The unknown third dimension: terrain elevations, water depths and fluvial dynamics of Austrian Danube river landscapes prior to regulation | Die unbekannte dritte Dimension: Geländehöhen, Gewässertiefen und Dynamik österreichischer Donaulandschaften vor der Regulierung

River landscapes are multidimensional ecosystems characterized by diverse spatio-temporal interrelations. Longitudinal interactions along the river continuum and lateral exchange processes between river and floodplain are common focal points in river restoration projects. Vertical interactions, such as between aquatic habitats and river bottom (hyporheic interstitial) or between groundwater and terrestrial habitats, are often only addressed in planning schemes, if the impairments of ecological functions and human uses are already evident. This partly reflects the dearth of well-founded basic data on the configuration of river landscapes prior to regulation in the vertical dimension. Consequently, only few reference data are available for designing adapted restoration measures.Several research projects in recent years have yielded new insights into the three-dimensional hydromorphological configuration of Austrian Danube landscapes in the early and mid-19th century. They enable the height of different ecomorphologically significant terrain zones within the floodplain to be specified in respect to characteristic Danube water levels. Each of these zones corresponds to a particular morphological stage of development and fulfills specific ecological functions. Reconstructions of the groundwater tables at mean water level within the modern, up to 500-years-old floodplain show that these were on average only 1.6-1.9 m below the terrain surface. In comparison, today the groundwater table depths amount to c. 3 m or more. The water bodies of the Danube were formerly not only much broader, but also much shallower. Accordingly, shallow, up to 1‑meter-deep water zones originally amounted to c. 8.1 ha per km linear distance at summer mean water level. In contrast, the current value is 1.2 ha, representing an 86 % decrease. This calculation omits the formerly numerous lotic side arms with comparable habitats that no longer exist today.Particularly interesting insights were obtained regarding the historical morphological dynamics of the Danube. In the course of avulsive shortenings of Danube main arms, an average 3 million m³ of sediments were eroded annually between 1812 and 1817, and 2.6 million m³ were redeposited in the same floodplain. About 0.4 million m³ were discharged each year from the system and transported further downstream. Importantly, such massive turnover processes did not occur permanently within a particular river section. In subsequent years, the eroded volumes dropped significantly, with deposition clearly exceeding erosion. The intensive morphological dynamics were reflected in significant drawdowns and uplifts of the water and groundwater levels within the floodplain. Despite the extensive changes in the river landscape, in total these hydrodynamics remained roughly balanced. Other examined parameters indicate that – viewed over larger areas and longer time periods – the hydromorphological configuration of the river landscape remained in dynamic equilibrium or a type of steady state.Today, along the Danube River and most other major rivers, floodplains show much higher terrain levels in relation to the water or groundwater level than prior to regulation. Riverbed deepening, combined with constantly increasing levels of the floodplain terrain due to flood-related sediment deposition, led to a progressing vertical decoupling of aquatic and terrestrial habitats in the river-floodplain systems. Depths of the groundwater table relative to the terrain surface increased and formerly wet, moist or fresh sites of softwood forests were successively lost. The examples from the history of the Austrian Danube River illustrate that a river-typical – and in the case of the Danube River high – intensity of fluvial dynamics is crucial in maintaining a heterogeneous habitat complex. These processes regenerate and rejuvenate large parts of the aquatic and terrestrial habitats in relatively short periods of time.