and slopes at a 0.004-degree gradient to the east. This area is also one of the distinct physiographic provinces of the Parana-Paraguay Plain division. The areas more hospitable to development are along the Paraguay, Bermejo, and Pilcomayo Rivers. It is a great source of timber and tannin, which is derived from the native quebracho tree. Special tannin factories have been constructed there. The wood of the palo santo from the Central Chaco is the source of oil of guaiac (a fragrance for soap). Paraguay also cultivates mate in the lower part of the Chaco. Large tracts of the central and northern Chaco have high soil fertility, sandy alluvial soils with elevated levels of phosphorus, and a topography that is favorable for agricultural development. Other aspects are challenging for farming: a semiarid to semihumid climate (600–1300 mm annual rainfall) with a six-month dry season and sufficient fresh groundwater restricted to roughly one-third of the region, two-thirds being without groundwater or with groundwater of high salinity. Soils are generally erosion-prone once the forest has been cleared. In the central and northern Paraguay Chaco, occasional dust storms have caused major topsoil loss. History The Chaco was occupied by nomadic peoples, notably the various groups making up the Guaycuru, who resisted Spanish control of the Chaco, often with success, from the 16th until the early 20th centuries. Prior to national independence of the nations that compose the Chaco, the entire area was a separate colonial region named by the Spaniards as Chiquitos. The Gran Chaco had been a disputed territory since 1810. Officially, it was supposed to be part of Argentina, Bolivia, and Paraguay, although a bigger land portion west of the Paraguay River had belonged to Paraguay since its independence. Argentina claimed territories north of the Bermejo River until Paraguay's defeat in the War of the Triple Alliance in 1870 established its current border with Argentina. Over the next few decades, Bolivia began to push the natives out and settle in the Gran Chaco, while Paraguay ignored it. Bolivia sought the Paraguay River for shipping oil out into the sea (it had become a land-locked country after the loss of its Pacific coast in the War of the Pacific), and Paraguay claimed ownership of the land. This became the backdrop to the Gran Chaco War (1932–1935) between Paraguay and Bolivia over supposed oil in the Chaco Boreal (the aforementioned region north of the