Depending on the morphology described now, from the hydrographic point of view, Argentina has only two sides: the main Atlantic, and a much smaller, peaceful one, largely created by catches in a fairly recent time. However, due to the great irregularity in the superficial distribution of the rains, more than for strictly morphological reasons, not all the waters that collect on the Atlantic side reach the sea, indeed most of the side consists of basins without outflow, in which the they are lost or evaporated before reaching a river course that flows into the ocean. The largest river system of Argentina is that of the Rio de la Plata, which through the Paraná, Uruguay and Paraguay collects the outflows of the Brazilian-Uruguayan plateau, of the Brazilian Amazonian region, the Matto Grosso, the rivers of the northernmost stretch of the Argentine Andes, which cross the Chaco and the pampas, of which only one descends from the Sierre Pampeane, as well as some rare brook flowing on the edge closest to the lower Parana della Pampa itself. The three master rivers Paraná, Uruguay and Paraguay receive their main nourishment from the copious summer tropical rains of the Brazilian Plateau. In their lower reaches both the Paraná and Uruguay are large, majestic slow-flowing currents, rich in water and deep, each with a shallow flow of around 6000 cubic meters. second, and that in floods they rise to 30,000 and 22,000 cubic meters respectively. per second. The soft ones (crecientes) of Paraná, are summer, from January to April, with the maximum in March; Uruguay, which collects the waters in a more temperate zone, has the soft ones from May to October, with the lean ones (bajantes) in January. The largest navigable line is formed by the Paraná-Paraguay system of over 2000 km. in length, which goes up from the Atlantic to the Asunción, and to the Matto Grosso in Brazil, a great line of river penetration in the heart of the continent. Both the Paraná upstream of its confluence with Paraguay and Uruguay, near the transition step from the rocky soil of Brasilia to the softer one of the central Argentine plain, cross some rapids (saltos), obstacles that have greatly delayed the knowledge of the countries crossed by the upper courses of the two rivers, and of their high basins.
According to EHUACOM, the two major rivers of the Chaco, the Pilcomayo and the Bermejo, bring to Paraguay the waters collected in the valleys descending from the Bolivian Plateau, from the Puna di Jujuy, and from the numerous sub-Andean valleys. This, however, only in periods of soft, because then only their courses, which are lost in countless channels between the marshes of the Chaco, reach the collector. These rivers, although of conspicuous flow in the soft ones, are not very suitable for navigation, and have so far disappointed the many hopes placed in them, of making them the access routes to the sub-Andean countries and approaching Bolivia. Up to now, only on the lower reaches of the Bermejo, at the cost of much effort, has it been possible to establish a modest vapor line that climbs it for a relatively short distance.
Another river, the Salado, is directly tributary of the Paraná, flowing down from the Sierras Subandine of the provinces of Catamarca, Salta and Tucumán, immediately to the South. of the two previous ones, but which avoids the Chaco by turning sharply in the meridian direction as it enters the plain. After various swamps (esteros) the Salado continues its very short course in the plain of the Pampa to reach the Paraná near Santa Fe. From the Sierre Pampeane a single river descends to the Paraná, the Carcaraña, flowing between Rosario and Santa Fe, which collects as much water avauza from the two major rivers Tercero and Cuarto, which arise in the Sierra de Córdoba.
The Paraná, downstream from Rosario, divides and subdivides into many branches giving rise to what is usually called the Paraná delta or even las Islas. After the confluence with Uruguay, in Martín García, the delta is succeeded by the very wide estuary of the Plata, a “sea of fresh water” consisting mostly of shallow waters, furrowed by deeper channels used for navigation, and equipped with signaling (balizas) to facilitate it. The tides of the Atlantic, not very strong due to the wide sea, have little influence on the flow of the river, so that the water remains constantly sweet, almost as far as Montevideo. The tides, but above all the winds, give rise to daily variations in the level of the vast mirror of the estuary, above which the maintenance of the access routes of the two very important river ports of Buenos Aires and La Plata is based, in a similar way to what occurs due to the tide alone in the lagoon port of Venice.
