The south-east drainage basin, which is smaller and economically less important than that of the Madeira, discharges into the Paraguay and extends from the Sierras de Chiquitos south to the Argentine frontier, and from the Cordillera Oriental east to the Paraguay. It possesses only one large river in Bolivia, the Pilcomayo, which rises on the east slopes of the Cordillera Oriental opposite the south end of Lake Pampa Aullaguas and flows east and south-east through the sierra region to the Bolivian Chaco. It flows through a nearly level country with so sluggish a current that its channels are greatly obstructed. 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 Pílaya or Camblaya, the latter formed by the Cotagaita and San Juan. The Bermejo, which is an Argentine river, receives one large tributary from the Bolivian uplands, the Tarija or Rio Grande, which drains a small district south-east of the Santa Victoria sierra. The Bolivian tributaries of the upper Paraguay are small and unimportant. The Otuquis, the most southern of the group, is formed by the San Rafael and Tucabaca, which drain both slopes of the Cerro Cochii range; but is lost in some great marshes 50 m. from the Paraguay. Another considerable stream of this region, which is lost in the great marshy districts of the Bolivian plain, is the Parapiti, which rises on the eastern slopes of the Sierra de Misiones and flows north-east through a low plain for about 150 m. until lost.
The third drainage basin is that of the great central plateau, or alta-planicie. This is one of the most elevated lacustrine basins in the world, and though it once drained eastward, now has no surface outlet. Lake Titicaca receives the waters of several short streams from the neighbouring heights and discharges through the Desaguadero, a sluggish river flowing south for 184 m. with a gradually diminishing depth to Lake Pampa Aullaguas or Poopo. The Desaguadero is navigable for small craft, and has two or three small tributaries from the west. Two small streams empty into Lake Pampa Aullaguas, which has a small outlet in the Lacahahuira flowing west for 60 m. to the Cienegas de (salt-swamps of) Coipasa. The drainage of this extensive district seems to be wholly absorbed by the dry soil of the desert and by evaporation. In the extreme south the Rio Grande de Lipez is absorbed in the same way.
Few of the Bolivian lakes are at all well known. The great lacustrine basin between the Beni and the Mamoré contains several lakes and lagoons, two of them of large size. These are Lake Rogagua whose waters find their way into the Beni through Rio Negro, and the Roguaguado lagoon and marshes which cover a large area of territory near the Mamoré. The latter has an elevation little, if any, above the level of the Mamoré, which apparently drains this region, and its area has been estimated at about 580 sq. m. Lake Conceptión, in the Chiquitos mountains, belongs to this same hydrographic area. In the south-east there are several large shallow lakes whose character and size change with the season. They fill slight depressions and are caused by defective drainage. Near the Paraguay there are several of these lakes, partly caused by obstructed outlets, such as Bahia Negra, Cáceres, Mandioré, Gaiba and Uberaba, some of them of sufficient depth to be navigable by small craft. Above the latter are the great Xarayes swamps, sometimes described as a lake. This region, like that of the north, is subject to periodical inundations in the summer months (November-March or even May), when extensive areas of level country are flooded and traffic is possible only by the use of boats. The two principal lakes of the plateau region are Titicaca and Pampa Aullaguas or Poopo. The former lies near the north end of the great Bolivian alta-planicie, 12,644 ft. above sea-level, being one of the most elevated lakes of the world. It is indented with numerous bays and coves; its greatest length is 138 m., and its greatest breadth 69 m. According to a survey made by Dr M. Neveau-Lemaire (La Geographie, ix. p. 409, Paris, 1904), its water surface, excluding islands and peninsulas, is 1969 sq. m., and its greatest depth is 892 ft. The level of the lake rises about 5 in. in summer; the loss in winter is even greater. The lake belongs to both Bolivia and Peru, and is navigated by steamers running between Bolivian ports and the Peruvian railway port of Puno. The outlet of the lake is through the Desaguadero river. It has several islands, the largest of which bears the same name and contains highly interesting archaeological monuments of a prehistoric civilization usually attributed to the Incas. Lake Pampa Aullaguas or Poopo is about 180 m. south-east of Titicaca, and is fed principally by its outflow. It lies 505 ft. below the level of Titicaca, which gives an average fall for the Desaguadero of very nearly 2¾ ft. per mile. The Pampa Aullaguas has an estimated area of 386 sq. m., and has one large inhabited island. The lake is shallow and the district about it is sparsely populated. Its outlet is through the Lacahahuira river into the Coipasa swamp, and it is estimated that the outflow is much less than the inflow, showing a considerable loss by evaporation and earth absorption.
