Paraguay falls into two main natural regions: the Paraneña region (a mixture of plateaus, rolling hills, and valleys) and the Chaco region (an immense piedmont plain).
About 95 percent of Paraguay's population resides in the Paraneña region, which has all the significant orographic features and a more predictable climate. The Paraneña region can be generally described as consisting of an area of highlands in the east that slopes toward the Río Paraguay and becomes an area of lowlands, subject to floods, along the river.
The Chaco consists predominantly of lowlands, also inclined toward the Río Paraguay, that are alternately flooded and parched.
The Eastern RegionThe Eastern region extends from the Río Paraguay eastward to the Río Paraná, which forms the border with Brazil and Argentina. The eastern hills and mountains, an extension of a plateau in southern Brazil, dominate the region. They reach to about 700 meters (2,297 ft) above sea level at their highest point. The Eastern region also has spacious plains, broad valleys, and lowlands. About 80% of the region lies below 300 meters (984 ft) in elevation; the lowest elevation, 55 meters (180 ft), occurs in the extreme south at the confluence of the Río Paraguay and Río Paraná.
The Eastern region is drained primarily by rivers that flow westward to the Río Paraguay, although some rivers flow eastward to the Río Paraná. Low-lying meadows, subject to floods, separate the eastern mountains from the Río Paraguay.
The Eastern region as a whole naturally divides into five physiographic subregions:
- the Paraná Plateau
- the Northern Upland
- the Central Hill Belt
- the Central Lowland
- the Ñeembucú Plain
The Northern Upland, the Central Hill Belt, and the Central Lowland constitute the lower terrain lying between the escarpment and the Río Paraguay. The first of these eroded extensions stretching westward of the Paraná Plateau—the Northern Upland—occupies the portion northward from the Aquidabán River (Río Aquidabán) to the Apa River on the Brazilian border. For the most part it consists of a rolling plateau about 180 meters (591 ft) above sea level and 76 to 90 meters (249 to 295 ft) above the plain farther to the south. The Central Hill Belt encompasses the area in the vicinity of Asunción. Although nearly flat surfaces occur in this subregion, the rolling terrain is extremely uneven. Small, isolated peaks are numerous, and it is here that the only lakes of any size are found. Between these two upland subregions lies the Central Lowland, an area of low elevation and relief, sloping gently upward from the Río Paraguay toward the Paraná Plateau. The valleys of the Central Lowland's westward-flowing rivers are broad and shallow, and periodic flooding of their courses creates seasonal swamps. This subregion's most conspicuous features, its flat-topped hills, project 6–9 meters (19.7–29.5 ft) from the grassy plain. Thickly forested, these hills cover areas ranging from a hectare to several square kilometers (acres to square miles). Apparently the weathered remnants of rock related to geological formations farther to the east, these hills are called islas de monte (mountain islands), and their margins are known as costas (coasts).
The remaining subregion—the Ñeembucú Plain— lies in the southwest corner of the Paraneña region. This alluvial flatland has a slight westerly-southwesterly slope obscured by gentle undulations. The Tebicuary River (Río Tebicuary)—a major tributary of the Río Paraguay — bisects the swampy lowland, which is broken in its central portion by rounded swells of land up to three meters in height.
The main orographic features of the Paraneña region include the Cordillera de Amambay, the Cordillera de Mbaracayú, and the Cordillera de Caaguazú. The Cordillera de Amambay extends from the northeast corner of the region south and slightly east along the Brazilian border. The mountains reach on average 400 meters (1,312 ft) above sea level, although the highest point reaches 700 meters (2,297 ft). The main chain, 200 kilometers (124 mi) long, has smaller branches that extend to the west and die out along the banks of the Río Paraguay in the Northern Upland.
The Cordillera de Amambay merges with the Cordillera de Mbaracayú, which reaches eastward 120 kilometers (75 mi) to the Río Paraná. The average height of this mountain chain is 200 meters (656 ft); the highest point of the chain, 500 meters (1,640 ft), lies within Brazilian territory. The Río Paraná forms the Salto del Guairá waterfall where it cuts through the mountains of the Cordillera de Mbaracayú to enter Argentina.
The Cordillera de Caaguazú falls where the other two main mountain ranges meet and extends south, with an average height of 400 meters (1,312 ft). Its highest point, Cerro de San Joaquín, reaches 500 meters (1,640 ft) above sea level. This chain is not a continuous massif but is interrupted by hills and undulations covered with forests and meadows. The Cordillera de Caaguazú reaches westward from the Paraná Plateau into the Central Hill Belt.
A lesser mountain chain, the Serranía de Mbaracayú, also rises at the point where the Cordillera de Amambay and Cordillera de Mbaracayú meet. The Serranía de Mbaracayú extends east and then south to parallel the Río Paraná; the mountain chain has an average height of 500 meters (1,640 ft).
The Western Region (Chaco)
The Paraguayan Chaco is subdivided into two parts. The Alto Chaco (Upper Chaco), also called Chaco Seco (Dry Chaco) is the western three-quarters of the region, bordering on Bolivia, while the Bajo Chaco (Lower Chaco) or Chaco Húmedo (Humid Chaco) borders on the Paraguay River. The low hills in the northwestern part of the Alto Chaco are the highest parts in the Gran Chaco. One prominent wetland of the Bajo Chaco is the Estero Patiño, which at 1,500 km2 (579 sq mi) forms the largest swamp in the country.
The Paraguay Chaco's western two-thirds belong to the semi-arid tropics with annual precipitations between 550 and 1,000 mm (21.7 and 39.4 in), vegetation being dry low scrub in the west to higher growth xerophytic (semi-arid impenetrable thorn) forest towards the east. The eastern third belongs to the semi-humid tropics, with rainfall between 1,000 and 1,300 mm (39.4 and 51.2 in), taller vegetation, and tropical semi-humid forest. A belt about 50 km (31 mi) in length along the Paraguay River again has a different evergreen vegetation of wetlands and palm tree forests (Bajo Chaco).
Annual evaporation is around 1,500 mm (59.1 in). The very pronounced dry season lasts from May to October, and a wet season occurs from November to April, when the vegetation turns green and abundant.
The soils of the Chaco are very deep sedimentary soils rich in nutrients, including luvisols, cambisols, and regosols, and are in general very fertile and apt for agriculture and pasture (always presuming responsible and sustainable techniques), more so than most of the world's semi-arid tropics. Limiting factors include a lack of ground freshwater in most of the Paraguay's Chaco, except in the north and the west. The lowlands facing the Paraguay River have insufficient drainage and seasonal flooding (which again increases soil fertility) as a constraint
The majority of Paraguay is flat, but in the far eastern part of the country, two mountain ranges connect, forming an arc. Although they reach an average elevation of only 1,300 feet, the Amambay Mountains feature jagged, pointed peaks. The Stevia plant, whose extracts are used in food products in the U.S. as a sugar substitute, is native to the Amambay Mountains. The range runs from north to south along the border with Brazil, then turns to the southwest, where it becomes known as the Mbaracayu Mountains. Paraguay's highest point, Cerro Pero at 2,762 feet, is located in the Mbaracayu Mountains.