Final answer:
The Madeira River is the tributary of the Amazon River formed by the confluence of the Mamore and Beni Rivers. It is one of the Amazon's largest tributaries.
Step-by-step explanation:
The tributary of the Amazon River created from the merging of the Mamore and Beni Rivers is known as the Madeira River. The Madeira is one of the largest tributaries of the Amazon and plays a significant role in water transport and biodiversity within the Amazon Basin.
This river, alongside others like the Rio Negro and the Xingu, contributes to the massive flow of water that moves through the area, making the Amazon Basin home to a myriad of species and critical to the environmental health of the region.