It wasn't all that long ago when there was a lot of contention over which river was really the longest. To a point, there still is. Most people have accepted that the two longest rivers in the world are the Nile and the Amazon. Which is really the longest, though, is what makes it interesting.
Of the two, the Nile is the better known. This stands to reason since it is so closely tied to the lives and success of the people living near its shores, and has for about 6,000 years. The importance of the seasons revolved around its yearly flooding, which added nutrients to the soil and made life possible.
Without the Nile, life in the region would be much more difficult, if not impossible. Yet the Nile gave rise to great civilizations such as the Egyptians. Besides watering the crops, the river allowed for transportation of people and goods.
A little over 4,184 miles long (~6,695 km) the Nile is now recognized as the world's longest river. The reason there has been question about this, however, has been because only in the past few decades have teams actually succeeded in tracing it to its source, and because the Amazon is so close to the same length but it is so much more difficult to measure its source and distance to it. The Nile actually winds its way through 9 countries before emptying into the sea.
In contrast, the Amazon is a much larger river, draining almost half of South America. Tracing the source of the Amazon has been far more difficult as previously stated, and it appears that the river is 4,170 miles or more in length. This depends on the source of the information.
The Amazon, though, carries about 20% of all the water that empties into the sea. This outflow dwarfs the amount of water carried by the Nile. During the wet season, in fact, so much water flows from the Amazon that a ship nearly 100 miles in the ocean from the mouth of the Amazon is floating in fresh water.
Like the Nile, the Amazon River is also extremely important to the plant life, animals, and people living nearby. A difference, though, is that it drains many hundred thousand square miles of land. This again dwarfs the Nile.
There are many other long rivers, but none come close to these two rivers, either in length, or in volume of water delivered to the ocean. For instance, the Yangtze in China is nearly 4,000 miles long and the Huang He, also in China, is around 500 miles shorter. Put together, they don't equal the volume of the Amazon.
Compare this with the Mississippi at right around 2,350 miles in length, and the understanding of the length begins to sink in. The Mississippi also drains a huge amount of land, yet the outflow is many times smaller than the Amazon.
To get the maximum benefit of just how long these rivers are, a person only needs to compare them to some of the shortest rivers in the world. For instance, Link River, flowing between Klamath Lake and Lake Ewana in Southern Oregon is somewhat shorter than a half mile in length.
Again, it depends on which source we want to refer to, when we want to determine the longest river in the world. However, there is very little doubt that in sheer volume, there is no river in the world that comes even close to the amount of water it carries than the Amazon. Other long rivers end up taking the place of 'moderate', both in length and in volume.