Abstract: Understanding both taxonomic and functional dimensions of biodiversity is essential for revealing how aquatic communities respond to environmental disturbances. Taxonomic diversity reflects the structural attributes of a community, whereas functional diversity provides insight into ecological strategies, resource utilization patterns, and resilience mechanisms. In recent years, extreme hydrological events have become more frequent in northern China, yet the impacts of catastrophic floods on macroinvertebrate communities in mountain rivers remain insufficiently studied. We investigated benthic macroinvertebrates in the mountain section of the Yongding River in Beijing during the spring preceding the Haihe River Basin “23·7” extraordinary regional flood event and during the autumn following the flood. Our objectives were to examine seasonal variations in taxonomic and functional diversity and to explore the relationships between macroinvertebrate communities and key environmental variables, thereby elucidating the response patterns of mountain river ecosystems to large-scale flood disturbances. A total of 76 species of benthic macroinvertebrates were identified across both seasons, belonging to 3 phyla, 6 classes, 18 orders, 44 families, and 60 genera. The assemblage was dominated by Arthropoda, Annelida, and Mollusca, with aquatic insects representing a major proportion of the taxa. Despite this rich composition, the results indicated no significant seasonal differences in taxonomic diversity indices. In contrast, functional diversity exhibited notable temporal variation, functional evenness showed a significant decline during autumn, suggesting a reduction in the uniformity of trait distribution within the community after the flood disturbance. Correlation analyses revealed that water depth, water transparency, and chlorophyll-a concentration were the most influential environmental factors shaping functional diversity patterns. The substantial decrease in chlorophyll-a in autumn implied reduced primary productivity, which likely constrained food resource availability for benthic consumers. Following the flood, the benthic community shifted toward strongly dominance by small-bodied, fast-developing arthropods with high dispersal capacities- traits that confer advantages for rapid recolonization and survival in unstable environments. In contrast, mollusks, which typically exhibit limited mobility and slower life-history strategies, showed a sharp decline in both abundance and diversity, reflecting their considerably slower recovery after disturbance. These findings highlight that evaluating both taxonomic and functional aspects of macroinvertebrate diversity provides a more comprehensive understanding of community dynamics under extreme hydrological disturbances. While species richness alone may mask substantial ecological shifts, functional metrics capture subtle yet ecologically meaningful changes related to species’ ecological roles, niche complementarity, and adaptive strategies. Our results further demonstrate that large floods can reorganize community structure by selectively favoring taxa with traits that enhance resilience and recolonization potential, ultimately altering functional composition even when species diversity appears unchanged. Overall, this study deepens our understanding of seasonal changes and flood responses in mountain river macroinvertebrate communities. The insights gained contribute to a more integrative perspective on maintaining ecosystem function and stability in river systems experiencing increasing hydrological variability. These findings also provide scientific support for ecological protection and restoration efforts within the Yongding River Basin and similar mountainous river ecosystems.