Although landlocked, Paraguay is bordered and criss-crossed by navigable rivers. The Río Paraguay divides the country into strikingly different eastern and western regions. Both the eastern region--officially called Eastern Paraguay (Paraguay Oriental) and known as the Paraneña region--and the western region--officially Western Paraguay (Paraguay Occidental) and known as the Chaco-- gently slope toward and are drained into the Río Paraguay, which thus not only separates the two regions but unifies them. With the Paraneña region reaching southward and the Chaco extending to the north, Paraguay straddles the Tropic of Capricorn and experiences both subtropical and tropical climates.
Paraguay is bounded by three substantially larger countries: Bolivia, Argentina, and Brazil. The northwestern boundary with Bolivia, extending through the low hills of the Chaco region, was set in 1938. The boundary between the Chaco and Brazil was defined in 1927; it continues from the confluence of the Río Apa and Río Paraguay northward along the course of the Río Paraguay to the border with Bolivia. The northern border of the Paraneña region, set in 1872, follows the course of the Río Paraná, the ridges of the mountains in the northeast region, and finally the course of the Río Apa until it empties into the Río Paraguay. Paraguay's southern border with Argentina is formed by the Río Pilcomayo, Río Paraguay, and Río Paraná. These boundaries were agreed to in 1876.
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Natural Regions
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Climate
The two main natural regions in Paraguay are 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 the 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 is predominantly lowlands, also inclined toward the Río Paraguay, that are alternately flooded and parched.
The Paraneña 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, whose highest point is about 700 meters above sea level. The Paraneña region also has spacious plains, broad valleys, and lowlands. About 80 percent of the region is below 300 meters in elevation; the lowest elevation, 55 meters, is found in the extreme south at the confluence of the Río Paraguay and Río Paraná.
The Paraneña 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 Paraneña region as a whole naturally divides into five physiographic subregions: the Paraná Plateau, the Northern Upland, the Central Hill Belt, the Central Lowland, and the Ñeembucú Plain. In the east, the heavily wooded Paraná Plateau occupies one-third of the region and extends its full length from north to south and up to 145 kilometers westward from the Brazilian and Argentine borders. The Paraná Plateau's western edge is defined by an escarpment that descends from an elevation of about 460 meters in the north to about 180 meters at the subregion's southern extremity. The plateau slopes moderately to east and south, its remarkably uniform surface interrupted only by the narrow valleys carved by the westward-flowing tributaries of the Río Paraná.
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 Río Aquidabán to the Río Apa on the Brazilian border. For the most part it consists of a rolling plateau about 180 meters above sea level and 76 to 90 meters above the plain farther to the south. The Central Hill Belt encompasses the area in the vicinity of Asunción. Although nearly flat surfaces are not lacking 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 is 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 are its flat-topped hills, which project six to nine meters from the grassy plain. Thickly forested, these hills cover areas ranging from a hectare to several square kilometers. 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--is in the southwest corner of the Paraneña region. This alluvial flatland has a slight westerly-southwesterly slope obscured by gentle undulations. The 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 average height of the mountains is 400 meters above sea level, although the highest point reaches 700 meters. The main chain is 200 kilometers long and 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 to the Río Paraná. The average height of this mountain chain is 200 meters; the highest point of the chain, 500 meters, is 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 Paraguayan territory.
The Cordillera de Caaguazú rises where the other two main mountain ranges meet and extends south, with an average height of 400 meters. Its highest point is Cerro de San Joaquín, which reaches 500 meters 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.
Separated from the Paraneña region by the Río Paraguay, the Chaco region is a vast plain with elevations reaching no higher than 300 meters and averaging 125 meters. Covering more than 60 percent of Paraguay's total land area, the Chaco plain gently slopes eastward to the Río Paraguay. The Gran Chaco, the entire western portion of the region, is subdivided into the Alto Chaco (Upper Chaco), bordering on Bolivia, and the Bajo Chaco (Lower Chaco), bordering on the Río Paraguay. The low hills in the northwestern part of the Alto Chaco are the highest parts in the Gran Chaco. The main feature of the Bajo Chaco is the Estero Patiño, the largest swamp in the country at 1,500 square kilometers.
Rivers have greatly influenced the character of the country. The Río Paraguay and Río Paraná and their tributaries define most of the country's borders, provide all its drainage, and serve as transportation routes. Most of the larger towns of the interior, as well as Asunción, are river ports.
The Río Paraguay has a total course of 2,600 kilometers, 2,300 of which are navigable and 1,200 of which either border on or pass through Paraguay. The head of navigation is located in Brazil, and during most years vessels with twenty-one-meter drafts can reach Concepción without difficulty. Medium-sized ocean vessels can sometimes reach Asunción, but the twisting course and shifting sandbars can make this transit difficult. Although sluggish and shallow, the river sometimes overflows its low banks, forming temporary swamps and flooding villages. River islands, meander scars, and oxbow (U-shaped) lakes attest to frequent changes in course.
The major tributaries entering the Río Paraguay from the Paraneña region--such as the Río Apa, Río Aquidabán, and Río Tebicuary--descend rapidly from their sources in the Paraná Plateau to the lower lands; there they broaden and become sluggish as they meander westward. After heavy rains these rivers sometimes inundate nearby lowlands.
