Chaco,provincia (province), northeastern Argentina, between the northwestern Argentine highlands and the Paraná River and bounded on part of the east by Paraguay. It has an area of 38,468 square miles (99,633 square km), most of which is low hardwood forest with patches of savanna trending to thorn scrub and savanna in the drier northwest. Heavy rainfall, in combination with the flat terrain, produces widespread flooding during the summer. The Bermejo and Guaycurú rivers cross the province but do not provide adequate drainage. The province composes part of the Gran Chaco, which includes adjoining lands of Argentina to the north and south, the northwestern half of Paraguay, and adjoining areas of Bolivia. Chaco National Park (37,000 acres [15,000 hectares]) in northeastern Chaco province includes extensive savannas and palm forests.

The region was explored in 1528 by Sebastian Cabot and settled in the 17th century by Jesuits, who established reducciones (work missions) for the large Indian population. The provincial capital, Resistencia (q.v.), was founded in 1875. Chaco, organized as a national territory in 1884, was renamed Presidente Juan Perón in 1950; it attained provincial status in 1951 and resumed its original name following the revolution of 1955.

The economic development of Chaco province was retarded in the 19th and early 20th centuries by the region’s humid, subtropical climate, the lack of markets and transportation facilities, poor drainage, severe evaporation problems, and pests, notably locusts. But the exploitation of the quebracho tree (a source of tannin) has increased, and the province provides most of Argentina’s cotton; sorghum and sunflowers are also grown. Pop. (1986 est.2001) 791984,000446.