The Rio Guaraguaçu is a lowland coastal river situated in the Lagamar region of Paraná state, flowing northward through a broad floodplain surrounded by marine coastal plain before draining into Paranaguá Bay. The river lies within the Atlantic Rainforest biome, and the broader Lagamar region — a network of estuaries spanning south and southeast Brazil — is recognized as both a UNESCO Biosphere Reserve and a World Heritage Site, reflecting its significant conservation value. The river's coordinates place it at approximately 25°32′43.1″S, 48°27′51.1″W.

The basin supports a range of ecosystem services including navigation and recreational activities, and the river is home to various fish species including fat snook, common snook, trahira, and blue runner, with nearby waters in Paranaguá Bay also holding lookdown and gafftopsail sea catfish. The presence of endemic fish species adds to the ecological interest of the area.

Paddlers should be aware of notable water quality concerns. The river basin faces documented pressures from high levels of untreated sewage and landfill leachates entering the waterway, which raise hygiene and safety considerations for anyone with direct water contact. Macro-drainage projects in the area have also introduced large volumes of saltwater into freshwater reaches, altering the river's natural character in some sections. Invasive non-native species have been recorded in the basin as well.

Additional environmental pressures from expanding port activities and rapid population growth in the coastal region of Paraná continue to affect the river. The State Environmental Institute (IAT) has proposed classifying stretches of the Guaraguaçu as Class 2 under Brazilian water quality standards, though no formal classification is currently in force for the entire river course. Paddlers planning a trip should factor in these water quality conditions and consult current local guidance before entering the water.