The answer to the question "What's the longest rivers in the world?" might sound simple--find the source, locate the mouth, and then measure. But between these steps, there are a myriad of definitions, fractals and most importantly, national pride. It's an issue so complicated that a number of hydrologists who were who were contacted via Mental Floss have explained that river length isn't even considered a reliable measure nowadays. What is the longest river in the world?

What is the length of a river?

According to Laurel Larsen, an associate professor in the University of California Berkeley's geography department, "The length of a particular river is the length that is the longest possible continuous length along thalweg starting from the headwaters (first order stream) to the mouth of a river."

To clarify those terms, the thalweg can be described as "the line that connects the smallest or deepest points of the bed of a valley or stream." First order streams are more difficult. There are two primary ways to classify stream order. The most popular is the Strahler method. This method states that streams with no tributaries that enter it, or streams that are only beginning to break out on the river -- are considered to be first order. If two streams of the first order connect, they create a second order stream, and when two streams from the second order join they form an order of third (but when a first order stream crosses a second order the main river remains as a second order stream).

The point of origin of a river is considered to be the distance to the source of a river that has no inputs--although, in practice it can be very difficult to establish. It's also not the most perfect system. (For reasons of historical significance the source of the Mississippi is often treated separately from the Missouri even though the traditional definition grouping them into one river system.)

As for the mouth? It's also a matter of contention. The mouth of certain rivers is easy to identify. However, for rivers that are large and enter the ocean, such as the Amazon the location of the mouth can make all the difference.

Are we going to be able to answer this question?

Typically, the Nile has been regarded as the longest rivers in the world one, with the Amazon ranking second. In 2007 however, a team of Brazilian researchers discovered that the Amazon was declared the winner after a brand new analysis. They got this by identifying an entirely new source, and more importantly, a new mouth. The mouth of Amazon was located on Marajo Island's northern side. The new report however wraps the river around the south-facing side of the island, extending it to the Para River and then out into the ocean.

The side of the island the mouth is on might not seem all that relevant however, Marajo Island is the size of Switzerland. The new source plus the new mouth gave an additional distance than that of the Nile. You can obtain new information on longest stream by browsing our website.

This was controversial. The Para River is usually associated with the Tocantins River, not with the Amazon. Recent studies tend to confirm that the Para even though it has certain Amazon water, is quite different from the Amazon. This means that the most reliable estimate of the longest river in the world is the Nile.

There's a bigger issue in measuring rivers: What exactly does length really mean?

What's the problem?

In the 20th century's early years, mathematician Lewis Fry Richardson made the observation that Spain and Portugal disagreed on their border length. The border was 987 kilometers in Spain, while Portugal claimed that it was 1214 km. The difference wasn't related to territorial disputes or any other such thing; Richardson explained that it was due to the length of the measuring stick. As the measuring stick becomes smaller, it's able to more accurately capture the curvatures and subtleties of bends and curves in intricate borders.

The same thing happens with rivers. The rivers are meandering and feature small curves. Zoom in further to view more bends and turns in the thalweg.

This is also known as the paradox of the coast. The length of something that is complex may be difficult to measure since it shrinks with every measurement.

Mental Floss has been told by numerous researchers that river length isn't important due to its inherent difficulties in determining the length. The most important thing is the drainage area. This is the land that provides water to the river. This measurement is less complicated than river length since it requires only a few elevation measurements. According to Nature, the Amazon is the world's biggest river that covers 6.3 million km of drainage. The basin would be 7th in the world If it were a country, just second only to Australia.