Final answer:
The Paraná River is the river that traverses Argentina's Chaco region before joining Rio de la Plata.
Step-by-step explanation:
The river that cuts through Argentina's Chaco region on its way to the Rio de la Plata is the Paraná River. The Paraná River is one of the major rivers in South America.
It originates in Brazil and flows through Paraguay and Argentina, eventually forming the estuary of the Rio de la Plata, which is shared by Argentina and Uruguay. The Chaco region, which extends from northern Argentina into western Paraguay, is characterized by its scrublands and subtropical forests with a distinctive wet and dry season, suitable for raising livestock and some farming.
The river plays a vital role in the ecosystem and the economy of the regions it traverses.