According to microedu, the structure of the Mexican territory and the disposition of the relief, together with the climate, determine the regime and the course of the surface waters that descend to the Gulf of Mexico or the Pacific; the external slopes, however, are limited, as the mountain ranges run close to the coasts and therefore the rivers have not been able to develop towards the interior nor do they have a very extensive basin; however, since the external slopes are rich in precipitation, especially on the Gulf side, many of these watercourses with regressive erosions have managed to penetrate the internal valleys, capturing their waters. Even these major rivers, however, have a torrential regime and precipitous profile and only in the short coastal strip do they take on the character of lowland rivers and become navigable, although unfortunately the mouth bars hinder access from the sea and therefore diminish its economic importance. Much of the inland territory, however, especially in the northern plateau, remains without an outlet to the sea and therefore constitutes a series of closed basins that welcome generally salty lakes in the lower parts.
On the Atlantic side or rather the Gulf of Mexico you first meet the Río Grande del Norte or Río Bravo, which is the longest river in Mexico measuring about 2800 km., But only the right bank territory belongs to Mexico, from Ciudad Juárez, where the river is still at 1150 meters above sea level, at the mouth of the Gulf downstream of Matamoros. In this stretch it receives the main Mexican tributary, the Río Conchos, which originates in the Sierra Madre Occidental and flows in the state of Chihuahua: both these rivers, crossing an arid and hot country, are scarce in water and with variable regimes and are of no use at all. as waterways, but are used instead for irrigation and hydroelectric plants.
Numerous other rivers cross the Tamaulipas, such as the San Lorenzo and the San Bernardo and the most important Río Soto la Marina which, born in the Sierra Madre Oriental with the name of Río Blanco and swollen by various tributaries, including the Río Victoria which bathes the city of this name, the capital of the state, after 600 km. it gets lost in the Laguna Madre and the Morales Lagoon, inaccessible from the sea.
This is followed by the Río Pánuco, which originates in the Sierra Madre Oriental with the name of Río Cuautitlán, but the Río de Tula now also brings it the waters of the Valley of Mexico, which it receives through the artificial canal of Tequisquiac. The course of the Panuco, which measures 510 km., Takes place in the region of Mexico richest in rainfall and therefore the river always has very abundant waters, but since it crosses the Sierra Oriental chain in very deep gorges and ravines, it is only navigable in the coastal lowland where it is swollen by the Tamesi and ends in the Gulf near Tampico.
In the Veracruzan coast we also meet numerous rivers rich in water, even if short in course, such as the Río Tuxpán, the Tecolotlán, the Río de Actopán, the Río Blanco (partly already used by hydroelectric plants) and the greater Papaloapán, whose system very extensive, it penetrates deeply inland into the mountainous area of Puebla and Oaxaca until it almost reaches the Río Las Balsas basin. Formed by the Río Salado, which descends from the south-eastern side of the Orizaba, and the Río Grande, which comes from the Zempoaltepec massif, the Papaloapán ends up in the Alvarado lagoon. This is followed by the Río Coatzacoalcos, the river of the Tehuantepec isthmus, rich in water and navigable starting from Minatitlán, 32 km away. from the coast, where there is a large oil refinery: at its mouth is Puerto México. The Tabasco rivers are the Río Grijalva and the Río Usumacinta which flow together into the sea and which flow down from the high lands of Chiapas and Guatemala; also the common mouth of these two rivers has been improved with dredging works that have made the port of Frontera accessible. The Río Palizada, the Candelaria and the Mamantel flow into the Terminos lagoon, in the state of Campeche, while in the Yucatán the surface currents are of little importance for the constitution of the peninsula’s soil.
On the whole, the rivers on the Pacific side are less important, which in addition to having small basins, because they are limited to the external slope of the Sierra Madre Occidental, are poor in water because they flow in regions subject to long dry seasons. As a consequence these rivers remain wild streams which diverge with very rapid and very deep courses and with large gravel beds, within which the waters disappear for various months of the year and only at the mouth do they sometimes become navigable; however, almost all of the upper reaches are used for artificial irrigation. These rivers also often flow into coastal lagoons that are inaccessible from the sea and, even when they open onto the open sea, they are hampered by the formations of sandy bars and the movement of the surf. We will therefore limit ourselves to mentioning only a few, such as the Río Suchiate, the Tehuantepec, the Papagayo and in particular the Río Las Balsas, which originates on the southern plateau and precisely in the state of Tlaxcala with the name of Atoyac, crosses the state of Puebla, where its waters are used for important industrial uses, and then enters the state of Guerrero, where it is called Mexcala, and finally flows into the Pacific with two arms, of which the eastern or of Zacatula is navigable. The Río Las Balsas, about 750 km long, in the middle and lower course is rich in waters and has navigable sections for small boats and for the rafts of the Indians (balsas) and its valley of very hot and humid climate provides rich tropical products.
Another important course due to the particular constitution of its basin is the Río Grande de Santiago, made up of two completely distinct parts: the Río Lerma, a purely plateau river which once had to constitute a closed basin, and the Río Santiago, a river of the slopes. westerners, which captured the Lerma, causing its waters to flow directly to the Pacific. The Lerma comes from the Monti de Las Cruces, in the state of Mexico, and flows in the territory of Michoacán and Guanajuato and then in the state of Jalisco through a very fertile region, the so-called bajío, famous for the production of cereals; it then enters Lake Chapala, about 90 km long, with an area of 3600 sq km, and a depth of just 10 m., from which, exiting with the name of Río Santiago, it forms the powerful Junacatlán waterfalls, the so-called Mexican Niagara; it then engulfs a large ravine between the Sierra Occidental chains and flows in wild gorges and is interrupted by numerous cataracts. Its mouth is inaccessible from the sea, but the small port of San Blas opens up to its valley, one of the safest shelters on the Pacific coast.
To an even greater extent, the rivers that flow into the Gulf of California are wild, from the Mezquital which cuts through the Sierra de Nayarit with a deep ravine, to the Culiacán which bathes the capital of Sinaloa, to the Río Fuerte and Yaqui which flow down from the Sierra Tarahumare and in their upper course flow within real cañones, sometimes more than 1000 m deep, to then flow into large coastal lagoons which they are filling with their debris. These latter rivers are completely devoid of surface water during the dry season, however the sub-river currents of the gravel bed are used for the irrigation service.
The internal or closed basins include a large part of the plateau, especially in the states of Chihuahua, Coahuila and Durango, and the NE part is usually distinguished. formed by the basins of Casas Grandes, Santa María and Río del Carmen, which are lost in the lagoons of Guzmal and Santa María not far from Ciudad Juárez, and the central part called the basin or Bolsón de Mapimí to which the Río Aguanaval descends, which ends in the Viesca lagoon, and the Nazas which is lost in the lake basin, largely drained, of Mayrán.
Another closed basin is that of the southern plateau and includes the Valley of Mexico and the minor basins of Michoacán, but it can be said that as a result of the drainage works the Mexican basin has lost its peculiar character of closed basin and has become tributary of the Río Pánuco and the Atlantic.