The western third of the country is the most geographically complex. Starting at the shore of the Pacific Ocean in the west and moving eastward at a latitude of 5 degrees north, a diverse sequence of features is encountered. In the extreme west are the very narrow and discontinuous Pacific coastal lowlands, which are backed by the Serranía de Baudó, the lowest and narrowest of Colombia's mountain ranges. Next is the broad region of the Río Atrato/Río San Juan lowland, which has been proposed as a possible alternate to the Panama Canal as a human-made route between the Atlantic and the Pacific oceans. Colombia occupies most of the Andes mountain range northern extremity sharing a bit with Venezuela which splits into three branches between the Colombia-Ecuador border.
In the 1980s, only 3% 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 Baudó, 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 region, although it could have devastating consequences on the fragile environment.

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