Having no sea-coast, Bolivia has no seaport except what may be granted in usufruct by Chile.
Geology.—The eastern ranges of ihe Bolivian Andes are formed of Palaeozoic rocks with granitic and other intrusions; the Western Cordillera consists chiefly of Jurassic and Cretaceous beds, together with the lavas and ashes of the great volcanoes; while the intervening plateau is covered by freshwater and terrestrial deposits through which rise ridges of Palaeozoic rock and of a series of red sandstones and gypsiferous marls of somewhat uncertain age (probably, in part at least, Cretaceous). The Palaeozoic beds have yielded fossils of Cambrian, Ordovician, Devonian and Carboniferous age. In southern Bolivia Cambrian and Ordovician beds form the greater part of the eastern Andes, but farther north the Devonian and Carboniferous are extensively developed, especially in the north-eastern ranges. The hills, known as the Chiquitos, which rise from the plains of eastern Bolivia, are composed of ancient sedimentary rocks of unknown age. The Palaeozoic beds are directly overlaid by a series of red sandstones and gypsiferous marls, similar to the formacion petrolifera of Argentina and Brazil. At the base there is frequently a conglomerate or tuff of porphyritic rocks. Marine fossils found by Gustav Steinmann in the middle of the series are said to indicate an age not earlier than the Jurassic, and Steinmann refers them to the Lower Cretaceous. It is, however, not improbable that the series may represent more than one geological system. No later marine deposits have been found either in the eastern Andes or in the plains of Bolivia, but freshwater beds of Tertiary and later date occupy a wide area. The recent deposits, which cover so large a part of the depression between the Eastern and the Western Cordillera, appear to be partly of torrential origin, like the talus-fans at the foot of mountain ranges in other dry regions; but Lakes Titicaca and Pampa Aullaguas (Poopo) were undoubtedly at one time rather more extensive than they are to-day. The volcanoes of Bolivia lie almost entirely in the Western Cordillera—the great summits of the eastern range, such as Illimani and Sorata, being formed of Palaeozoic rocks with granitic and other intrusions. The gold, silver and tin of Bolivia occur chiefly in the Palaeozoic rocks of the eastern ranges. The copper belongs mostly to the red sandstone series.
Climate.—Bolivia lies wholly within the torrid zone, and variations in temperature are therefore due to elevation, mountain barriers and prevailing winds. The country possesses every gradation of temperature, from that of the tropical lowlands to the Arctic cold of the snow-capped peaks directly above. This vertical arrangement of climatic zones is modified to some extent (less than in Argentina) by varying rainfall conditions, which are governed by the high mountain ranges crossing one corner of the republic, and also by the prevailing winds. The trade winds give to S. Bolivia a wet and dry season similar to that of N. Argentina. Farther north, and east of the Cordillera Oriental, rains fall throughout the year, though the summer months (November-March) are usually described as the rainy season. On the west side of the Cordillera, which extracts the moisture from the prevailing easterly winds, the elevated plateaus have a limited rainfall in the north, which diminishes toward the south until the surface becomes absolutely barren. Brief and furious rain-storms sometimes sweep the northern plateau, but these are not frequent and occur during a short season only. Electrical wind storms are frequent in these high altitudes.
