Tn area of lakes and swampy land formed from alluvial sedimentation make up this wildlife refuge, which has been included on the Ramsar List of Wetlands of International Importance. The seasonal lake Caño Negro, some 800 ha in area an 3 m deep , is a dammed part of the Frío river. In the dry season, between February and may, the lake almost completely disappears.
In this refuge, there are five main habitats. The vegetation on the edges of the lake and along the channels is mainly herbaceous, such as gamalote grass (Paspalum fasciculatum) and dormilona (Mimosa pigra). What is more, there are several species of small trees like the provision tree (Pachira aquatica) and the swamp wood (Zygia longifolia). Towards the peripheri of the lagoon, there is a lot of juncus (Juncus spp.). the flooded primary forest is located in places that are permanently or almost permanently flooded; these present a great variety of arboreal species, such as the emery (Vochysia guatemalensis) and tamarind (Dialium guianense).
The camibar forests are also in flooded areas, but are less rich in species; camibar (Copaifera aromatica), and crabwood (Carapa guianensis) predominate. The patches of Santa María forests are areas with very homogenous vegetation, pincipally made up of Santa María (Calophyllum brasiliense) and the royal Palm (Attalea butyracea). In the holillo stands, the holillo palm (Raphia taedigera) predominates.
The bird life is rich and varied. Some of the most abundant or noteworthy aquatic birds are: the anhinga (Anhinga anhinga), roseate spoonbill (Ajaia ajaja), white ibis (Eudocimus albus), jacana (Jacana spinosa), woodstork (Mycteria americana), black-bellied tree duck (Dendrocygna autumnalis) and the jabiru (Jabiru mycteria). The colony of olivaceous cormorants (Phalacrocorax brasilianus) that nest here is the biggest in the country.
The refuge holds the only permanent population of Nicaraguan grackle (Quiscalus nicaraguensis), en endemic bird of the basin of Nicaragua Lake. The mammals and reptiles living in this area include the puma (Puma concolor), jaguar (panthera onca), ocelot (Leopardus pardalis), Baird's tapir (Tapirus bairdii) and cayman (Caiman crocodylus), which is very common in the channels.
Caño Negro is situated on the Los Guatusos plain near the border with Nicaragua. It has a centre for ecological wetland research with laboratories, an incubation area, secue area and living area. It can be reached via San José - Ciudad Quesada - Los Chiles - Caño Negro (201 km). Bus services operate between San José and Caño Negro, and Los Chiles and Caño Negro. Boats can be hired in Caño Negro for trips on the river and the lake, and there are cabins and grocery stores. In Los Chiles there are boarding houses, markets and taxis for hire.