The Tárcoles River is formed by the convergence of two rivers, the Virilla and the Grande de San Ramón, and empties into the Pacific ocean. In total, the river's watershed covers over 2000 km. It forms the northern border of the Carara Biological Reserve, and helps irrigate important agricultural areas of the country. In addition to its agricultural significance, the river also helps generate electricity. It is, however, not a good place to swim, as it is highly polluted.

The river is perhaps best known for its abundance of American crocodiles. It's said that the Tárcoles River has one of the highest populations of crocodiles in the entire world - 25 crocodiles per square kilometer. Several tour operators take advantage of this fact by offering river tours that guarantee croc sightings. Much of the time these large reptiles (which can grow to a length of seven meters) can be spotted swimming through the river or sunbathing along the banks.

In addition to the crocodiles, the river also supports more than 50 species of migratory, native and coastal birds, including a variety of heron and egrets, crested caracas, roseate spoonbills, scarlet macaws and more. Iguanas can often be spotted scampering about the sides of the river as well.