Upriver from Bahía Negra, the Negro River flows into the Paraguay River at the tri-border with Bolivia and Brazil. At Puerto Caballo, a Paraguayan Navy outpost and the place where Bahía Negra's first settlers lived, you can visit Boundary Marker XI (Hito XI) and gaze out over endless marshland. Inland stretches the Río Negro National Park Reserve, some 122,700 hectares of forests and flooded savannas with no infrastructure: it's best appreciated from the boat or from its edges, with local boatmen who know every bend of the river.

The area was established in 2001 as the “Río Negro National Park Reserve Area” when the Chaco Biosphere Reserve was created (Decree 13,202). It is still in the early stages of implementation, with no permanent Ministry of the Environment base. It protects a fragment of the Pantanal that is home to rare species such as the giant otter and, according to the WWF and Eco Pantanal guide, the area is on UNESCO's Tentative List.