Rio Guamá is a significant river system flowing through the state of Pará in northern Brazil, draining into the inner Amazon Estuary near the city of Belém. The river occupies a fascinating ecological position as a transition zone between freshwater and estuarine environments, where limnic and mesohaline waters meet and mix. This dynamic mixing zone creates an exceptionally productive fishery, with the estuarine waters of the lower Guamá allowing enough light penetration to support large algae production and high primary productivity. The river's lower reaches function as a critical nursery for juvenile fish, particularly large-bodied siluriform catfishes, which feed and grow in these nutrient-rich waters before moving into adjacent systems. The Guamá basin is an ample hydrological system that directly furnishes nutrients based on organic matter produced within its flood forests and surrounding landscape. Downstream, the river's influence merges with the Pará River delta, where a broad area of mixed salt and freshwaters hosts an extraordinary diversity of fish species.