Geographers have devised different ways to divide Colombia into regions. It is most appropriate to divide the country into four geographic regions: the Andean highlands, consisting of the three Andean ranges and intervening valley lowlands; the Caribbean lowlands coastal region; the Pacific lowlands coastal region, separated from the Caribbean lowlands by swamps at the base of the Isthmus of Panama; and eastern Colombia, the great plain that lies to the east of the Andes Mountains.
Near the Ecuadoran frontier, the Andes Mountains divide into three distinct, roughly parallel chains, called cordilleras, that extend northeastward almost to the Caribbean Sea. Altitudes reach more than 5,700 meters, and mountain peaks are permanently covered with snow. The elevated basins and plateaus of these ranges have a moderate climate that provides pleasant living conditions and in many places enables farmers to harvest twice a year. Torrential rivers on the slopes of the mountains produce a large hydroelectric power potential and add their volume to the navigable rivers in the valleys. In the late 1980s, approximately 78 percent of the country's population lived in the Andean highlands.
The Cordillera Occidental in the west, the Cordillera Central in the center, and the Cordillera Oriental in the east have different characteristics. Geologically, the Cordillera Occidental and the Cordillera Central form the western and eastern sides of a massive crystalline arch that extends from the Caribbean lowlands to the southern border of Ecuador. The Cordillera Oriental, however, is composed of folded stratified rocks overlying a crystalline core.
The Cordillera Occidental is relatively low and is the least populated of the three cordilleras. Summits are only about 3,000 meters above sea level and do not have permanent snows. Few passes exist, although one that is about 1,520 meters above sea level provides the major city of Cali with an outlet to the Pacific Ocean. The relatively low elevation of the cordillera permits dense vegetation, which on the western slopes is truly tropical.
The Cordillera Occidental is separated from the Cordillera Central by the deep rift of the Cauca Valley. The Río Cauca rises within 200 kilometers of the border with Ecuador and flows through some of the best farmland in the country. After the two cordilleras converge, the Cauca Valley becomes a deep gorge all the way to the Caribbean lowlands.
The Cordillera Central is the loftiest of the mountain systems. Its crystalline rocks form an 800-kilometer-long towering wall dotted with snow-covered volcanoes. There are no plateaus in this range and no passes under 3,300 meters. The highest peak in this range, the Nevado del Huila, reaches 5,439 meters above sea level. The second highest peak is a volcano, Nevado del Ruiz, which erupted violently on November 13, 1985. Toward its northern end, this cordillera separates into several branches that descend toward the Caribbean coast.
The Caribbean lowlands consist of all of Colombia north of an imaginary line extending northeastward from the Golfo de Urabá to the Venezuelan frontier at the northern extremity of the Cordillera Oriental. The semiarid Guajira Peninsula, in the extreme north, bears little resemblance to the rest of the region. In the southern part rises the Sierra Nevada de Santa Marta, an isolated mountain system with peaks reaching heights over 5,700 meters and slopes generally too steep for cultivation.
The Caribbean lowlands region is in roughly the shape of a triangle, the longest side of which is the coastline. Most of the country's commerce moves through Cartagena, Barranquilla, Santa Marta, and the other ports located along this important coast. Inland from these cities are swamps, hidden streams, and shallow lakes that support banana and cotton plantations, countless small farms, and, in higher places, cattle ranches.
The Caribbean region merges into and is connected with the Andean highlands through the two great river valleys. After the Andean highlands, it is the second most important region in economic activity. Approximately 17 percent of the country's population lived in this region in the late 1980s.
In the 1980s, only 3 percent of all Colombians resided in the Pacific lowlands, a region of jungle and swamp with considerable but little-exploited potential in minerals and other resources. Buenaventura is the only port of any size on the coast. On the east, the Pacific lowlands are bounded by the Cordillera Occidental, from which numerous streams run. Most of the streams flow westward to the Pacific, but the largest, the navigable Río Atrato, flows northward to the Golfo de Urabá, making the river settlements accessible to the major Atlantic ports and commercially related primarily to the Caribbean lowlands hinterland. To the west of the Río Atrato rises the Serranía de Baudo, an isolated chain of low mountains that occupies a large part of the region. Its highest elevation is less than 1,800 meters, and its vegetation resembles that of the surrounding tropical forest.
