Statistical approach of contemporary hydrogeomorphological channel changes of the middle Allier River, France: morphostructural controls, human impacts and flow regime

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The Allier River, France, known for its very dynamic river sections in its lower course, was nevertheless subjected to human impacts. The present study aims to better explain the recent hydrogeomorphological channel changes of the middle Allier River considering morphostructural constraints, human impacts and flow regime since the second half of the 20th century. Channel changes are analyzed from six historical aerial photographs (1954, 1974, 1992, 2000, 2010 and 2016) and four longitudinal water surface profiles (1934/35, 1999, 2005 and 2020), based on descriptive statistical analyses and a Principal Component Analysis followed by a Hierarchical Cluster Analysis. The influence of morphostructural controls and human impacts are tested using a Correspondence Factor Analysis, whereas temporal trends of the flow regime are analyzed using Mann Kendall and Pettitt tests. Overall, results show that during the last sixty to eighty years the channel bed incised by 0.86 m, narrowed by 25% and that lateral mobility decreased by 68%. Four morphological evolutionary trajectories were identified for the alluvial plain sections, laterally more mobile than the naturally confined section. These evolutionary trajectories were caracterized by a more or less important presence of human impacts on the channel, such as gravel mining activities and, more locally, in-channel engineering works. Moreover, hydrological changes recorded at the end of the 20th century, characterized by a decrease of the frequency, the duration and the intensity of floods, has led to a general increase of annual abandoned surfaces and, conversely, to a decrease of annual eroded surfaces. In conclusion, future river management schemes will have to consider past and present human impacts on channel morphodynamics as well as current climate change and its consequences on the flow regime.