Six decades of ecogeomorphological adjustment to multi-scale disturbance in central Spanish Pyrenean headwater rivers

Mountain basins are fundamental sources of water and sediment for downstream systems, yet their fluvial dynamics are highly sensitive to land use and land cover (LULC) changes and hydrological disturbance. Since the mid-20th century, the central Spanish Pyrenees have undergone widespread land abandonment and afforestation, leading to modified streamflow regimes and complex ecogeomorphological adjustments. Fully understanding and managing these ecosystems demands an integrated analysis that links key drivers at basin scale and its consequences on fluvial dynamics at reach scale. We combined historical streamflow records, LULC basin changes, in-channel vegetation encroachment evolution (NDVI time series), topographic differencing (DoDs), and active channel width analysis to reconstruct morphological trajectories of four mountain rivers in the Upper Aragón River basin (Central Spanish Pyrenees) from 1956 to 2021. Our results reveal a persistent decline in mean and peak discharges and in the frequency of high magnitude flows, concurrent with widespread vegetation growth. Active channel width narrowed continuously since 1956, with no sustained recovery and progressive riparian colonization of former active bars. DoD analyses reveal pervasive material loss and incision on lateral bars, while limited deposition is confined to the main channel. Overall, the studied reaches exemplify a broader Mediterranean and European trend toward narrower, incised, and stabilized mountain channels. This morphodynamic simplification poses significant risks due to local erosion to infrastructure, bridges, roads and levees, and undermines ecological resilience by reducing habitat diversity. Long-term monitoring thus emerges as an essential tool for designing adaptive landscape strategies that focuses not only on water availability but also on sediment availability and vegetation dynamics under ongoing Global Change.