Rio Guaíba is one of the most distinctive and debated water bodies in South America — a massive estuarine system in Rio Grande do Sul, Brazil, that locals and scientists alike argue over whether to call a river or a lake. Covering approximately 496 square kilometers, it sits at the heart of the state's hydrological network, bordered by the city of Porto Alegre to the east. Fed by four major river systems — the Jacuí, Sinos, Gravataí, and Caí rivers — it drains southward into the Lagoa dos Patos, the largest lagoon in South America. This unique position as a transitional water body between freshwater river systems and a vast coastal lagoon creates an exceptionally diverse aquatic environment, drawing migratory species from the coast while also harboring resident freshwater species year-round.