In general, Costa Rica is a land of high volcanic peaks and thick tropical forests, but in the far north, adjacent to the country’s border with Nicaragua, the Caño Negro wetlands spread out in a tangled expanse of marshes, reed islands and sinuous channels. The ecosystem is fed by the Frío River, which meanders its way across the international frontier into Lake Nicaragua, at 3,190 square miles the largest lake in Central America.
During the rainy season (May to October) the river overflows its banks to form the Caño Negro Lagoon, a huge cloud mirror that is located directly beneath the flyway for migrant North and South American birds. Vast flocks of waterfowl, along with storks, spoonbills, ibis, anhingas and cormorants, arrive to overwinter at the beginning of the dry season each December. For the next four months, the water level falls gradually until all that is left is the main channel of the Frío. Then, the migrants depart.
As well as more than 400 recorded species of birds, the Caño Negro National Wildlife Refuge is home to elusive mammals such as jaguars, pumas, ocelots, tapirs and peccaries. Even the dorsal fins of large bull sharks can sometimes be seen slicing through the surface of the lagoon (owing to the fact that Lake Nicaragua is joined to the Caribbean Sea by the 112-mile-long San Juan River).