The eastern part of Paraguay contains luxuriant hills, meadows, and forests. The western three-fifths is a waterless prairie covered with dry grass and sparsely dotted with shadeless trees. The southward-flowing Paraguay River, the nation's most important waterway, divides the two sections; this river, which for a long time was Paraguay's principal contact with the outside world, rises in southwestern Brazil and extends for a total length of 2,549 km (1,584 mi). The Pilcomayo River, which rises in the mountains of southern Bolivia and extends about 1,600 km (1,000 mi), flows southeast, forming the southwestern border between Argentina and Paraguay, and joins the Paraguay near Asunción.
The eastern sector of Paraguay comprises the western part of the great Paraná Plateau, varying from 300 to 610 m (1,000 to 2,000 ft) in altitude. The Paraná River—called Upper (Alto) Paraná in Paraguay—flows southward from south-central Brazil through the center of the plateau, dropping in the Guairá Falls at the easternmost point in the Paraguay-Brazil frontier. Between the Guairá Falls and the confluence with the Paraguay River at the southwestern tip of the country, the Paraná passes through a deep canyon that forms the eastern and southern frontier with Argentina.
Just west of the plateau is an area of gently rounded hills descending to the low plains that stretch westward to the Paraguay River. These hills occur in two series, one extending northwestward to the Paraguay River just north of Concepción, and the other meeting the river at Asunción. The remaining territory east of the Paraguay River is composed of lowland plain, much of it subject to annual floods.
West of the Paraguay River is the Chaco, part of the larger Gran Chaco, which includes portions of Argentina, Bolivia, and Brazil. The Gran Chaco, a vast alluvial plain composed of unconsolidated sands and clays, is crossed by the Pilcomayo and Bermejo rivers, but over much of the area there are no surface streams. The water table, however, is only a few feet below the surface, and patches of alkali frequently appear during the long dry season. In many places the groundwater is salty.