Sizing up the Candidates

Even though they are separated from each other by only about 170 kilometers as the crow flies, the Mezcalapa-Grijalva and the Usumacinta are very different rivers in many ways:

• Hydroelectric potential. Although the Usumacinta carries more water, the Mezcalapa-Grijalva has much higher hydroelectric potential because it has steeper gradients over much of its course, which causes faster currents. The Mezcalapa-Grijalva has 3 large and 1 small hydroelectric dams and the Usumacinta has none (although some of the higher elevation tributaries of the Usumacinta have been dammed). 
• Tropical climate. Both rivers are within tropical latitudes, but the altitudes and rain shadows affecting the Mezcalapa-Grijalva give it a sub-tropical climate throughout most of its course. The Usumacinta, on the other hand, is definitely a tropical river. In fact, geographers generally define the Usumacinta basin as the northern edge of the Central America tropics. This means that compared with the Usumacinta, the Mezcalapa-Grijalva runs through country that generally has:
• Cooler temperatures
• Lower humidity
• Less dense vegetation.
• Seasonality. Both rivers run higher during the rainy season and lower during the dry season, but the seasonal change in water volume is relatively greater in the Mezcalapa-Grijalva. The Usumacinta does not fluctuate as much because its catchment area receives greater amounts of precipitation spread more evenly throughout the year.
• Navigability. The Usumacinta is navigable for about 330 kilometers upstream. The Mezcalapa-Grijalva is only navigable for less than half that distance.
• Obstructions to movement. The middle Mezcalapa-Grijalva is home to the world-famous Sumidero with high cliffs on either side of a 13-kilometer-long canyon. The Usumacinta does flow through some canyons, but they are modest in comparison. 
• Length. The Usumacinta is somewhat longer than the Mezcalapa-Grijalva.
• Sinuosity. The Mezcalapa runs relatively straight, particularly in its upper and middle sections, while the Usumacinta is famously convoluted with hundreds of large meanders.
• Basin size. The Usumacinta drains an area more than twice as large as the Mezcalapa-Grijalva.
• Ambient elevation. Along most of its course, the Mezcalapa-Grijalva is surrounded by high mountains.  The Usumacinta, on the other hand, runs through generally flatter country.
• Visibility. From the banks of the Mezcalapa-Grijalva, one often has visibility for kilometers. The banks of the Usumacinta, on the other hand, are generally so heavily forested that visibility through the jungle is limited.
• Forest Canopy height. The forest canopy surrounding the Usumacinta is much higher on average than the trees near the Mezcalapa-Grijalva. In fact, in many places, the vegetation surrounding the Mezcalapa-Grijalva is brushy scrub dotted with the occasional tall tree.
• Population. Many more people live along the Mezcalapa-Grijalva than along the Usumacinta. The same was true anciently.
• Population distribution. The smaller numbers of people who live along the Usumacinta are concentrated  in pockets with relative wilderness between the enclaves. The larger numbers of people who live along the Mezcalapa-Grijalva are spread more evenly. This same pattern generally held anciently. 
• Language. The Mezcalapa-Grijalva begins in Maya country, but it runs through Mixe-Zoque country for much of its length. The famed Chiapa de Corzo is the quintessential Zoque site, and barrios of Zoque speakers reside in Tuxtla Gutierrez today.
• Breadth. The Mezcalapa-Grijalva basin is relatively narrow and confined in western Chiapas and western Tabasco. The Usumacinta basin, on the other hand, encompasses the entire Lacantun, Pasion, and San Pedro basins. It includes central Chiapas on the west, most of the Peten, and a tiny part of Belize on the east.
• Flood plain. The Usumacinta flood plain is much larger than that of the Mezcalapa-Grijalva. There are many lakes and wetlands alongside the actual Usumacinta river. In the lower Usumacinta, the extensive Pantanos de Centla wetlands that cover almost one-fourth of the state of Tabasco (the largest wetlands in Mexico and a United Nations Biosphere Reserve) alternate between swamps in the dry season and lakes in the rainy season. 
• Roads. In both ancient and modern times, extensive networks of trails and roads have been important avenues of transportation and communication throughout the Mezcalapa-Grijalva basin. Because of the jungles and swamps, trails and roads have been less of a factor in the Usumacinta basin, and more movement has been on the waterways themselves.