Flooded forests represent an important part of Amazonian diversity, yet the distribution, ecology, and evolutionary history of the avifauna of these forests have received little attention. We conducted ornithological surveys in the Rio Branco Basin, which is entirely in the Brazilian State of Roraima. In this paper, we discuss the presence of 20 bird species recorded along the lower Rio Branco, 16 of which represent new records for the State of Roraima and the entire Rio Branco Basin. Among our most interesting records are four species of white-water river specialists (Synallaxis propinqua, Stigmatura napensis, Serpophaga hypoleuca, and Conirostrum bicolor) that have populations on the lower Rio Branco, isolated from other Amazonian white-water river systems by the black waters of the Rio Negro where they do not occur. We also discovered new localities for the endemic and endangered Rio Branco Antbird (Cercomacra carbonaria), doubling the size of its known range. We discuss the implications of these records in a biogeographic perspective for better understanding the distributional patterns of the flooded-forest avifauna in Amazonia.