The Frontier Biological Corridor Wildlife Refuge extends for two kilometers south of and along the entire border with Nicaragua, coast to coast. Boats ply the Río San Juan, connecting Puerto Viejo de Sarapiquí with Barra del Colorado and Tortuguero (and west with San Carlos, in Nicaragua).

The nature-viewing is fantastic, with birds galore, monkeys and sloths in the trees along the riverbank, and crocodiles and caimans poking their nostrils and eyes above the waters.

You can even visit El Castillo de la Inmaculada Concepción, built by the Spanish in 1675 on a hill dominating the river and intended to repel pirates and English invaders. The ruins are in Nicaragua, three kilometers west of where the Costa Rican border moves south of the river (you’ll need your passport).

The transboundary park was birthed in 1985, when Nicaraguan president Daniel Ortega seized on the idea as a way to demilitarize the area, at the time being used by anti-Sandinista rebels. Ortega proposed the region be declared an international park for peace and gave it the name Sí-a-Paz—Yes to Peace. Efforts by the Arias administration to kick the rebels out of Costa Rica’s northern zone led to demilitarization of the area, but lack of funding and political difficulties prevented the two countries from making much progress.

The end of the Nicaraguan war in 1990 allowed the governments to dedicate more money to the project. In 2003, the efforts led to formalization of the boundaries of a new national park, the 30,000-hectare Maquenque National Park, a massive swath to encompass heavily logged and denuded terrain between the Sarapiquí and San Carlos Rivers and extending northward from Braulio Carrillo National Park to the Indio Maíz Biological Reserve, which protects nearly half a million hectares of rainforest in the southeast corner of Nicaragua.

Maquenque will link the Frontier Biological Corridor Wildlife Refuge with the San Juan–La Selva Biological Corridor, covering 340,000 hectares and 29 protected areas, including Tortuguero National Park and Barra del Colorado Wildlife Refuge.

The Ministro de Ambiente y Energía (Ministry of Environment and Energy, tel. 506/2471-2191, refugio.fronterizo [at] sinac [dot] co [dot] cr, 8 a.m.–4 p.m. Wed. and Fri. only), in Los Chiles, has responsibility for administering the park; call or visit for information.

Getting to Frontier Biological Corridor Wildlife Refuge

A water-taxi departs the dock in Puerto Viejo at 1:30 p.m. for Trinidad (on the east bank of the Río Sarapiquí at its junction with the Río San Juan, $5). Water-taxis are also for hire, from slender motorized canoes to canopied tour boats for 8–20 passengers. Trips cost about $10 per hour for up to five people.

A full-day trip to the Río San Juan and back costs from $100 per boat. Expect to pay $500 for a charter boat (for up to 20 people) all the way down the Río San Juan to Barra del Colorado and Tortuguero National Parks.