Sustainable management of rivers and waterways requires reliable data on the transport of substances, their influencing factors, and the expected trends, which are accelerated by anthropogenic environmental change. The high dynamics of rivers, which is reflected in a large spatio-temporal variability of (suspended) matter flows, places high demands on surface water monitoring. Therefore, it becomes more and more important to clearly describe existing scientific uncertainties, to reduce them if possible, and to communicate them to decision makers. The aim of the online symposium “Uncertainty in river water monitoring – New insights on spatio-temporal variability of suspended river water components” was to address these challenges within the scope of an international platform. The symposium is part of the BfG research project URSACHEN, (Uncertainties in the determination of spatio-temporal variable material loads in rivers), implemented until 2023. It was held as an online event on Nov. 30 and Dec. 1, 2022. With 230 registered participants from 45 countries, the symposium was well received internationally. Over the two days, a total of 27 presentations addressed spatial, temporal, and methodological uncertainties in the determination of substance loads. Joint discussions highlighted both the challenges and opportunities for surface water monitoring. In this context, the results of the URSACHEN project were presented as well. The virtual platform enabled the participation of scientists “around the globe”,  reflected by the keynotes given by speakers such as Ton Hoitink (Wageningen, the Netherlands), Magdalena Bieroza (Uppsala, Sweden) and Marwan Hassan (Vancouver, Canada).

The symposium was opened by Harald Koethe, the director of ICWRGC (International Centre for Water Resources and Global Change), who presented the interdisciplinary URSACHEN project, while emphasising the international networking of BfG and ICWRGC. He also made reference to the UNESCO International Sediment Initiative (ISI). In addition, Mr. Koethe underlined the global importance of the topic by emphasizing that an understanding of suspended sediment transport plays a central role for the monitoring of water quality as well as for the stability of river systems and deltas and thus for millions of people living along rivers.

The overarching topic of “uncertainty” in river water component measurements was addressed in three “sessions”, each focusing on methodological, spatial and temporal aspects. In the first session on methodological uncertainties, moderated by Harald Koethe (ICWRGC), the possibilities, but also the challenges and limitations of new and already established, direct and indirect sampling and monitoring methods were presented. Based on examples of sediment concentration determination using acoustic and optical sensors and in discussions, it became clear that even established methods can only provide reliable results if the controlling factors for suspended sediment concentration such as hydraulic conditions and sediment properties are included and adequate calibration of the sensors takes place.

Thomas Hoffmann (M3) and Lars Duester (G4) led through the following programme. The second and third sessions dealt with the recording of the spatial and temporal variability of suspended matter. The “Spatial Session” focused on the problem of the representative determination of substance concentrations at certain water body (cross)sections,while i.a. remote sensing methods were also presented. The “Temporal Session” focused primarily on the importance of continuous monitoring series. On the one hand, the analysis of seasonal courses and hysteresis curves allows conclusions to be drawn about the possible sources and influencing factors of suspended sediment loads, while on the other hand, a minimum time interval can be determined, which is necessary for a reliable determination of suspended compounds in water and their loads.

The need for further research concerning uncertainties in river monitoring was actively discussed and its importance has been recognized. The significance of the topic was also mirrored by the number of participants and the presented diverse research approaches along with their results. The symposium succeeded in promoting the international and interdisciplinary exchange in the “river monitoring community” and offered valuable chances and opportunities to establish new professional contacts and deepen existing ones.