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The Pampas (; from Quechua pampa 'plain'), also known as the Pampas Plain, are fertile low that cover more than and include the Argentina provinces of Buenos Aires, La Pampa, Santa Fe, Entre Ríos, and Córdoba; all of Uruguay; and Brazil's southernmost state, Rio Grande do Sul. The vast plains are a natural region, interrupted only by the low Ventana and Tandil hills, near Bahía Blanca and Tandil (Argentina), with a height of and , respectively. This ecoregion has been changed by humans, especially since the release of animals like cattle, pigs, and especially sheep onto these plains.
The climate is temperate, with precipitation of that is more or less evenly distributed throughout the year, making the soils appropriate for agriculture. The area is also one of the distinct physiography provinces of the larger Paraná–Paraguay river plain division.
It is considered that the limit of the Pampas plain is to the north with the Atlantic Forest and the Gran Chaco, to the west with the Pampas Mountains and the Cuyo Region, and to the south with Patagonia.
Winters are generally mild, but cold waves often occur. Typical temperatures range from during the day, and from at night. With strong northerly winds, days of over can be recorded almost everywhere, and during cold waves, high temperatures can be only . Frost occurs everywhere in the Pampas, but it is much more frequent in the southwest than around the Parana and Uruguay Rivers. Temperatures under can occur everywhere, but values of or lower are confined to the south and west. Snow almost never falls in the northernmost third and is rare and light elsewhere, except for exceptional events in which depths have reached . Springs are very variable; it is warmer than fall in most areas (especially in the west) but significantly colder along the Atlantic. Violent storms are more common as well as wide temperature variations: days of can give way to nights of under or even frost, all within only a few days.
Precipitation ranges from in the northeast to about or less in the southern and western edges. It is highly seasonal in the West, with some places recording averages of monthly in the summer, and only monthly in the winter. The eastern areas have small peaks in the fall and the spring, with relatively rainy summers and winters that are only slightly drier. However, where summer rain falls as short, heavy storms, winter rain falls mostly as cold drizzle, and so the amount of rainy days is fairly constant. Very intense are common in the spring and summer, and it has among the most frequent lightning and highest convective cloud tops in the world. The severe thunderstorms produce intense , both and , and the most consistently active tornado region outside the central and southeastern US.
Mammals that are still fairly present include Brazilian guinea pig, southern mountain cavy, coypu, Pampas fox, Geoffroy's cat, lesser grison, white-eared opossum, Molina's hog-nosed skunk, big lutrine opossum, big hairy armadillo and southern long-nosed armadillo. Bird species of the pampas are ruddy-headed goose, pampas meadowlark, hudsonian godwit, maguari stork, white-faced ibis, white-winged coot, southern screamer, dot-winged crake, curve-billed reedhaunter, burrowing owl and the rhea. Invasive species include the European hare, wild boar and house sparrow.
Why the pristine pampas were treeless regions has been much debated. Perhaps the most commonly cited explanation is seasonal drought. A related hypothesis is that grass roots compete for water and exclude tree seedlings. The effect might be increased by heavy, clayed soils which limit tap root penetration. Other causes that have been proposed are fires set by indigenous peoples for land clearance; the existence of heavy-bodied herbivores; and that the pampas are relicts of drier past climates. These explanations have been criticised as mono-causal. "Overall, we expect that low propagule pressure, abiotic stresses, biotic resistance, and a paucity of specific symbionts might have exerted a synergistic influence in slowing tree invasion rates ".
, pp. 122-7.The World Wildlife Fund divides the Pampas into three distinct ecoregions. The Uruguayan Savanna lies east of the Paraná River, and includes all of Uruguay, most of Entre Ríos and Corrientes provinces in Argentina, and the southern portion of Brazil's state of Rio Grande do Sul. The Humid Pampas include eastern Buenos Aires Province, and southern Entre Ríos Province. The Semi-arid Pampas includes western Buenos Aires Province and adjacent portions of Santa Fe, Córdoba, and La Pampa provinces. The Pampas are bounded by the drier Argentine Espinal grasslands, which form a semicircle around the north, west, and south of the Humid Pampas.
Winters are cold to mild, and summers are hot and humid. Rainfall is fairly uniform throughout the year but is a little heavier during the summer. Annual rainfall is heaviest near the coast and decreases gradually further inland. Rain during the late spring and summer usually arrives in the form of brief heavy showers and thunderstorms. More general rainfall occurs the remainder of the year as and storm systems move through. Although cold spells during the winter often send nighttime temperatures below freezing, snow is quite rare. In most winters, a few light snowfalls occur over inland areas.
Central Argentina boasts a successful agricultural business, with crops grown on the Pampas south and west of Buenos Aires. Much of the area is also used for cattle, and more recently, to cultivate vineyards in the Buenos Aires wine region. The area is also used for farming honey using European Honey bee. These farming regions are particularly susceptible to flooding during thunderstorms. The weather averages out to be year-round in the Pampas.
As European settlers established frontier settlements, the Mapuche raided them for cattle or looted their produce. They drove off the cattle stolen in the incursions ( malones) and took them to Chile through the mountain passes. The main trail for this trade was called Camino de los chilenos. In the 1870s, to counter the cattle raids (and the native peoples on horseback), Argentina constructed a deep trench, called Zanja de Alsina, to prevent cattle from being driven west and establish a boundary to the raiding tribes in the pampas.
The lonco Calfucurá crossed the Andes from Chile to the Pampas around 1830. In 1859, Calfucurá attacked Bahía Blanca in Argentina with 3,000 warriors. In 1872, Calfucurá and his 6,000 warriors went across the Pampas to attack the cities of General Alvear, Veinticinco de Mayo and Nueve de Julio. After this, Argentina organized its forces to launch what it called the Conquest of the Desert.
However, many immigrants eventually moved to more permanent employment in cities as industrialization picked up after the 1930s. As a result, Argentina's immigration history in Buenos Aires Province is typically associated with cities and urban life, unlike in Entre Ríos Province and Santa Fe Province, where European immigration took on a more rural profile.
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