Dividing the Rivers

Before we begin illustrating the physical characteristics of the Mezcalapa-Grijalva and the Usumacinta from satellite imagery and other spatial data sets, it is important to establish some riverine nomenclature. Both of the rivers we will be studying have an upper, middle and lower section.

The upper Mezcalapa-Grijalva begins at the confluence of the Selegua and the Cuilco and runs through the Central Depression of Chiapas past Tuxtla Gutierrez, ending at the formidable Sumidero Canyon. The Angostura dam, aka Belisario Dominguez (1976), created a huge reservoir that floods dozens of important archeological sites in this part of the river. The upper Mezcalapa-Grijalva flows 178 km and falls from 539 meters elevation to 388 meters with a slope gradient of .000848.

Upper Mezcalapa-Grijalva with the Nenton on the north flowing into the
Selegua in the middle which then joins with the Cuilco coming in from
the south to form the Mezcalapa-Grijalva.

The middle Mezcalapa-Grijalva begins at the Sumidero and runs to the head of navigation at Plan de Ayala upstream from Huimanguillo. The Chicoasen dam (1980), just downstream from Sumidero Canyon, is Mexico's highest and houses the largest hydroelectric plant in the country. Malpaso dam, aka Netzahualcoyotl (1966), created a large reservoir that also floods many important ancient sites. Penitas dam (1987), the smallest on the river, lies just upstream from Plan de Ayala. The middle Mezcalapa-Grijalva flows 175 km and falls from 388 meters elevation to 51 meters with a slope gradient of .001926.

Middle Mezcalapa-Grijalva

The lower Mezcalapa-Grijalva begins at the head of navigation and runs through the Tabasco coastal plain  to the Gulf of Campeche. In Book of Mormon times, the outlet was at Barra Tonala where the Tonala River flows today. In Book of Mormon times, the lower Mezcalapa-Grijalva flowed 229 km and fell from 51 meters elevation to sea level with a slope gradient of .000223.

Lower Mezcalapa-Grijalva as it flowed in Book of Mormon Times

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The upper Usumacinta begins at the confluence of the Salinas and the Pasion and runs through the jungle on the Mexico/Guatemala border, ending at Boca del Cerro. The upper Usumacinta flows 227 km and falls from 109 meters elevation to 17 meters with a slope gradient of .000405.

Upper Usumacinta with the Salinas coming up from the south joining
with the Pasion coming in from the east.

The middle Usumacinta begins at Boca del Cerro and runs through the higher ground of the Tabasco coastal plain above the area of annual flooding, ending at Teclopan. The middle Usumacinta flows 200 km and falls from 17 meters elevation to 4 meters with a slope gradient of .000065.

Middle Usumacinta

The lower Usumacinta begins at Teclopan, flows through the Pantanos de Centla (the delta area of permanent flooding), joins with the Original Grijalva and empties into the Bay of Campeche at Frontera. The lower Usumacinta flows 188 km and falls from 4 meters elevation to sea level with a slope gradient of .000021.

Lower Usumacinta with distributaties the Palizada that flows eastward
into the Laguna de Terminos and the San Pedro y San Pablo that
flows northward in the middle of the delta. The main channel
joins with the Original Grijalva flowing northward from Villahermosa.

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Does this mean that Book of Mormon authors also divided the river Sidon into upper, middle and lower sections? No. These are modern geographical terms that archaeologists have adopted. The Nephites were very familiar with the "Head of the river Sidon" Alma 22:27, Alma 50:11 which will be a topic for another day.
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In summary, the Mezcalapa-Grijalva in Book of Mormon times flowed 582 km and fell from 539 meters elevation to sea level with a steep slope gradient of .000926. The Usumacinta, which has not changed course significantly since Book of Mormon times, flows 615 km and falls from 109 meters to sea level with a mild slope gradient of .000177.