This study investigates the interplay between groundwater and surface runoff in controlling suspended sediment dynamics within the Galabre catchment, a 20 km² Mediterranean mountainous catchment characterized by extensive badland areas. Using an End-Member Mixing Analyzing framework that accounts for event-specific variability in end-member chemistry, the study quantifies surface runoff and groundwater contributions during 86 flood events spanning three hydrological years.</p> <p>Hydrological analyses reveal clear seasonal contrasts in the generation of surface runoff and groundwater flow rates. Surface runoff contributions during floods vary from 0 to 50 % of the instantaneous flow rate, with higher proportions during the dry spring/summer season (May–September) and lower values in autumn/winter. Surface runoff and groundwater flow rates are strongly correlated with event rainfall accumulation, while groundwater-related variables also show sensitivity to antecedent rainfall over 15 days, highlighting their dependence on long-term hydrological connectivity.</p> <p>Results further demonstrate that suspended sediment concentrations correlate more strongly with surface runoff flow rate than with flow rate, emphasizing the dominant role of surface runoff in sediment detachment and transport on hillslopes. Marked seasonal differences in hydrosedimentary processes were observed. Spring/summer floods, driven by short and intense rainfall, produce low-flow responses with high surface runoff contribution, high suspended sediment concentration, and exhibit anticlockwise flow concentration hysteresis loops. This suggests that high amounts of fine sediments are mobilized on hillslopes during these floods, leading to high suspended sediment concentrations and moderate flow rates in the river network, associated with sediment deposition. Conversely, autumn/winter floods, governed by prolonged low-intensity rainfall, and characterized by enhanced groundwater contributions, produce high flow responses, with low suspended sediment concentration, and clockwise flow concentration hysteresis loops. These floods are associated with riverbed sediment re-mobilization. These findings reveal a fundamental seasonal shift from primary (i.e. hillslope mobilization) to secondary (i.e. riverbed re-mobilization) erosion processes, controlled by the dynamic balance between surface runoff and groundwater inputs to the riverbed throughout the year.