The river Desaguadero is the divisional line between the provinces of San Luis and Mendoza:--this river is the drain of a singular chain of lakes known by the name of Guanacache, formed by the confluence of the river Mendoza, which runs into them from the south, and the San Juan river, which, after passing the town or city so called, is discharged into them from the north. The Desaguadero, after receiving these rivers, runs first in an easterly direction, and afterwards south, into a vast lake called the Bevedero, below the town of San Luis:--a portion, also, of the waters of the river Tunuyan are lost in the same great sack-like lake, which thus becomes the reservoir of the greater part of the streams which issue from the Andes between the thirty-first and the thirty-fourth degree of latitude. It is said that in old times the Tunuyan also, like the rivers of Mendoza and San Juan, had no other outlet, but that river, at a later period, opened for itself a new channel, and though a portion of its waters are still carried into the Bevedero, the greater part of them turn off to the south before reaching it in a stream called the Rio Nuevo by Bauza, and the Desaguadero by Cruz,[71] which runs in that direction a considerable distance, till the Diamante and Chadi-leubú rivers join it, and together they form another great inland water without any outlet, called the Urré-lauquen, or Bitter Lake, from its extreme saltness, as described in chapter eight. The account of this lake given to Cruz by the Indians who accompanied him in his journey across that part of the Pampas in 1806, has been verified of late years by General Aldao, who personally examined it in an expedition which he commanded against the savages in 1833, when he rode round it, and ascertained that it had no outlet. The river Tunuyan rises from the base of the mighty mountain of Tupungato, and at first runs south through a wide and rich valley in the Cordillera; passing eastward of the volcano of Maypú, or Peuquenes, it afterwards finds its way through the eastern chain of the Andes by a deep chasm or opening, which it seems to have burst for itself through the mountains seven or eight miles below the Portillo Pass, and nearly opposite to where the Maypú leaves the Cordillera on the western side: thence its course through the plains is north, and afterwards eastward, in the direction of the great lake Bevedero, as already stated.