Arroyo Piray Miní is a small, clear-flowing stream in the northern part of Misiones Province, Argentina, draining as a left-bank tributary into the Río Paraná. Situated between the Río Iguazú and the Arroyo Urugua-í drainages, this stream occupies a biogeographically isolated corner of the Paraná River basin, lending it a remarkable degree of ecological uniqueness. The stream runs through a subtropical landscape typical of northeastern Argentina, with variable water depths ranging from roughly 0.20 to 1.40 meters and a mixed substrate of mud, sand, and stones. Submerged aquatic vegetation is sparse, and the current is notably rapid in many stretches, creating the kind of clear, oxygenated water that supports a distinctive community of native freshwater fish found nowhere else on earth.