This nearly 10,000-hectare refuge is important as a wintering site for migrant waterfowl, as well as a year-round habitat for resident wetland species. From October to April, when the migrant birds are at the refuge, the variety of species and sheer quantity of individual birds creates a spectacle that even non-birdwatchers will marvel at. Among the many birds usually present then are: wood stork, white ibis, glossy ibis, black-necked stilt, anhinga, black-bellied whistling-duck, snail kite, American widgeon, northern shoveler and blue-winged teal. One species that birdwatchers hope to see here is the Nicaraguan grackle, since in Costa Rica it is found only in marshy areas just south of Lake Nicaragua.

During the winter residents' visit, the habitat at Caño Negro undergoes a dramatic seasonal change. The water accumulated in the rainy season forms a shallow lake measuring some 800 hectares. As the dry season progresses from December to March, the lake dries up. By April, all that is left is the main channel of the Río Frio (Cold River).

To get there: Most of the increasing number of visitors to the area come from hotels in the San Carlos region and drive to the town of Los Chiles, near the Nicaraguan border. A number of boats are kept at the town dock in Los Chiles, which can be hired for the approximately 15-kilometer trip upriver to Playuelas on the northern edge of the refuge. However, in March 1998, a bridge was completed across the Río Frio, making it possible to drive to the village of Caño Negro. The turn off to the bridge is several kilometers before Los Chiles.

An alternative approach is to drive to Upala (especially for those coming from Guanacaste) and continue east on a gravel road for 36 kilometers until reaching the village of Caño Negro, where dugout canoes can be hired for exploration of the marsh area. (Given the nature of the principal kind of habitat protected at Caño Negro, there are no hiking trails.)

Buses from San José service both the towns of Upala and Los Chiles. There is a bus that runs from Upala to Caño Negro.

Fishing: The Río Frio is just now becoming discovered by vacationing fishermen, and although it doesn't yet have much infrastructure available, it has good fishing for snook, guapote, alligator gar, drum, and huge tarpon (up to 90 kg., by some sight estimates). The legal fishing season for this area is from September 1 to March 31.

Climate: Daytime temperatures are quite warm and it is usually humid. Although Caño Negro is on the Atlantic side of Costa Rica, it is far enough inland in the northern plains to have a weather pattern with a short but distinct dry season from January to April.