A calculation has been made by Colonel Monasterios, author of an excellent paper on this river, printed in the Statistical Register of Buenos Ayres for 1822, that no less than 4000 square leagues of country are annually covered by the waters during the periodical inundations of the Paranã.
From the almost uninterrupted level of the country which intervenes between the eastern ranges of the Cordillera and the Paraguay, many rivers which descend from them are either partially or entirely lost, after long and tortuous meanderings, in swamps and lakes, the waters of which are absorbed by evaporation during the heats of summer. This is strikingly exemplified in the river Pasages, or Salado, which, from the great extent of its course, and the many other streams it collects in its long course from the province of Salta to Santa Fé, would be a river of the first importance, were not the greater part of its waters lost in the level plains through which it runs. The Dulce, which, passing by Tucuman and Santiago, runs parallel to it, is lost in the great lake called the Porongos, in the pampas of the province of Santa Fé. The Primero and Segundo, which rise in the province of Cordova, disappear in the same plains. The Tercero, the most important river of that province, with difficulty finds its way during part of the year to the bed of the Carcaraña, which falls into the Paranã, near San Espiritu, below Santa Fé. The Quarto and the Quinto, and, still further south, the waters of the rivers from Mendoza and San Luis, are lost in the swamps and lakes which form so striking a feature in the maps of that part of the continent.
The Uruguay, which contributes with the Paranã to form the great estuary of La Plata, takes its name from the numerous falls and rapids which mark its course. The whole extent of its course is little less than 300 leagues. It rises in latitude 27° 30´, in the mountains on the coast of Brazil, opposite the island of St. Catharine's, and for a long distance runs nearly due west, receiving, besides many rivers of less importance, the Uruguay-Mini (or Little Uruguay) from the south, and the Pepiry-Guazú (or Great Pepiry) from the north. As it approaches the Paranã it changes its course, inclining southward through the beautiful territories of the old Jesuit Missions. Opposite to Yapeyú, the last of those establishments, it receives, in latitude 29° 30´, the Ybicuy, a considerable stream from the east. In 30° 12´ the Mirinay pours into it from the west a great part of the drainage of the great lake or swamp of Ybera. Its principal tributaries afterwards are the Gualeguaychú, from the province of Entre Rios, and the Negro, the largest river of the Banda Oriental, soon after the junction of which it falls into the Plata with the Paranã, in about 34° south latitude. Flowing through a country the geological formation of which totally differs from that through which the Paraguay takes its course, its navigation is broken by many reefs and falls, only passable when the waters are at their highest, during the periodical foods, or by portages in the dry season. Of these the Salto Grande and Chico (the great and small falls), a little below the 31° of latitude, are the first and worst impediments met with in ascending the river. The former consists of a rocky reef running like a wall across its bed, which at low water is at times crossed by the gauchos of the country on horseback, though during the floods it is passable in boats, by which, and canoes, the river is navigable without further danger as high up as the Missions.
Beautiful specimens of silicified wood and variegated pebbles are found in the upper parts of the bed of this river, of which I brought many to this country.
The Negro, which runs into it from the Banda Oriental, derives its name (the black river) from the sarsaparilla plant, which, at a particular season, rots upon its banks, and falls into the stream in such immense quantities as to discolour its waters, which are found to be highly medicinal, and much in request in consequence. The little village of Mercedes, near its mouth, has of late years been much resorted to by invalids from Buenos Ayres to drink these waters.
The river Paraguay, as high as the Jaurú, was carefully laid down after the treaty of 1750; and the Spanish officers appointed to determine the boundaries, in virtue of that subsequently signed in 1777, surveyed the Paranã as high as the Tieté, as well as the whole of the Uruguay, and determined the courses of all their most important affluents in the course of the eighteen years during which they were employed in laying down the southern division only of this survey. The results of their labours, which were only stopped by the renewal of war, may justly be ranked amongst the most beautiful and perfect geographical works ever produced. Copies of the whole existed at Buenos Ayres during my time in the hands of Colonel Cabrer, one of the officers originally attached to the commissioners; and the Government of Buenos Ayres were in treaty for the purchase of them for the use of the topographical department of the state, where, it is to be hoped, they will not be buried in unprofitable obscurity.
When the war with Brazil for the Banda Oriental broke out, in 1826, Colonel Cabrer drew a MS. map from these materials for the use of General Alvear, the Buenos Ayrean Commander-in-chief, which he was afterwards kind enough to present to me. By a curious coincidence, about the same time, I obtained possession of one upon a large scale of the southern provinces of Brazil, drawn, by the Emperor's order, from the best data to be collected at Rio de Janeiro, for the Marquis of Barbacena, who commanded the Brazilian army, and lost it at the battle of Ituzaingo. They have, I believe, afforded Mr. Arrowsmith data for materially improving his last maps of that part of South America.