In June 2024 a severe flood occurred in the headwater regions of the Danube River in Germany and Austria. This event gave rise to the idea to sample the floodwave along the river for simple and easy to preserve geochemical parameters. Such a spontaneous sampling offered itself as an exceptional opportunity particularly through the DALIA project. This is because  many partners live in close vicinity or directly at the river.  This setup allowed us to sample the flood peak as it moved down the river in an international effort throughout several countries. The sampling had to be spontaneous and would have been impossible to carry out  by one team alone. It would also have taken too long to activate  several regional and national environmental agencies due to the narrow time window of only several days. The crowd sampling activity was organized by partner FAU who is responsible for water sampling across the entire Danube River system. The response by the DALIA partners was overwhelming and we were able to collect a total of 48 samples of this extreme event. From this collection we used only the 20 most interesting samples that best represented the peak of the floodwave as it moved down the Danube.

Demo Site 2 grabbed the chance to turn the Danube River into a practical and interactive learning experience when a significant flood struck the river in June 2024. DALIA partners and also the general public were given the opportunity of going out in the floodplain and collecting water samples in several areas, getting a chance to engage directly with the river and observe the science firsthand. A crowd sampling activity showcasing collective effort and awareness raising on science. While sampling took place in June 2024 sending, processing and interpretation of the samples were carried out in the following months. 

The activity was not only an educational experience but also a good adventure that was memorable and outlined the excitement of fieldwork in a real-life environment. 

It also helped to connect participants in the DALIA consortium to work on a common system. 

Water samples collected were analysed for robust and easy to preserve parameters. These were electrical conductivity and the water stable isotope composition (expressed in permille (‰) in a delta notation. Even though only these two parameters were chosen for analyses, this offers an exceptional data set that allows a rare observation of a floodwave along a large river system. 

An initial examination of the samples has already shown interesting facts. Both the d18O water isotopes and electrical  conductivity were lower during the flood in comparison to seasonal averages as the flood took place (figure 1). These results are starting to explain the reaction of the Danube to extreme hydrological events. Such lowering of these geochemical tracers indicates the higher degree of dilution by excessive input of rainwater in the headwater region. Interestingly, severe flooding took place in the headwater region of the Danube (i.e. in Germany and Austria), while the river water levels were high further downstream but remained within the river boundary (i.e. no flooding of roads, houses and floodplains). Yet the dilution effect could be observed throughout the entire river system down to the mouth of the river near Galati. This persistence of dilution effects was somewhat unexpected because along its  course, the river becomes  influenced by several other inputs such as from tributaries and groundwater. These effects were of course still present but seem to have been unable to mask the dilution input by the flood. The implication of such a finding is that likely not only water but also transported material including mobilised pollutants undergo enhanced mobility during such an extreme event.  

Figure 1: The Danube River basin with blue lines indicating the Danube river and its tributaries. Red dots indicate the sampling locations of the peak of the flood wave (±1 day).

Figure 2: δ¹⁸O values of H₂O plotted against the Danube river kilometers with 0 km representing the Danube Delta. Blue points indicate the seasonal averages (spring, summer, autumn and winter) with their standard deviations and red points indicate the δ¹⁸O values during the flood event. Note that from the 48 samples collected only the 20 most interesting ones (that best represent the flood peak) were selected.

Figure 3: Conductivity values (in µS/cm) plotted against the Danube river kilometers with 0 km representing the Danube Delta. Blue points indicate the seasonal averages (spring, summer, autumn, and winter) with their standard deviations, while red points represent the conductivity values during the flood event. Note that from the 48 samples collected only the 20 most interesting ones (that best represent the flood peak) were selected.

One question that remains open is the role of groundwater during such flooding. By concept the river is always to variable degrees fed by incoming groundwater. Generally this important input to the river is more intense during low water stands and becomes reduced during times of higher runoff.  The same holds true for tributary input. Unfortunately, neither groundwater samples nor tributary samples were available during this time of flooding. Also geochemical data from the literature were too variable to establish a homogeneous groundwater and tributary input signals to the river during flooding. This lack of input data did not allow a mass balance calculation that would have quantified the degree of dilution in relation to groundwater and tributary input. Such a quantification would have been scientifically very interesting because change of flow dynamics during extreme events in large river systems are so far hardly quantified in the hydro(geo)logic community. In order to solve such quantifications more background data on groundwater and tributaries would be necessary. Nonetheless, the fact remains that dilution and transport by  flood waters must have been very strong.  

In the meantime, this crowd-sourced project has demonstrated a success story by itself and showed that interactive field work can be highly useful in terms of science. The fast and voluntary reaction of a large group of project partners shows a very good approach of   data gathering. Such activities also serve as an international and large group learning process, a team-building activity, and for all participants an adventure on how to work together on a large and complex river system.