Organic and inorganic materials migrate downstream and have important roles in regulat-ing environmental health in the river networks.However,it remains unclear whether and how a mixture of materials(i.e.,microbial species)from various upstream habitats con-tribute to microbial community coalescence upstream of a dam.Here we track the spa-tial variation in microbial abundance and diversity in the Three Gorges Reservoir based on quantitative PCR and 16S rRNA gene high-throughput sequencing data.We further quanti-tatively assess the relative contributions of microbial species from mainstem,its tributaries,and the surrounding riverbank soils to the area immediately upstream of the Three Gorges Dam(TGD).We found an increase of microbial diversity and the convergent microbial dis-tribution pattern in areas immediately upstream of TGD,suggesting this area become a new confluence for microbial diversity immigrating from upstream.Indeed,the number of shared species increased from upstream to TGD but unique species decreased,indicating immigration of various sources of microbial species overwhelms local environmental condi-tions in structuring microbial community close to TGD.By quantifying the sources of micro-bial species close to TGD,we found little contribution from soils as compared to tributaries,especially for sites closer to TGD,suggesting tributary microbes have greater influence on microbial diversity and environmental health in the Three Gorges Reservoir.Collectively,our results suggest that tracking microbial geographic origin and evaluating accumulating ef-fects of microbial diversity shed light on the ecological processes in microbial communities and provide information for regulating aquatic ecological health.