How do you measure a river? It’s not as simple as using a ruler

Measuring the world’s longest river is actually not as simple as it sounds. That’s because the process is far more complicated than finding the source and the mouth then measuring the distance in between. Rivers often join together in river systems making it very difficult to pinpoint where an individual river begins and where it ends.

Today, most hydrologists seem to agree that the most accepted method is to measure the longest possible along-thalweg continuous distance from the headwaters of the 1st order stream to the mouth of a river. The thalweg is a line connecting the lowest points of successive cross-sections along the course of a valley or river. A 1st order stream is a stream without any tributaries entering. When two 1st order streams meet, they form a 2nd order stream and when two or more 2nd order streams meet they form a 3rd order stream and so on. In other words, you have to measure the length of the longest continuous river channel in a given river system.

This is grossly simplifying things because in practice things can get very tricky. For instance, for most rivers, the mouth is easily determined and measured but for very large rivers like the Amazon which flows into the ocean, where the mouth is placed can make all the difference. Another example of murky measurement is the Mississippi whose headwaters are considered by the USGS to be Lake Itasca in Minnesota yet if its longest tributary is taken into account — the Jefferson and Missouri rivers — then it becomes three times as long.

I know what you’re thinking — get to the damn question already.

Well, there are two ways to think about ‘what’s the largest river’. One is length, where the Nile takes the crown, the other is volume, where the Amazon clearly stands out among all other river systems in the world.

The Nile — the longest river in the world


According to the U.S. National Park Service, the river Nile is the longest in the world spanning 4,135 miles (6,650 kilometers). Though it mostly runs through Egypt, from its source in Burundi to its delta on the Mediterranean Sea, the Nile also passes through nine other African countries: Sudan, Eritrea, Ethiopia, Uganda, Kenya, Tanzania, Rwanda, Burundi and the Democratic Republic of Congo.

The biggest lake in Africa, Lake Victoria, was largely regarded as the source of the river Nile. A waterfall known as Ripon Falls on the northern edge of the lake pours water through a narrow opening which some claim this to be the very beginning of the Nile. But if that’s the case, what’s the source of Ripon Falls? Lake Victoria is surrounded by mountains riddled with streams which tumble down into the lake. The largest tributary of Lake Victoria is the Kagera river which has its headwater in Burundi. It is from here that the Nile is measured as the world’s longest river.

Some 300 million people depend on this river for their water supply and to irrigate food crops.

There’s even a dam that harnesses the Nile’s energy at Aswan High Dam. After it was completed in 1970, for some years it used to proved half of Egypt’s electricity demand though this figure has steadily shrunk as the nation required more and more demand. It now supplies around 20 percent of the country’s electricity. The dam also controls summer flooding.

Officially, the shortest river is the D River, Oregon USA, which is just 37 meters long.


The Amazon — the largest river in the world by water volume


Credit: Maps of the World.

It’s not even close. The Amazon is considered the 2nd longest river in the world spanning 3,980 miles (6,400 kilometers). However, it holds the title of the world’s largest river by volume. On average 120,000 cubic metres (about 20 swimming pools’ worth) of water flows out of its mouth every second. It contains a staggering 20 percent of all of the whole world’s fresh water and parts of the river can exceed 120 miles (190 kilometers) in width when the Amazon gets swollen during the wet season. Even in dry conditions, the Amazon is so wide throughout its length that to this day, no bridge spans it.

From its source in Peru the Amazon, or Rio Amazonas in Portuguese and Spanish, flows mostly through Brazil and empties into the Atlantic Ocean. The Amazon also forms the world’s largest river drainage basin that includes Brazil, Bolivia, Peru, Ecuador, and Colombia.

The source itself of the Amazon has been hard to pin down over the centuries. Scientists and explorers have attempted to establish the river’s source ever since the 1600s. Over the years, five rivers in southwestern Peru were given the honor and for nearly a century the headwaters of the Apurímac River on Nevado Mismi had been considered as the Amazon’s most distant source. But then in 2014 study found it to be the Cordillera Rumi Cruz at the headwaters of the Mantaro River in Peru.

However, some geographers have disputed Contos’s claims. According to them, Mantaro stays dry for about five months in a year when the Tablachaca dam, built in 1974, diverts its water through a 12-mile (20km) tunnel. And to make the dispute even more interesting, if Mantaro River really is the source, that would add 47 to 57 miles (75 to 92 kilometers) to the length of the Amazon.

Does being the longest even matter?

In 2007, Brazilian researchers announced they had identified a new source but also a new mouth which would make the Amazon 4,225 miles (6,800 kilometers) long, toppling the Nile as the longest river. The mouth of the Amazon is traditionally thought to be located on the north side of the Marajó Island, which is about the size of Switzerland. The rather hefty area means which side of the island the mouth is on can matter a lot when measuring the Amazon’s length. The Brazilian study, which was not peer-reviewed and immediately proved controversial, put the mouth on the south side of the island to the Pará River then out into the ocean. After more recent studies, experts seem to agree that, although there’s indeed some of the Amazon’s water in the Pará, the latter river is distinct from the Amazon. The Nile is still king for now but as new sources are discovered and mouth areas are redefined, the crown could get swapped between the two rivers — and possibly more than once.

At the end of the day, though, maybe being ‘the longest river’ doesn’t even matter all that much. As the constant juggling of measurements throughout the centuries show, there will always be some researcher or team that will claim they’ve made some more precise readings. Of course, they will, because there’s such a thing as the coastline paradox which basically says measuring something with a complex geometry, such as a coastline, is not possible because the length actually increases the more granular the measurement gets. Huh? I know that sounds shocking but this counterintuitive concept has been proven mathematically and arises from the properties of fractal-like geometries, which includes rivers. Rivers have a lot of curves and the more you zoom in, you’ll register more bends and twists.

So, keeping the coastline paradox in mind, a lot of scientists have long ago stopped carrying about measuring river length. What’s far more interesting and scientifical — not to mention a lot easier and precise — is to look at the drainage area, which is any area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, etc. By this measure, the Amazon is clearly the largest river in the world with a drainage area of 6.3 million square kilometers while the Nile makes it only to the fifth spot, behind Congo, the Mississippi, and the Ob.

What do you think?


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