Final answer:
The largest tributary of the Amazon River and the largest Blackwater river in the world is the Rio Negro, which contributes to the Amazon's extensive drainage system covering 40 percent of South America.
Step-by-step explanation:
The largest tributary of the Amazon River and the largest Blackwater river in the world is the Rio Negro. The Rio Negro, alongside the Madeira and the Xingu, is one of the significant tributaries moving massive amounts of water through the Amazon Basin.
The Amazon has over 1,100 tributaries, and the Rio Negro stands out not only for its size but also for the distinctive dark color of its water, which gives it the 'blackwater' designation.
The Amazon River, known for being one of the longest rivers in the world and having the greatest discharge, has a highly extended network of tributaries that surpass many of the world's other rivers in size.
The Rio Negro contributes to the Amazon's vast drainage basin, which covers approximately 40 percent of South America and during the rainy season can extend over 100 miles in width. This extensive system supports an incredibly diverse ecosystem within the largest tropical rain forest in the world.