The main river of the Pantanal wetlands in South America is the Paraguay River.
The Paraguay rises on Brazil’s Parecis Plateau in Mato Grosso and flows generally south through the Pantanal, one of the world’s largest tropical wetlands. Its broad, slow-moving upper basin helps collect and regulate seasonal rainfall, allowing the Pantanal to expand dramatically during the wet season and contract during the dry season.
After passing through Brazil, the river forms or approaches international borders involving Bolivia and Paraguay, then continues through Paraguay and along part of the Paraguay–Argentina border. Near the border region, it joins the Paraná River, becoming part of the Río de la Plata drainage system and ultimately reaching the Atlantic.
The Paraná is a frequent wrong answer because it is the larger neighbouring river and receives the Paraguay. The Uruguay is another major Río de la Plata river, but it lies farther east. The Paraguay is also central to transport in the region: its navigable channel has long connected inland communities with ports farther south.