Application of structure from motion (SfM) algorithms for the historical analysis of changes in fluvial geomorphology

This work presents the workflow used to obtain spatial information to analyze geomorphological changes in the upper reach of the Cinca River (Southern Pyrenees) during the 1927-2015 period. The final products after application of the SfM-MVS methodology are orthomosaics with a root mean square error (RMSE) of 0.5-1 m, and point clouds with an RMSE of 1-2 m. The results of the study indicate that the study reach has suffered a clear reduction of the active channel width (i.e. 52%) with a high incision processes associated (e.g. >5m in some points), and a simplification in the channel pattern (i.e. reduction of the braiding index and changes from a multi-channel pattern to a single-channel one). These processes are directly influenced by the anthropogenic impacts associated with in-channel gravel mining and the associated impacts related to channel embankment (reach scale, localized impacts), and also influenced by the impacts on the production and transfer of sediment (catchment scale) due to land use changes since the 50s of the 20th century. The methodology presented in this paper is very useful for the diagnosis of the morphosedimentary state of river systems. In the particular case of the Alto Cinca River, the results are of great interest for the improvement of the understanding of the cause-effect relationships in the morphosedimentary dynamics observed for the period 1927-2015. This improvement can help modify basin management plans through a more comprehensive view of contemporary processes.