The status of the Danube River sediment regime and morphology as a basis for future basin management

Hydromorphological alterations of large rivers are evident and have to be related to multiple anthropogenic pressures. Here we present the results of an integrated study concerning the actual status of the hydromorphology of the Danube River Basin that shows the sediment regime features a heavily disturbed system at various scales. Combined impacts of flood protection, navigation, and hydropower measures applied over a long period of time have been identified based on the river scaling concept as being responsible for these specific alterations (lack of bed load and suspended load in the remaining free-flowing sections). Moreover, long sections of the Danube River have been narrowed, channelized, disconnected from floodplains, and morphologically degraded over the last 200 years. This has caused increased bottom shear stresses, increased sediment transport capacities, and in addition a lack of lateral self-forming processes and corresponding reduced morphodynamics in the non-impounded sections. As a consequence of both, longitudinal and lateral disturbances of the sediment supply and additional impacts of the channelization, in opposition to a surplus of sediments in impoundments the remaining free-flowing sections are subject to various forms of river bed degradation. Such degradation or river bed incision leads to a loss of instream structures in general, with a disappearance of gravel bars at the Upper Danube and changes to sand bars in the Lower Danube. Hence, for river systems and large river basins, it has to be stated that the preservation and restoration of sediment continuity and morphodynamics is one of the most relevant issues for river engineering and ecology. This has to be considered especially for the implementation of legal directives and/or future river basin management plans.