Rio Guapimirim flows through the municipality of Guapimirim in the state of Rio de Janeiro, draining the oceanic slopes of the Serra do Mar and emptying toward the Baía de Guanabara. As part of the Guanabara-Sepetiba bioregion — the second most species-diverse freshwater bioregion in the state — this river system supports a remarkably rich assemblage of native fish, making it one of the more biologically significant waterways in southeastern Brazil. The surrounding landscape is characterized by Atlantic Forest, with the river cutting through lowland coastal terrain before reaching the bay. The municipality of Guapimirim itself ranks among the highest in the state for fish species richness, with documented records of approximately 30 native species across its waters, reflecting the ecological health and connectivity of this drainage system.