In the extreme south are the Bolivian Chaco and the llanos (open grassy plains) of Manzo, while above these in eastern Chuquisaca and southern Santa Cruz are extensive swamps and low-lying plains, subject to periodical inundations and of little value for agricultural and pastoral purposes.
Nothing definite is known of its tributaries in the Chaco, but in the sierra region it possesses a number of small tributaries, the largest of which are the Cachimayo, Mataca and Pilaya or Camblaya, the latter formed by the Cotagaita and San Juan.
The Buenos Aires treaty of the 3rd of February 1876 fixed the frontier between Argentina and Paraguay, and assigned to Paraguay the portion of the Gran Chaco between Rio Verde and Bahia Negra; the appropriation of the portion between Rio Verde and the Pilcomayo was submitted to the arbitration of the president of the United States, who in 1878 assigned it to Paraguay.
From this centre Spanish adventurers pushed east to La Guayra, beyond the Parana, and west into the Gran Chaco; and before long vast numbers of the less warlike natives were reduced to serfdom.
Grubb, Among the Indians of the Paraguayan Chaco (London, 1904); E.