Final answer:
The river formed from the merging of the Uruguay and Parana rivers is the Rio de la Plata.
Step-by-step explanation:
The river formed from the merging of the Uruguay and Parana rivers is the Rio de la Plata. The Paraná River flows south from Brazil, and the Uruguay River flows west from Brazil into the Rio de la Plata, which is an estuary between Argentina and Uruguay. The Rio de la Plata is a major waterway in South America and serves as a natural border between the two countries.