The second great river system, by extension if not by richness of water, is that of the Desaguadero, the most expressive of the various names that the common collector of the waters descending from the innumerable valleys of the eastern side of the Andes and the western Sierre Pampeane takes., flowing with a meridian direction in the very long interposed furrow. This collector of over 1000 km. in length it subsequently bears the names of Bermejo, Desaguadero, Bañados del Atuel, Salado, Chadi Leufú and Curacó; due to the poverty of the rains and the dryness of the climate, however, it does not reach the sea, and pours the modest and occasional surplus of the waters lost by way, in the first of the Patagonian rivers, the Colorado. This, despite the conspicuous tributaries coming from the snowfields of the Andes, such as the Río de San Juan, Río Mendoza, Río Tunayán, Río Diamante, Río Atuel. The valley of this collector completely disproportionate in amplitude compared to the waters that now flow in its lower part, was evidently formed during a climatic cycle with abundant rainfall, different from the current one.
The Río Colorado and the Río Negro are usually considered as forming a separate system in the Government of Neuquén, a transitional territory between the Pampa and Patagonia, rivers whose source basins are located in the Andes chain in the stretch of 700 km. approximately, between 35 ° lat. S. and the lake of Nahuel Huapí. The presence of more numerous glaciers and lakes ensures greater permanence to the waters of the two rivers, which however, having come out of the Andes, take on the character of the Patagonians and cross the plateaus in wide alluvial valleys, without receiving any contribution from them. forming they closed basins. Colorado, the more northerly of the two, has in its lower course very low flow rates, which give it a torrential character. It should be remembered the catastrophe that occurred in 1914 in its upper valley, where the Carri Lauquén lake (Green lake) formed upstream of a prehistoric landslide, which blocked the course of the Río Barrancas, emptied itself in one night, due to the sudden collapse of the natural embankment, losing a volume of about 2 billion cubic meters of water, and 95 meters of level. The Río Negro results from the joining of the Río Neuquén to N. and the Río Limay to S., the latter emissary of the great lake Nahuel Huapí. The Río Negro therefore has not only a much greater but more constant flow than the Colorado; its valley through the plateaus was therefore able to be equipped with irrigation works, using water derived from a dam near the junction of the two upper rivers, according to Cipolletti’s projects.
Further to the South., on the Atlantic side of Patagonia there is a series of watercourses that affect the high plateaus of the country, arid or even desert, and convey the melting waters of the glaciers of the Andes, or those of the downpours of the region. Andean, with river characteristics, except for those that receive contributions from the great lakes of southern Patagonia (Argentino lake and Viedma lake).
The Pacific side is divided into two parts: the southern one, resulting from hydrographic catches, with the springs on the western edge of the Patagonian plateau, and that after crossing more or less large lakes such as Buenos Aires, Puyrredón and San Martín, they cut the Patagonian Andes with deep gorges; the northern one which includes the rare watercourses of the western slope of the Puna. The set of areas without outflow embraces more than half of the Republic, and even when they constitute a continuous whole, such as that formed by the Puna, the Sierre Pampeane, and the central Pampa, they are actually divided into innumerable individual basins independent from each other..
Some end up in permanent bodies of water, albeit of variable extension, such as the lagoon of los Porongos, and the Mar Chiquita east of the Sierre Pampeane; others instead in salt marshes, such as the Salinas Grandes in the NO. of the Sierra de Córdoba, and many others of the Sierre Pampeane itself or of the Puna plateaus. The same happens in the Patagonian highlands, except for the lower frequency and extension of the salt pans.
The vast Argentine plain hosts numerous stagnant waters, known by the general name of lagunas, with variable surfaces according to the seasons, now sweet, now brackish, and distinguished with a rich terminology, as well as in actual lagoons, in esteros (stagnations) along the rivers; cienagas (muddy pools), bañados (marshes), cañadas (river beds), etc., often passing through salt flats. Properly called lakes are known only in the Andean region, and as is well known, the largest in Argentina are located at the foot of the Patagonian Andes, such as the often mentioned lake Nahuel Huapí, lake Fontana, Buenos Aires, Puyrredón, San Martín, Viedma, Argentina, in which the last two glaciers of the Patagonian Andes descend, in the same way with which in the Quaternary the retreating alpine glaciers had their fronts in the waters of our Lombard lakes. Some closed lakes in the Patagonian highlands have a special character, such as the two lakes Colhué Huapí and Musters.