<p>River sediments are primary sinks for microplastics (MPs) while the MPs hazards are of great concern. River depth and anthropogenic activities are reported to influence MPs distribution, but emerging evidence suggests that extreme events and biological interactions also alter MPs fate. Here, we conducted a one-year monitoring program in a river section downstream of a reservoir to investigate the impacts of a flood event caused by increased reservoir discharge and aquatic plants on the MPs characteristics in sediments. A significant increase in MPs abundance was observed in July (post-flood; 551–4174 items/kg) and September (during peak plant proliferation; 4104–4908 items/kg). MPs abundance showed a positive correlation with aquatic plant biomass and density. The Granger causality test suggested that other environmental factors simultaneously influence MPs accumulation. Redundancy analysis further confirmed that current velocity and discharge remained significant factors shaping MPs composition, and the flood was significantly associated with an increase in small-sized MPs. Our findings demonstrated that storm events could induce a short-term increase in MPs abundance and aquatic plant communities can enhance MPs deposition and acting as a biological sieve. Future control and management of MPs pollution can introduce aquatic plants and need to consider the far-reaching impacts of extreme events.</p>