In dammed rivers, the conservation of free-flowing tributaries is considered an important strategy to mitigate the negative effects of the dam on fish communities. In this study, we evaluated the importance of a free-flowing tributary of a dammed river as a reproductive migration route. From August 2012 to July 2013, hydroacoustics data were collected alongside active fish sampling using cast nets and ichthyoplankton nets in the São Francisco and Abaeté rivers. Cast net sampling captured 738 individuals and hydroacoustics detected 42196 fishes. In almost all samples and sampling sites, preferential movements to the Abaeté River and the main São Francisco River were observed. The increase in the flow rate of the Abaeté coincided with a greater incidence of fish movements to upstream areas of the tributary. A higher density and proportion of larger fishes, as well as a higher density of ichthyoplankton, were observed in the Abaeté River compared with the main river. Because this tributary is an alternative route for migratory fishes, its protection is essential for fish conservation and therefore maintenance of local fisheries