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What Is the Longest River in the World?

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The answer to the question “What’s the longest river in the world?” might sound simple—find the source, the mouth, and measure—but in between those steps are strange definitions, fractals, hydrology, and especially national pride. It’s an issue so complicated that several hydrologists contacted by Mental Floss explained that river length isn't even considered a useful measure anymore. So, what is the longest river in the world?

HOW LONG IS A RIVER?

According to Laurel Larsen, assistant professor at the University of California Berkeley’s geography department, “The length of a particular river is the longest possible along-thalweg continuous distance from the headwaters (1st order stream) to the mouth of a river.”

To unpack those terms, the thalweg is “the line connecting the lowest or deepest points along a stream bed or valley.” First order stream is more complicated. There are two major ways to classify stream order, but the most common is the Strahler method. In this method, streams without any tributaries entering—streams that are just starting out the river—are first order. When two first order streams join together they form a second order stream, and when two second order streams join they form a third order (but if a first order stream intersects with a second order, the main river stays a second order stream).

The source of a river then is considered to be the furthest distance to the source of a stream that has no inputs—although, in practice, this can be extremely difficult to determine. And it's not a perfect system. (For historical reasons, the source of the Mississippi is often treated separately from the Missouri, despite the conventional definition grouping them into one river system.)

As for the mouth? That’s also contentious. For some rivers, the mouth is relatively simple to determine. But for large rivers entering the ocean, like the Amazon, where the mouth is placed can make all the difference.

ARE WE GOING TO ANSWER THE QUESTION?

The longest river has generally been the Nile, with the Amazon coming in second. But in 2007 Brazilian scientists announced that a new analysis put the Amazon on top. They got this by identifying a new source, but more importantly a new mouth. Traditionally, the mouth of the Amazon has been located on the north side of Marajó Island. But this new report wrapped the river around the south side of the island to the Pará River and then out into the ocean.

What side of an island the mouth is on might not seem relevant, but Marajó Island is the size of Switzerland. The new source plus the new mouth gave a distance longer than the Nile.

This was controversial. The Pará River is usually associated with the Tocantins River, not the Amazon. And more recent studies have tended to agree that, although there is some Amazon water in it, the Pará is distinct from the Amazon. Which means that the current best guess for the longest river in the world is still the Nile.

But there’s another, more fundamental issue with measuring a river: What does length even mean?

SO WHAT’S THE PROBLEM?

The early 20th century mathematician Lewis Fry Richardson made the observation that Spain and Portugal disagreed on their border length. Spain said it was 987 km and Portugal maintained it was 1214 km. The disagreement wasn’t down to disputed territories or anything like that; Richardson explained that it was the length of the measuring stick. As the measuring stick gets smaller, it’s able to more accurately capture the curves and nuances of bends and curves in complex borders.

The same trick appears with rivers. Rivers meander and have small curves. And if you zoom in further, more little bends and twists might appear in the thalweg.

It’s called the coastline paradox—the length of something complex is basically impossible to determine because the length keeps increasing the smaller the measurement goes.

For both this reason and the inherent difficulties of determining length, several researchers have told Mental Floss that river length just isn’t something that particularly matters, and what is really important is drainage area, which is the area of land that contributes water to the river. Unlike river length, a few elevation measurements make this a much easier metric to calculate. And according to an article in Nature, using this metric, the Amazon is dramatically the largest river in the world, with a drainage area of 6.3 million square kilometers. If it were a country, the drainage basin would be the seventh largest in the world just behind Australia.

The Nile falls down to fifth, behind the Congo, the Mississippi, and the Ob. So in the measurement that matters to hydrologists today, the Amazon wins it.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Are There Number 1 Pencils?
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Almost every syllabus, teacher, and standardized test points to the ubiquitous No. 2 pencil, but are there other choices out there?

Of course! Pencil makers manufacture No. 1, 2, 2.5, 3, and 4 pencils—and sometimes other intermediate numbers. The higher the number, the harder the core and lighter the markings. (No. 1 pencils produce darker markings, which are sometimes preferred by people working in publishing.)

The current style of production is profiled after pencils developed in 1794 by Nicolas-Jacques Conté. Before Conté, pencil hardness varied from location to location and maker to maker. The earliest pencils were made by filling a wood shaft with raw graphite, leading to the need for a trade-wide recognized method of production.

Conté’s method involved mixing powdered graphite with finely ground clay; that mixture was shaped into a long cylinder and then baked in an oven. The proportion of clay versus graphite added to a mixture determines the hardness of the lead. Although the method may be agreed upon, the way various companies categorize and label pencils isn't.

Today, many U.S.  companies use a numbering system for general-purpose, writing pencils that specifies how hard the lead is. For graphic and artist pencils and for companies outside the U.S., systems get a little complicated, using a combination of numbers and letters known as the HB Graphite Scale.

"H" indicates hardness and "B" indicates blackness. Lowest on the scale is 9H, indicating a pencil with extremely hard lead that produces a light mark. On the opposite end of the scale, 9B represents a pencil with extremely soft lead that produces a dark mark. ("F" also indicates a pencil that sharpens to a fine point.) The middle of the scale shows the letters and numbers that correspond to everyday writing utensils: B = No. 1 pencils, HB = No. 2, F = No. 2½, H = No. 3, and 2H = No. 4 (although exact conversions depend on the brand).

So why are testing centers such sticklers about using only No. 2 pencils? They cooperate better with technology because early machines used the electrical conductivity of the lead to read the pencil marks. Early scanning-and-scoring machines couldn't detect marks made by harder pencils, so No. 3 and No. 4 pencils usually resulted in erroneous results. Softer pencils like No. 1s smudge, so they're just impractical to use. So No. 2 pencils became the industry standard.

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Big Questions
What Are Curlers Yelling About?
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Curling is a sport that prides itself on civility—in fact, one of its key tenets is known as the “Spirit of Curling,” a term that illustrates the respect that the athletes have for both their own teammates and their opponents. But if you’re one of the millions of people who get absorbed by the sport once every four years, you probably noticed one quirk that is decidedly uncivilized: the yelling.

Watch any curling match and you’ll hear skips—or captains—on both sides barking and shouting as the 42-pound stone rumbles down the ice. This isn’t trash talk; it’s strategy. And, of course, curlers have their own jargon, so while their screams won’t make a whole lot of sense to the uninitiated, they could decide whether or not a team will have a spot on the podium once these Olympics are over.

For instance, when you hear a skip shouting “Whoa!” it means he or she needs their teammates to stop sweeping. Shouting “Hard!” means the others need to start sweeping faster. If that’s still not getting the job done, yelling “Hurry hard!” will likely drive the point home: pick up the intensity and sweep with downward pressure. A "Clean!" yell means put a brush on the ice but apply no pressure. This will clear the ice so the stone can glide more easily.

There's no regulation for the shouts, though—curler Erika Brown says she shouts “Right off!” and “Whoa!” to get her teammates to stop sweeping. And when it's time for the team to start sweeping, you might hear "Yes!" or "Sweep!" or "Get on it!" The actual terminology isn't as important as how the phrase is shouted. Curling is a sport predicated on feel, and it’s often the volume and urgency in the skip’s voice (and what shade of red they’re turning) that’s the most important aspect of the shouting.

If you need any more reason to make curling your favorite winter sport, once all that yelling is over and a winner is declared, it's not uncommon for both teams to go out for a round of drinks afterwards (with the winners picking up the tab, obviously). Find out how you can pick up a brush and learn the ins and outs of curling with our beginner's guide.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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