management measures and navigation development. If traditional river training activities continue, riverbed erosion will persist or increase in the future, resulting in slow, but continuous drying of floodplains, that are very important nature conservation areas. Consequently, the majority of floodplain reconstruction works’ effects might become negligible, while navigation problems will remain unsolved. In the EU Floods Directive, natural flood management is an important issue, with a focus on increasing water retention capacities by e.g. the re-connection of rivers with their floodplains and restoration of wetlands which can store flood water and help “slow the flow” of flood waters. In this respect, lateral connectivity is one of the most essential issues, as it is for the species inhabiting floodplains and rivers. However, floodplain lateral connectivity is already severed by the decreasing frequency and extent of inundation. The studied hydrological regime is the most important determinant of floodplain habitats.