The long-term-memory of a typical mid-European upland to lowland river : or why we struggle to reach a good ecological state for our rivers

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023; Aachen : RWTH Aachen Univsersity 1 Online-Ressource : Illustrationen, Diagramme, Karten (2023). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023
Anthropogenic impacts on rivers date back several millennia BC, starting with agricultural land use and deforestation. In Europe, many of today’s measures, like mill ditches, still used by industries for process and cooling water today, originate from medieval times. With Industrialization, rivers were regulated, and large dams were constructed. Until today, several resulting morphodynamic processes from former and recent measures overlap, and it is almost impossible to determine how the river’s morphology will develop in the future. Thus, systematically evaluating the river’s morphological behavior supports sustainable water resources management. The Rur River (North Rhine-Westphalia, Germany) is a model example for a typical mid-sized mid-European low-mountain to lowland river whose catchment has been affected by industrial development, including the construction of seven reservoirs. In this thesis, anthropogenic impacts are evaluated qualitatively and quantitively on different spatial scales. We identify the morphological developments from different eras affecting the Rur River until today, specific measures of water resources management impacting the sediment connectivity, and long-term impacts of damming to assess boundaries for a river’s morphological development. Morphological changes over the last 200 years are determined in GIS. Changes in river straightening and structural diversity are described by structural indicators and correlated with eras of water resources management. The most severe measures affecting sediment transport are identified by modifying the sediment rating curve. A hybrid examination of field measurements and numerical modeling investigates the impact of large dams on the downstream’s morphology. We identified five eras characterized by priority shifts in water resources management during the last 200 years: The Pre- Industrial Era (mid-18th – mid-19th century), the Industrial Era (mid-19th century – WWI), the Agricultural Era (after WWI – 1980s), the Era of Ecological Improvement (1980s – 2000), and the Era of EU-WFD (from 2000 to the present). The Industrial and Agricultural Era led to river straightening, even without direct hydraulic measures. The suspended sediment transport as the main morphodynamic driver could be described more accurately with the sediment rating curve when incorporating parameters for the length of a river’s section and damming. Results are river specific as they are closely linked to connectivity behavior. For the Rur River, the construction of large dams leads to a sediment deficit, increased mean sediment diameters downstream of the reservoir, and an alteration of the flow regime. The local lithostratigraphy superimposes the sedimentological changes downstream of the dam. The changes in the flow regime, however, lead to severe floodplain decoupling. Natural boundaries pose the foundation on which a river develops. Geomorphological boundaries primarily operate on different scales and, amongst other boundaries, lead to a specific fingerprint in a river’s connectivity, which determines responses to anthropogenic measures. However, the socio-economic era determines priorities in water resources management. To achieve sustainable river development, we need a shift in the socio-economic priorities of the present era and the implementation of water resources management in current political agendas. Further, evaluating a river’s connectivity behavior on different scales needs to be incorporated into water resources management to achieve development within the river’s natural boundaries.
Published by RWTH Aachen Univsersity, Aachen