Illegal alluvial mining has expanded rapidly across the Amazon basin, increasing pressure on river ecosystems, although its effects on fluvial dynamics during extreme hydrometeorological events remain insufficiently understood. This study examined land use and land cover (LULC) changes after a flooding and overflow event in the Nangaritza River and evaluated their influence on river dynamics. The objective was to determine the impacts of illegal mining on the fluvial behavior of the Nangaritza River in the southern Ecuadorian Amazon. Sentinel-2 Level-2A multispectral imagery was processed in Google Earth Engine to generate cloud-free mosaics for pre- and post-event conditions. Spectral indices including NDVI, MNDWI, NDTI, NDMI, and NDBaI improved classification accuracy. LULC maps were produced using the Random Forest supervised classification algorithm. Accuracy was evaluated with confusion matrices, while transition matrices and Map Algebra in ArcGIS Pro quantified spatial changes. Fluvial dynamics were assessed through variations in water bodies, mining areas, and sandbanks. Results showed a marked expansion of illegal mining along the river corridor through the conversion of forested, sedimentary, and aquatic surfaces into mining zones. These changes were accompanied by deforestation and transitions among clear water, turbid water, and sandbanks, indicating active sediment redistribution. Exchanges between water bodies and exposed sediments reflected shifts in erosion and deposition processes. Overall, illegal mining combined with extreme rainfall altered river morphology, sediment dynamics, and riparian land cover in the Amazonian fluvial system. The findings highlight growing environmental risks for biodiversity, water quality, and long-term stability of riparian ecosystems.