Historic changes in river course and morphodynamics of the Ahr river within geological and anthropogenic boundaries

The narrow valley parts of the Ahr river are located in a low mountain region on the edge of the Volcanic Eifel. In July 2021, a flash flood formed, and energetic water masses with high flow velocities drove significant morphodynamic change and sediment displacements along the Ahr river. The riverbed and the banks experienced erosion, and floodplains showed slope erosion by surface runoff. In other locations, we observed floodplain sedimentation and riverbed infill. Large sediment lenses formed on floodplains, and gravel bank deposits, partly high in organic content, changed the local morphology of the Ahr river. Sorting of the grain size of deposited sediments from coarse to fine was observed providing clues to the complex flow conditions. The change of the river-course was overall limited due to the high urban development and man-made riverbank constraints by infrastructure. In this context, the prediction of future morphological changes in the Ahr river is essential for sustainable water resource management, especially in the context of reaching the goals set by the European Water Framework Directive (EU-WFD). If not properly accounted for, future maintenance and engineering measures affecting the river-course may not only be costly but also introduce new hazards. This study investigates morphological changes in different sections of the Ahr river as a result of the July 2021 mega-event at different spatial scales. Those changes were evaluated in a historic context to compare the observations to theoretical morphological developments of the river unaffected by anthropogenic development. The Lateral Mobility Index (LMI) is used to evaluate stream bed changes. In the upper reaches of the Ahr river, confining bedrock limits morphological riverbed changes as a natural geological boundary. Thus, most morphological changes in the Ahr river occurred in the lower reaches. The LMI caused by the mega-event in the 2.7 km long lower river reach up to the Rhine confluence is 1.2, which indicates severe changes in the river course. However, the LMI between the early 19^th century and the mid-19^th century is 2.2, which could result from an increase of confining anthropogenic boundaries within the last 200 years, which further caused reduced river structures and a reduced alluvial corridor. Anastomosis structures with a total channel width of up to 250 meters in the early 19^th century are non-existent anymore, and the total channel width was reduced to 20 meters on average before the flood event. Although the total channel widened up to 90 meters after the flood event, the structural diversity only slightly increased. Concluding, morphological changes by the flood event in mid-July 2021 did not generally develop towards near-natural river structure, likely resulting from the anthropogenic hard boundaries constraining flow. River engineering actions in the course of rehabilitation after the flood event need to be evaluated on different spatial scales to assess sustainable water resource management for the future.