Manual de Plantas de Costa Rica | The physical environment | 61 
de Tigre (708 m), near Térraba; Cerro Bola (ca. 145 m), near Ciudad Neily; and along 
the Rio Lori, on the Caribbean slope of the Cordillera de Talamanca (Tournon & AI- 
varado, 1997). Cocos Island is also of volcanic origin. 
The Caribbean slope 
Three major river systems (and numerous secondary ones) drain Costa Rica’s Carib- 
bean slope. These are, from north to south: the Rio San Juan, along the border with 
Nicaragua; the Rio Reventaz6n (known upstream as the Rio Grande de Orosi), in the 
central part of the country; and the Rio Sixaola (known upstream as the Rio Telire), 
which debouches into the sea at the Panamanian border. The Rio San Juan is the largest 
of these, and has the most extensive drainage basin, including the eastern slopes of the 
Cordilleras de Guanacaste and Tilaran and most of the Cordillera Central. The broad 
plains of the lower Rio San Juan basin in Costa Rica are divided into three inconsis- 
tently delimited regions: the Llanura de Los Guatusos, the Llanura de San Carlos, and 
the Llanura de Sarapiqui (many systems, including the Manual, omit the last of these, 
combining it with the Llanura de San Carlos, de Tortuguero, or de Santa Clara). 
The Llanura de Los Guatusos may be generously defined as the region crossed by 
those tributaries draining the Cordillera de Guanacaste and flowing northward into 
Lake Nicaragua or the Rio San Juan (which empties the lake). The area so defined ex- 
tends from the Rio Sapoa, in extreme northwestern Costa Rica, eastward to the Rio 
Pocosol. The Rio Sapoa actually originates on the western slopes of Volcan Orosi, the 
northernmost peak in the Guanacaste chain. Here, the Continental Divide descends, ul- 
timately to less than 180 m elevation near La Cruz, where it approaches to within less 
than 3.5 km of the Pacific Ocean. In the eastern portion of the Llanura de Los Guatu- 
sos, the Rio Frio overflows during the rainy season to form Lago Cafio Negro, an im- 
portant wetland and (seasonally) one of the largest natural bodies of fresh water in 
Costa Rica. 
The rivers crossing the Llanura de San Carlos and the Llanura de Sarapiqui flow 
directly into the Rio San Juan. The Llanura de San Carlos is dominated by the Rio San 
Carlos, collecting the waters of its many tributaries, e.g., the Rio Arenal (emptying 
Lake Arenal), Rio Pefias Blancas, Rio San Lorenzo, and Rio Aguas Zarcas. This sys- 
tem drains the entire eastern slope of the Cordillera de Tilaran, as well as the southern 
extreme of the Cordillera de Guanacaste and the western end of the Cordillera Central. 
The Llanura de Sarapiqui may be defined as that region traversed by the river of the 
same name, with its important tributaries, viz., the Rio Toro, Rio Puerto Viejo, and Rio 
Sucio. Most of the waters falling on the northern slopes of the Cordillera Central flow 
to the sea via this route. However, some from the western end spill into Rio San Carlos, 
as mentioned above, and some from the eastern half of the range are shunted into the Rio 
Chirrip6 (see below), which empties into the Rio Colorado (a lower effluent of the Rio 
San Juan). The Rio Chirrip6 is a hybrid, a joint effluent of the Rio Sucio and the Rio Toro 
