South of the Sao Lourenco, the important rivers entering the Paraguay from the east are the Taquary, which rises in the Serra Cayap6, on the southern extension of the Matto Grosso table land ; the Mondego, with many branches, draining a great area of extreme southern Matto Grosso, and the Apa, which forms the boundary between Paraguay and Brazilian Matto Grosso.
The Pilcomayo rises among the Bolivian Andes north of Potosi and north-west of Sucre, races down the mountains to their base, crosses the Chaco plains, and pours into the river Paraguay near Asuncion. It does not receive any branch of importance until it reaches about 21° S., where it is joined from the south-west by the river Pelaya, upon which Tupiza, the most southerly city of Bolivia, is situated. Just below the junction is the fall of Guarapetendi, 23ft. high. From this point to the mouth of the Pilcomayo the distance in a straight line is 480m., although by the curves of the river, which is extremely tortuous, it is about double that distance. According to Storm, who quotes Captain Baldrich, the river bifurcates at 21° 51' S., but again becomes a single stream at 23° 43', the right channel being the greater in volume. It is probable that between 23° and 24° S. it throws three great arms to the river Paraguay, the upper por tions of which have yet to be explored, but the lower parts have been examined for 'co to 200M. up from the Paraguay. From i8o to 200M. above its mouth the Pilcomayo filters through a vast swamp about loom. in diameter, through which there is no prin cipal channel. This swamp, or perhaps shallow lagoon, is probably partly drained by the river Confuso, which reaches the Paraguay between the Pilcomayo and Math. A northern branch of the Pil comayo, the Fontana, the junction being at 24° 56' S., is probably also a drainage outlet of the same great swamp.
For the first loom. below the Fall of Guarapetendi the Pilco mayo is from 600 to i,000ft. wide, but it so distributes its waters through its many bifurcations, and loses so much from infiltration and in swamps, and by evaporation from the numerous lagoons it forms on either side of its course, that its channel is greatly contracted before it reaches the Paraguay. From Sucre to the Andean margin of the Chaco, a distance of about 350m. by the river, the fall is at least 8,000ft.—a sufficient indication that its upper course is useless for purposes of navigation. Its lower course
is of little value for commercial purposes.
The Bermejo river flows parallel to the Pil comayo, and enters the Paraguay a few miles above the junction of this with the Parana. Its numerous sources are on the eastern frontage of the inland Andes, between the Bolivian town of Tarija and the Argentine city of Jujuy. In 23° 50' S. the Bermejo re ceives its main affluent, the San Francisco, from the south-west. The latter has its source in about 22° 30' 5., and, under the name of Rio Grande, runs directly southwards, in a deep mountain valley, as far as Jujuy. It then turns eastwards for 5om., and is joined by the Lavayen from the south-west. These two streams form the San Francisco, which, from their junction, runs north eastwards to the Bermejo. From its junction with the latter stream the Bermejo flows south-eastwards to the Paraguay with an average width in its main channel of about 65oft., although narrowing at times to 16o and even ioo. In its course, however, it bifurcates and ramifies into many channels, forming enormous islands, and frequently leaves old beds for new ones.
Since the exploration of the Bermejo by Patino in 1721, it has often been examined from its sources to its mouth, with a view to ascertaining its navigability. Like the Pilcomayo, the Bermejo is of little use owing to swift currents, shoals, quicksands, snags and fallen trees.
The Salado, about 250m. south-west of and ap proximately parallel to the Bermejo, is the only great tributary which the Parana receives from the west below its confluence with the Paraguay. Its extreme head-waters, the Santa Maria and Calchaqui, which unite near the town of San Carlos and form the river Guachipas, drain a much broken Andean region lying between 24° and 26° 3o' S. in the Argentine province of Salta. Having received the Arias, the Guachipas runs north-eastwards about 50m., and then it changes its name to the Juramento, which is retained until the river reaches the Chaco plains at the base of the foot-hills of the Andes. Here it becomes the Salado. It joins the Parana near Sante Fe in 35° 39' S. and 6o° 41' West. Explorers of the Salado, inclusive of Captain Page in 1855, claim that its lower half is navigable, but the many efforts which have been made to utilize it as a commercial route have all resulted in failure.