Rio das Velhas is one of the most significant rivers in Minas Gerais, Brazil, flowing through the central portion of the state as the longest tributary of the rio São Francisco. The river originates in the highlands near Belo Horizonte — the state capital — and winds northward through a diverse landscape shaped by the ancient geology of the Quadrilátero Ferrífero (Iron Quadrangle), a region characterized by quartzites, phyllites, iron formations, and dolomites. The watershed drains a vast area and supplies water to approximately 6 million people in the greater Belo Horizonte metropolitan region. Despite facing considerable anthropogenic pressures over the decades, the rio das Velhas basin remains one of the most ichthyologically rich river systems in central Brazil, harboring more than 115 known fish species across its headwaters, upper, middle, and lower reaches.