The Rio São Francisco is a major river system in Brazil, stretching 2,870 kilometers from its headwaters in the Serra da Canastra to the Atlantic Ocean. It is the fourth longest river in South America and encompasses a vast and diverse basin. The river's character changes significantly along its course. The upper basin features fast-flowing, well-oxygenated waters with rapids and falls. The middle reaches slow down and are subject to seasonal flooding, while the lower course is a plains river with slow flow, extensive floodplains, and many marginal lagoons. The basin includes over 33 reservoirs, with Sobradinho and Itaparica being among the largest on the main stem. This immense system supports a remarkably rich and unique fish fauna, with a high degree of endemism.