About 4,700 kilometers long, the Río Paraná is the second major river in the country. From Salto del Guairá, where the river enters Paraguay, the Río Paraná flows 800 kilometers to its juncture with the Río Paraguay and then continues southward to the Río de la Plata Estuary at Buenos Aires, Argentina. In general, the Río Paraná is navigable by large ships only up to Encarnación but smaller boats may go somewhat farther. In summer months the river is deep enough to permit vessels with drafts of up to three meters to reach Salto del Guairá, but seasonal and other occasional conditions severely limit the river's navigational value. On the upper course, sudden floods may raise the water level by as much as five meters in twenty-four hours; west of Encarnación, however, the rocks of the riverbed sometimes come within one meter of the surface during winter and effectively sever communication between the upper river and Buenos Aires.
The rivers flowing eastward across the Paraneña region as tributaries of the Río Paraná are shorter, faster-flowing, and narrower than the tributaries of the Río Paraguay. Sixteen of these rivers and numerous smaller streams enter the Río Paraná above Encarnación.
Paraguay's third largest river, the Río Pilcomayo, flows into the Río Paraguay near Asunción after demarcating the entire border between the Chaco region and Argentina. During most of its course, the river is sluggish and marshy, although small craft can navigate its lower reaches. When the Río Pilcomayo overflows its low banks, it feeds the Estero Patiño.
Drainage in the Chaco region is generally poor because of the flatness of the land and the small number of important streams. In many parts of the region, the water table is only a meter beneath the surface of the ground, and there are numerous small ponds and seasonal marshes. As a consequence of the poor drainage, most of the water is too salty for drinking or irrigation.
Because of the seasonal overflow of the numerous westwardflowing streams, the lowland areas of the Paraneña region also experience poor drainage conditions, particularly in the Ñeembucú Plain in the southwest, where an almost impervious clay subsurface prevents the absorption of excess surface water into the aquifer. About 30 percent of the Paraneña region is flooded from time to time, creating extensive areas of seasonal marshlands. Permanent bogs are found only near the largest geographic depressions, however.
Paraguay experiences a subtropical climate in the Paraneña region and a tropical climate in the Chaco. The Paraneña region is humid, with abundant precipitation throughout the year and only moderate seasonal changes in temperature. During the Southern Hemisphere's summer, which corresponds to the northern winter, the dominant influence on the climate is the warm sirocco winds blowing out of the northeast. During the winter, the dominant wind is the cold pampero from the South Atlantic, which blows across Argentina and is deflected northeastward by the Andes in the southern part of that country. Because of the lack of topographic barriers within Paraguay, these opposite prevailing winds bring about abrupt and irregular changes in the usually moderate weather. Winds are generally brisk. Velocities of 160 kilometers per hour have been reported in southern locations, and the town of Encarnación was once leveled by a tornado.
The Paraneña region has only two distinct seasons: summer from October to March and winter from May to August. April and September are transitional months in which temperatures are below the midsummer averages and minimums may dip below freezing. Climatically, autumn and spring do not really exist. During the mild winters, July is the coldest month, with a mean temperature of about 18°C in Asunción and 17°C on the Paraná Plateau. There is no significant north-south variation. The number of days with temperatures falling below freezing ranges from as few as three to as many as sixteen yearly, and with even wider variations deep in the interior. Some winters are very mild, with winds blowing constantly from the north, and little frost. During a cold winter, however, tongues of Antarctic air bring subfreezing temperatures to all areas. No part of the Paraneña region is entirely free from the possibility of frost and consequent damage to crops, and snow flurries have been reported in various locations.
Moist tropical air keeps the weather warm in the Paraneña region from October through March. In Asunción the seasonal average is about 24°C, with January--the warmest month--averaging 29°C. Villarrica has a seasonal mean temperature of 21°C and a January mean of 27°C. During the summer, daytime temperatures reaching 38°C are fairly common. Frequent waves of cool air from the south, however, cause weather that alternates between clear, humid conditions and storms. Skies will be almost cloudless for a week to ten days as temperature and humidity rise continually. As the soggy heat nears intolerable limits, thunderstorms preceding a cold front will blow in from the south, and temperatures will drop as much as 15°C in a few minutes.
Rainfall in the Paraneña region is fairly evenly distributed. Although local meteorological conditions play a contributing role, rain usually falls when tropical air masses are dominant. The least rain falls in August, when averages in various parts of the region range from two to ten centimeters. The two periods of maximum precipitation are March through May and October to November.
For the region as a whole, the difference between the driest and the wettest months ranges from ten to eighteen centimeters. The annual average rainfall is 127 centimeters, although the average on the Paraná Plateau is 25 to 38 centimeters greater. All subregions may experience considerable variations from year to year. Asunción has recorded as much as 208 centimeters and as little as 56 centimeters of annual rainfall; Puerto Bertoni on the Paraná Plateau has recorded as much as 330 centimeters and as little as 79 centimeters.
In contrast to the Paraneña region, the Chaco has a tropical wet-and-dry climate bordering on semi-arid. The Chaco experiences seasons that alternately flood and parch the land, yet seasonal variations in temperature are modest. Chaco temperatures are usually high, the averages dropping only slightly in winter. Even at night the air is stifling despite the usually present breezes. Rainfall is light, varying from 50 to 100 centimeters per year, except in the higher land to the northwest where it is somewhat greater. Rainfall is concentrated in the summer months, and extensive areas that are deserts in winter become summer swamps. Rainwater evaporates very rapidly.