Located in the eastern portion of Panama which borders Colombia, Darién Province basically forms a long and flat lowland valley mostly fringed by mountains. Darién National Park is along 90% of the international border and extends from the Pacific coast to the continental divide, which is just 16 km from the Caribbean Sea.<br> The extensive central valley receives the drainages of the country's largest watershed (c. 13,371 km2), primarily from the south-east-flowing Chucunaque River and north-west-flowing Tuira River as well as the Balsas River, which join as the Tuira River to flow westward to the large Gulf of San Miguel on the Pacific Ocean (Map 31). Brackish to freshwater marshes and swamps extend well inland along the Tuira River's lower portion and the lower Chucunaque River. Twice a day strong Pacific tides (ranging 26 m) affect the Tuira and Chucunaque for many km inland.<br> The spine of Panama along the north-eastern border of the province is a fairly continuous mountain system, the Cordillera de San Blas and higher Serranía del Darién. The serranía emerges in the north with elevations of 300-600 m and in the south-east rises to 1875 m at Cerro Tacarcuna (8°10'N, 77°15'W), which is the highest mountain between the Andes and western Panama and only c. 32 km inland from Colombia's Caribbean coast at the Gulf of Urabá (which gives way to the vast lowland swamps and lagoons of the Atrato River).<br> The Darién is one of the most diverse and species-rich regions in Central America. It includes almost ten major vegetation types, from the littoral strand and coastal dry forests to mangroves, brackish and freshwater swamps, and various lowland and premontane to lower montane rain- forest life zones, including cloud forests and elfin forests at rather low elevations because of the isthmian effect. Five Holdridge life zones are represented in Darién National Park.<br> Mangrove forests occur along the Pacific and Atlantic coasts; their greatest representation is in the Gulf of San Miguel area, which has c. 46% of the mangrove forest of the entire country. The major Pacific brackish water community is nearly pure stands of Rhizophora brevistyla to 41 m, which ranks them among the world's tallest mangroves.