Amazon River

The Amazon is undeniably one of the world's greatest rivers. It is the largest river [blank] volume, and its basin is home to the Amazon Rainforest, the world's richest [blank] most-varied biological reservoir. The Amazon might also be the world's longest river - depending on whom you ask. Most scientists believe the South American river is [blank] least 4,000 miles (6,400 km) long - still shorter than the Nile, which is widely held to be the world's longest river at about 4,132 miles (6,650 km). Others, however, have asserted that Amazon is actually much longer. One would think a definitive measurement would be easy to obtain [blank] modern technology; if its namesake online company can deliver cans of unicorn meat by drone, then determining the Amazon's length shouldn't be too hard. However, it's actually proven very difficult.