The Argentine Republic: Its Development and Progress

by Pierre Denis

The two rivers, of which the Rio de la Plata forms the common estuary, differ considerably in their features. The Uruguay has irregular floods, especially in autumn (May) and at the end of the winter (August-October). Low water is in summer (January-February). Its basin belongs to the temperate zone, and does not extend northward as far as the area of tropical summer-rain. The Uruguay also differs from the Paraná in its low capacity for transport and alluvial deposit. While the Paraná has built up a vast deltaic plain, the Uruguay ends in an ordinary estuary, with rocky or sandy bed and clear water. The estuary of the Uruguay is 130 miles long and five or six miles wide. The eastern shore is rocky and broken. The Argentine shore is low. It is formed in the south by the deposits of the delta of the Paraná, while further north, from Gualeguacha to Concepción, the hills of Entre Rios are hidden behind a screen of flat islands covered with palms, formed by the stuff brought by the streams of Entre Rios. The river-floods are lost in the great sheet of the estuary. The tide in the estuary or a flood in the Paraná is enough to turn the current.

Maritime navigation goes beyond the estuary and beyond Paysandu, as far as the rapids which prevent further advance at Salto. The twin towns of Concordia (right bank) and Salta (left bank) mark the limit of navigation on the inner course of the river. It begins again above the falls, at Monte Caseros, from which the river-boats go to San Tomé and occasionally to Concepción. Small ships go higher, as far as Salto Grande in Misiones (27° 20′ S. lat.).[121]

The navigable system of the Paraná is four times as large. The first survey of the river was made about the middle of the nineteenth century by the British Navy. At the beginning of the twentieth century the Argentine Government took up the study of the bed and the peculiarities of the Paraná, and the Ministry of Public Works published a map, on the scale 1:100,000, of the course of the river between Posadas and San Pedro, at the beginning of the delta. A precise survey was made, and twenty-six fluvio-metrical scales were established, the zero of which represents mean low-water.[122] Transverse soundings were taken at equal distances of 670 and 1,000 feet, the distance being reduced to 160 and even 80 feet at critical points. Thanks to this work, the Paraná is now, no doubt, the best known of all rivers of that size.

Its output is estimated at 6,000 cubic metres a second at mean low-water, in the latitude of Rosario, and 25,000 to 30,000 cubic metres a second during flood at a height of six metres above low-water.[123] Its features bear the mark of its tropical origin. The tropical character is typical on the Paraguay, which is, by its situation in the central South-American plain, the real continuation of the lower Paraná. The slightness of the fall of the Paraguay, however, and the extent of the marshes over which it spreads in Brazil and Paraguay, have the effect of regulating and retarding the flood, which only attains its maximum at Asunción in May. The flood of the Paraguay extends the period of high water on the lower Paraná until the end of autumn. The upper Paraná has most of its basin in the tropical zone of summer rain. But its behaviour is also influenced by the spring or autumn rains of the southern part of the Brazilian tableland. Its floods are sudden and violent. They reach a height of sixty or seventy feet in the region of the confluence of the Yguassu. They sweep rapidly down stream, and reach the lower Paraná before the flood of the Paraguay, which they hold back.

From Posadas the flood-waves reach Corrientes in five days (235 miles). From Corrientes they reach Paraná in eight days (380 miles), travelling about two miles an hour. That is one-third the speed of the current, as the flood is retarded, and more or less absorbed, by the ramifications of the broader bed in which it moves.

At Bajada Grande the lowest water is in September. The flood appears in December or January, though sometimes in October or November. The maximum is in March or April. The rise is rapid at first, but it gradually moderates, and the level of the water is raised about one metre per month during three months. It then sinks in corresponding order. The ebb is often interrupted in June, and sometimes as late as August, by a sudden leap upward of the curve, representing an ascensional movement of the water three times as rapid as that of the main flood (one metre in ten days). The level reached in this late flood is sometimes higher than that of the normal flood in April or May. The range of the ordinary flood-movements is from ten to sixteen feet. Exceptional floods rise to a height of twenty-three feet above the low-water mark.

The curves established for the years 1908 to 1910 by the Argentine hydrographical service enable us to analyse the mechanism of the flood with a good deal of confidence. The beginning of the flood at Bajada Grande in October corresponds to the first flood of the upper Paraná. During this first phase the curve of the Bajada is parallel (thirteen days later) to that of Posadas. There is the same parallelism in November, December and January. If the summer rains are light on the upper Paraná, the flood is late on the lower Paraná, and the water is still low there in December (0.20 below the low-water mark on December 31, 1910). At the beginning of March, before the maximum of the flood, the curve of Bajada Grande differs from the curve of Posadas. It is the time when the flood of the lower river is caused by the rise of the Paraguay. The secondary floods of June and July again have their origin in the upper Paraná, but, as they are added to the flood of the Paraguay on the lower river, they reach a higher level there than at Posadas; the difference gradually disappears as the flood of the Paraguay subsides. It is the addition of the late floods of the upper Paraná to the flood of the Paraguay that causes on the lower river the abnormal floods that occur there at irregular intervals (in 1825, 1833, 1858, 1878, 1905 and 1917).

Below the Bajada the height of the floods progressively declines. On the estuary they are no longer perceptible; variations of level are due entirely to the tides. In the channels of the delta of the Paraná the tide does not reverse the current as it does in the estuary of the Uruguay, but it causes a slight rise of the water; and this has been observed sometimes, at very low water, as far as Rosario.