evidence: short section of the boundary with Paraguay, just west of Salto das Sete Quedas (Guaira Falls) on the Rio Parana, is in dispute; two short sections of boundary with Uruguay are in dispute - Arroio Invernada (Arroyo de la Invernada) area of the Rio Quarai (Rio Cuareim) and the islands at the confluence of the Rio Quarai and the Uruguay River