The Gran Chaco of South America The Gran Chaco is a vast region extending through Western Paraguay, Eastern Bolivia and Northeastern Argentina.  Its boundaries include the Mato Grosso plateau on the north, the Andean foothills on the west, the Paraguay and Paraná rivers on the east, and the Salado River Basin on the south.  This markedly seasonal region is typically characterized by a patchwork of savannah grasslands and semi-arid forest, with gallery forests along rivers and creeks.  Rainfall is highest in the east and decreases markedly to the west.  The low degree of topographical decline leads to the formation of extensive wetlands, which generally dry out during the dry season (winter and spring) and become over-flooded with the summer rains.   Summer temperatures reach over 40º C, with considerable daily variation increasing towards the west.  There are occasional frosts during the dry season (June-July).