evidence: As the Guadiana River makes its way toward the Gulf of Cádiz, it twice marks the boundary between Spain and Portugal. The first is between Ribeira de Cuncos and River Cala, and the second is between River Chança till the Guadiana River’s mouth. However, the Guadiana River is not entirely used to define the boundary between the two nations. A part of the boundary that falls between the Táliga Ravine and Olivenza Ravine is a disputed segment that has been claimed ‘de jure’ by both nations and administered ‘de facto’ by Spain as a portion of the landlocked Spanish autonomous community of Extremadura. The Guadiana River is navigable for 42 miles from the Atlantic Ocean to Mértola. About 17km north of Mértola on the Guadiana River is the Pulo do Lobo waterfall, considered the highest waterfall in Southern Portugal.