Bolivia has a wide range of temperature between places of the same latitude. The natives designate the Bolivian climatic zones as yungas, valle or medio yungas, cabezera de valle, puna and puna brava. The yungas comprises all the lowlands and the mountain valleys up to an elevation of 5000 ft. The temperature is tropical, winter is unknown and the atmosphere is exceedingly humid. The mean temperature, according to official estimates, is 70° F., but this probably represents the average between the higher elevations and the low country. The valle zone includes the deep valleys from 5000 to 9500 ft., has a warm climate with moderate variations in temperature and no cold weather, is sub-tropical in character and productions, and is sometimes described as a region of perpetual summer. The cabezera de valle, as the name indicates, includes the heads of the deep valleys above the valle zone, with elevations ranging from 9500 to 11,000 ft.; its climate is temperate, is divided into regular seasons, and is favourable to the production of cereals and vegetables. The puna, which lies between 11,000 and 12,500 ft., includes the great central plateau of Bolivia. It has but two seasons, a cold summer or autumn and winter. The air is cold and dry, and the warmer season is too short for the production of anything but potatoes and barley. The mean temperature is officially estimated as 54° F. The puna brava extends from 12,500 ft. up to the snow limit (about 17,500 ft.), and covers a bleak, inhospitable territory, inhabited only by shepherds and miners. Above this is the region of eternal snow, an Arctic zone within the tropics. In general, the sub-tropical (valle) and temperate (cabezera de valle) regions of Bolivia are healthy and agreeable, have a plentiful rainfall, moderate temperature in the shade, and varied and abundant products. There is a high rate of mortality among the natives, due to unsanitary habits and diet, and not to the climate. In the tropical yungas the ground is covered with decaying vegetation, and malaria and fevers are common. There are localities in the open country and on exposed elevations where healthy conditions prevail, but the greater part of this region is considered unhealthy. The prevailing winds are easterly, bringing moisture across Brazil from the Atlantic, but eastern Bolivia is also exposed to hot, oppressive winds from the north, and to violent cold winds (surazos) from the Argentine plains, which have been known to cause a fall of temperature of 36° within a few hours. According to the Sinópsis Estadistica y Geográfica de la República de Bolivia (La Paz, 1903), the average mean temperature and the annual rainfall in eastern Bolivia are as follows: 10° S. lat., 90.8° F. and 31.5 in. rainfall; 15° S. lat., 86° F. and 30.7 in. rainfall; 20° S. lat., 81° F. and 30 in. rainfall; and 25° S. lat., 76.8° F. and 29.3 in. rainfall.
Fauna.—The indigenous fauna of Bolivia corresponds closely to that of the neighbouring districts of Argentina, Brazil and Peru. Numerous species of monkeys inhabit the forests of the tropical region, together with the puma, jaguar, wildcat, coati, tapir or anta, sloth, ant-bear, paca (Coelogenys paca) and capybara. A rare species of bear, the Ursus ornatus (spectacled bear) is found among the wooded Andean foothills. The chinchilla (C. laniger), also found in northern Argentina and Chile, inhabits the colder plateau regions and is prized for its fur. The plateau species of the viscacha (Lagidium cuvieri) and the widely distributed South American otter (Lutra paranensis) are also hunted for their skins. The peccary, which prefers a partially open country, ranges from the Chaco to the densely wooded districts of the north. There are two or three species of deer, the most common being the large marsh deer of the Chaco; but the deer are not numerous. The armadillo, opossum, ferret and skunk are widely distributed. The amphibia are well represented throughout the lower tropical districts. Alligators are found in the tributaries of the Paraguay and their lagoons, lizards and turtles are numerous, and the batrachians are represented by several species. Snakes are also numerous, including rattlesnakes and the great boa-constrictors of the Amazon region.
The most interesting of all the Bolivian animals, however, are the guanaco (Auchenia huanaco) and its congeners, the llama (A. llama), alpaca (A. pacos) and vicuña (A. vicugna), belonging to the Camelidae, with the structure and habits of the African camel, but smaller, having no hump, and inhabiting a mountainous and not a level sandy region. They are able to go without food and drink for long periods, and inhabit the arid and semi-arid plateaus of the Andes and the steppes of Patagonia. The guanaco is supposed to be the original type, is the largest of the four, and has the greatest range from Peru to Tierra del Fuego. The llama and alpaca were domesticated long before the discovery of America, but the guanaco and vicuña are found in a wild state only. The llama is used as a pack animal in Bolivia and Peru, and its coarse wool is used in the making of garments for the natives. The alpaca is highly prized for its fine wool, which is a staple export from Bolivia, but the animal is reared with difficulty and the product cannot be largely increased. The vicuña also is celebrated for its wool, which the natives weave into beautiful and costly ponchos (blanket cloaks) and other wearing apparel. The guanaco is hunted for its skin, which, when dressed, makes an attractive rug or robe. The slaughter of the guanaco and vicuña is rapidly diminishing their number. The rearing of llamas and alpacas is a recognized industry in the Bolivian highlands and is wholly in the hands of the Indians, who alone seem to understand the habits and peculiarities of these interesting animals.