What Is the Longest River in the World?


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?


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.


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?


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.

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Why Are Marathons 26.2 Miles Long?


What's the reason behind the cursed distance of a marathon? The mythical explanation is that, around 490 BCE, the courier Pheidippides ran from Marathon to Athens to delivers news that the Greeks had trounced the Persians at the Battle of Marathon. The trouble with that explanation, however, is that Pheidippides would have only covered a distance of approximately 25 miles. So what accounts for the extra 1.2 miles?

When the modern marathon appeared in the late 19th century, the race distance was inconsistent. During the first Olympic games in 1896, runners jogged along Pheidippides’s old route for a distance of 40,000 meters—or 24.85 miles. (That race, by the way, was won by a Greek postal worker.) The next Olympic games saw the distance bumped to a pinch over 25 miles. And while subsequent marathons floated around the 25 mile mark, no standard distance was ever codified.

Then the Olympics came to London. In 1908, the marathon, which stretched between Windsor Castle and White City Stadium in London, lasted 26.2 miles—all for the benefit of England's royal family.

It wasn't supposed to be that way. Like previous races, the original event was supposed to cover a ballpark of 25 miles. The royal family, however, had other plans: They wanted the event to start directly in front of Windsor Castle—as the story goes, the royal children wanted to see the start of the race from the castle nursery. Officials duly agreed and moved the starting line, tacking on an extra mile to the race.

As for the pesky final 0.2? That was the royal family’s fault, too. The finish line was extended an extra 385 yards so the race would end in front of the royal family’s viewing box.

Those extra 1.2 miles proved to be a curse. The race’s leader, an Italian pastry chef named Dorando Pietri, collapsed multiple times while running toward the finish line and had to be helped to his feet. One of the people who came to his aid was a journalist named Arthur Conan Doyle. Afterward, Conan Doyle wrote about Pietri's late-race struggles for the Daily Mail, saying, "Through the doorway crawled a little, exhausted man ... He trotted for a few exhausted yards like a man galvanized into life; then the trot expired into a slow crawl, so slow that the officials could scarcely walk slow enough to keep beside him."

After the London Olympics, the distance of most marathons continued to hover between 24 and 26 miles, but it seems that Conan Doyle's writing may have brought special attention to the distance of 26.2, endowing it with a legendary "breaker-of-men" reputation. Indeed, when the International Amateur Athletic Federation convened to standardize the marathon, they chose the old London distance of 26 miles and 385 yards—or 26.219 miles.

Writing for Reuters, Steven Downes concluded that, "the marathon race may have been as much a Conan Doyle creation as Sherlock Holmes."

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What Are the Santa Ana Winds?

Satellite image of Santa Ana winds in Southern California.
Satellite image of Santa Ana winds in Southern California.
NASA/JPL-Caltech, Wikimedia Commons // Public Domain

Two massive wildfires burning in California have now become the state's deadliest and most destructive. In Northern California, the Camp Fire near Chico decimated the town of Paradise and killed 29 people as of November 12, 2018. In Southern California, the Woolsey Fire started near Simi Valley northwest of Los Angeles, and has torched hundreds of homes in Malibu and other communities.

The National Weather Service says that a combination of high temperatures, low humidity, and gusty Santa Ana winds have created perfect conditions for cataclysmic fires.

What are these Santa Ana winds and why do they help create fire conditions?

Santa Anas are dry, warm (often hot) winds that blow westward through Southern California toward the coast. They're usually seasonal, and typically occur between October and March and peak in December. They originate when high pressure systems form over the high-elevation deserts of the Great Basin between the Sierra Nevadas and the Rocky Mountains. Air from the system flows clockwise, so winds on the southern side of the system push west towards the Pacific Ocean.

The winds pass over the mountains between coastal California and the inland deserts. As they flow downslope, the air gets compressed and rises in temperature at a rate of almost 29 degrees per mile of descent. While air's temperature rises, its relative humidity drops, commonly to less than 20 percent and sometimes to even less than 10 percent. The winds also increase dramatically in speed when they're forced through narrow mountain passes and canyons.

By the time the winds hit the coastal areas, they're very dry, warm, and moving fast. This is what makes them problematic. They dry out vegetation, making it better fuel for a fire—and once a fire starts, the winds fan the flames and help spread them.


So, why are the winds called "Santa Ana winds"?

"While the origin and cause of the Santa Ana winds are not in dispute," writes Robert Fovell, currently a professor of atmospheric and environmental sciences at SUNY Albany, "the origin of the name is."

One fairly popular explanation is that the name comes from a Native American word, santana, which means "devil wind" and was corrupted into Santa Ana. But according to Fovell, the Los Angeles Times, and other sources, no one has found any words similar to santana with that definition in any of the native languages of the area.

Another explanation is that the winds were named for Mexican politician and general Antonio López de Santa Anna, possibly in reference to dust storms kicked up by the cavalry he commanded. Santa Anna never operated in southern California, though, and spelled his name with two n's. The Oxford English Dictionary dismisses this etymology as having no foundation.

In the early 1930s, an article in the United States Naval Institute Proceedings suggested that the name might have originated with early Spanish explorers, who had a "custom of naming places and events for the saint's day on which they happened or were discovered." In this case, they might have noted the winds on St. Anne's day and named them for her. This also seems unlikely to historians, though, because a few Santa Ana winds, experienced for the first time, probably wouldn't have warranted naming—and the winds aren't recorded with any name until much later, anyway. St. Anne's feast day is also July 26, when a Santa Ana wind is unlikely.

The most common and accepted etymology, says Fovell, is that the winds' name simply derives from the Santa Ana canyon in Orange County.

This article was originally published in 2014 and has been updated.