The Atrato Swamp--in Chocó Department adjoining the border with Panama--is a deep muck sixty-five kilometers in width that for years has challenged engineers seeking to complete the Pan American Highway. This stretch, near Turbo, where the highway is interrupted is known as the Tapón del Chocó (Chocon Plug). A second major transportation project involving Chocó Department has been proposed. A second interoceanic canal would be constructed by dredging the Río Atrato and other streams and digging short access canals. Completion of either of these projects would do much to transform this somnolent region.
The area east of the Andes includes about 699,300 square kilometers, or three-fifths of the country's total area, but Colombians view it almost as an alien land. The entire area, known as the eastern plains, was home to only 2 percent of the country's population in the late 1980s (see fig. 3). The Spanish term for plains (llanos) can be applied only to the open plains in the northern part, particularly the piedmont areas near the Cordillera Oriental, where cattle raising is practiced.
The region is unbroken by highlands except in Meta Department, where the Macarena Sierra, an outlier of the Andes, is of interest to scientists because its vegetation and wildlife are believed to be reminiscent of those that once existed throughout the Andes. Many of the numerous large rivers of eastern Colombia are navigable. The Río Guaviare and the streams to its north flow eastward and drain into the basin of the Río Orinoco, the largest river in Venezuela. Those south of the Río Guaviare flow into the basin of the Amazon. The Río Guaviare divides eastern Colombia into the llanos subregion in the north and the tropical rainforest, or selva, subregion in the south.
Climate: The striking variety in temperature and precipitation results principally from differences in elevation. Temperatures range from very hot at sea level to relatively cold at higher elevations but vary little with the season. At Bogotá, for example, the average annual temperature is 15°C, and the difference between the average of the coldest and the warmest months is less than 1°C. More significant, however, is the daily variation in temperature, from 5°C at night to 17°C during the day.
Colombians customarily describe their country in terms of the climatic zones: the area under 900 meters in elevation is called the hot zone (tierra caliente), elevations between 900 and 1,980 meters are the temperate zone (tierra templada), and elevations from 1,980 meters to about 3,500 meters constitute the cold zone (tierra fría). The upper limit of the cold zone marks the tree line and the approximate limit of human habitation. The treeless regions adjacent to the cold zone and extending to approximately 4,500 meters are high, bleak areas (usually referred to as the páramos), above which begins the area of permanent snow (nevado).
About 86 percent of the country's total area lies in the hot zone. Included in the hot zone and interrupting the temperate area of the Andean highlands are the long and narrow extension of the Magdalena Valley and a small extension in the Cauca Valley. Temperatures, depending on elevation, vary between 24°C and 38°C, and there are alternating dry and wet seasons corresponding to summer and winter, respectively. Breezes on the Caribbean coast, however, reduce both heat and precipitation.
Rainfall in the hot zone is heaviest in the Pacific lowlands and in parts of eastern Colombia, where rain is almost a daily occurrence and rain forests predominate. Precipitation exceeds 760 centimeters annually in most of the Pacific lowlands, making this one of the wettest regions in the world; in eastern Colombia, it decreases from 635 centimeters in portions of the Andean piedmont to 254 centimeters eastward. Extensive areas of the Caribbean interior are permanently flooded, more because of poor drainage than because of the moderately heavy precipitation during the rainy season from May through October.
The temperate zone covers about 8 percent of the country. This zone includes the lower slopes of the Cordillera Oriental and the Cordillera Central and most of the intermontane valleys. The important cities of Medellín (1,487 meters) and Cali (1,030 meters) are located in this zone, where rainfall is moderate and the mean annual temperature varies between 19°C and 24°C, depending on the elevation. In the higher elevations of this zone, farmers benefit from two wet and two dry seasons each year; January through March and July through September are the dry seasons.