Isthmus of Tehuantepec

Geography

The isthmus includes the part of Mexico lying between the 94th and 96th meridians west longitude, or the southeastern parts of Veracruz and Oaxaca, including small areas of Chiapas and Tabasco. The states of Tabasco and Chiapas are east of the isthmus, with Veracruz and Oaxaca on the west.

At its narrowest point, the isthmus is 200 km (124 mi) across from gulf to gulf, or 192 km (119 mi) to the head of Laguna Superior on the Pacific coast. The Sierra Madre del Sur mountain range breaks down at this point into a broad, plateau-like ridge, whose elevation, at the highest point reached by the Ferrocarril Transistmico railway at Chivela Pass, is 224 m (735 ft). The northern side of the isthmus is swampy and densely covered with jungle, which has been a greater obstacle to railway construction than the grades in crossing the sierra.

The Selva Zoque in the eastern-central region of the isthmus is an area of great ecological importance, the largest remaining area of tropical rainforest in Mexico and holding the majority of the terrestrial biodiversity in Mexico.

The Sierra Madre de Oaxaca mountains flatten to form Chivela Pass before the Sierra Madre de Chiapas mountains resume to the south, so geographically the isthmus divides North America from Central America. The southern edge of the North American tectonic plate lies across the Motagua Fault in Guatemala, so geologically, the division between North America and Central America (on the Caribbean Plate) is much farther south than the isthmus of Tehuantepec.

History

The Isthmus of Tehuantepec, a region located in the south of Mexico, is the narrowest area between the Atlantic and Pacific oceans in the whole country. Only 220 kilometers separate the two oceans between the gulfs of Mexico and Tehuantepec. During the first half of the 16th century, it seemed as if nature could not withstand the strenuous effort of the Spanish conquistadores to find, within the tropics, a strait connecting the two oceans. This meant that in the conquest explorations during the 16th century, this region was used as an interoceanic passage. The exploration of this region between the 'North Sea' and the 'South Sea' was, since the conquest of Mesoamerica, a fixed idea of the Spanish monarchy and its representatives. Charles V made this project one of the priority missions of the explorers whose objective was to put the kingdom of New Spain in communication with that of Peru. Hernán Cortés, in 1520, used this route to transport equipment and supplies across the isthmus from the Gulf of Mexico to his shipyard located on the Pacific coast, near the town of Santo Domingo de Tehuantepec. The advantage lay in the ability to make a good part of the journey by river. Approximately two-thirds of the Isthmus is crossed by the Coatzacoalcos River, which is navigable for approximately 200 kilometers. Cortés saw this river as a means of fluvial communication across the Isthmus itself. During the last third of the 16th century, Spanish expeditions arrived at the port of Veracruz, in the Gulf of Mexico, and from there they moved by boat to the mouth of the Coatzacoalcos River. The materials necessary to build ships in the shipyards located along the Pacific coast were loaded together with artillery into shallow canoes and brought upstream. They were subsequently transported on the backs of indigenous people from the sources of the Coatzacoalcos River to the Gulf of Tehuantepec on the Pacific coast. Many of these expeditions later moved along the coast to the port of Acapulco from where they sailed along the western coast of North America and then set off westwards, seeking a passage to the Philippines and southern China. The advantages of this itinerary would lead to the proposal, as early as the 16th century, of an interoceanic canal. However, a series of difficulties caused the most feasible option, Panama, to eclipse Tehuantepec as an interoceanic passage.

Biogeography

The Isthmus of Tehuantepec, a valley otherwise surrounded by montane habitats, has also been noted as an important biogeographical barrier among montane taxa, such as Mexican birds. Population diversification has been observed among not only avian fauna, but other organisms as well, including toads and the Central American river turtle. As a result, the Isthmus presents a case of allopatric speciation wherein a geographic divide gives rise to population divergence and a significant decrease in gene flow.

Climate

The predominant climates in the region are tropical savanna (primarily in the south) and tropical monsoon (primarily in the north). There are also small central areas with a temperate climate due to elevation. The annual rainfall on the Atlantic or northern slope is 3,960 mm (156 in) and the maximum temperature about 35 °C (95 °F) in the shade. The Pacific slope has a light rainfall and dryer climate.

The narrowness of the isthmus, and the gap in the Sierra Madre, allow the trade winds from the Gulf of Mexico to blow through to the Pacific. Normally, these winds are not particularly strong, but periodically, a surge of denser air originating from the North American continent will send strong winds through the Chivela Pass and out over the Gulf of Tehuantepec on the Pacific coast. This wind is known as the Tehuano. The region has one of the best wind resources in Mexico, with several wind farms.