A century of dynamic changes in the lower Gállego River (NE Spain) in response to natural and human interactions

The lower course of the Gállego River (NE Spain) has undergone significant geomorphological transformations over the last century due to the complex interplay between natural dynamics and human activities. This study analyzes the evolution of the fluvial system between 1927 and 2026, using four representative sectors located between Zuera and San Juan de Mozarrifar, near the city of Zaragoza (Spain). The methodology relies on diachronic geomorphological mapping comparing an exceptional 1927 photogrammetric survey with 2025-2026 satellite imagery and extensive fieldwork. The results reveal a transition from highly active braided and free-meandering systems in 1927—characterized by mobile gravel bars and sparse riparian vegetation—to a more unstable and anthropized landscape in 2026. While the northernmost sectors near Zuera and San Mateo maintains relatively natural dynamics, downstream areas have experienced intense degradation. Key drivers of this change include river regulation through dams and irrigation networks, alongside industrial and urban expansion since the 1960s. Nonetheless, the most critical alteration is a progressive river incision, reaching depths of up to 10 meters in the southernmost sectors. This process was initially triggered by massive gravel mining and the narrowing of the channel through the creation of waste dumps. Crucially, this incision has exhumed the Quaternary Qt9 terrace and its associated paleodoline infills. These karstic features, composed of silts and peats, now act as loose levels that enhance erosive processes, creating deep pools, rapids, and sharp 90° bends that dictate the modern fluvial pattern. This study concludes that the lower Gállego has shifted from a high-energy natural system in 1927 to a highly regulated and incised reach at present where ancient geological structures now play a decisive role in contemporary dynamics.