Amazon Estuary.

Why the Amazon is the widest river in the world

Lenght isn’t everything so today, we’re taking a look at which river can boast being the widest.

Amazon sunset.

Sunset over the Amazon River.
Image credits Oscar Castillo.

Earlier this week, we looked at which river is the longest in the world — a deceptively tricky question to answer, as my colleague Tibi showed. Width, by contrast, should be much more straightforward, shouldn’t it?

Well, not really; but let’s get into it.

Widest of the wide

Before we bite into the issue, let’s take a moment to talk estuaries.

Rivers that flow out to sea in tidal regions tend to form wide mouths known as estuaries. The Thames, the Loire, La Plata, Saint-Lawrence, Ob-Irtysh, the Tagus rivers, are some of the rivers that form estuaries. Due to tidal movements on the one hand and the river’s outflow on the other, estuaries tend to hold brackish water — a mixture of sweet and salt water brought in by the tides. The same motions that mix these two also flush out silt and other sediment flowing out to sea. In effect, this prevents the river from forming new deposits at its mouth.

Amazon Estuary.

Satellite image of the Amazon River estuary (Brazil), with Marajó Island in the center, and the cities (in red) of Macapá (left) and Belém (right).
Image credits NASA / LandSat / Geocover 1990 via Wikimedia.

Estuaries, then, resemble deltas sans the solid bits. This is quite relevant in the context of the discussions we’re having today, as deltas tend to be wide, fan or funnel-shaped structures.

Since estuaries aren’t technically rivers, I’m not going to include them in the comparison to follow (sorry, estuaries). To give you an idea of how massively wide such structures can get, the St. Lawrence River (on the Canada-US border) estuary — the widest in the world — boasts over 140 km in width. That’s wider than the state of New Hampshire, and simply dwarfs anything a river can reach.

With that out of the way, we’re left with five candidates for the title of “World’s Widest River”: the Missouri, Amazon, Mekong, Huanghe, and Congo rivers.

Function of circumstance

The length of a river may be tricky to measure because there are many ways to go about it. Their width, however, is hard to gauge because it changes over time.

A river’s length is mostly a product of geography — where it springs, where it flows out to sea, and the terrain between these two points. Their width is mostly a function of volume — i.e. how much water it carries. Geography tends to stay constant; volume likes some variety.

However, since volume weighs so heavily, a likely victor appears: the Amazon.

The Amazon has the largest drainage system (the surface it draws water from) in the world, covering a large part of South America — some 350,000 sq kilometers. Even better, that drainage system includes vast expanses of rainforest, which tend to be very humid environments that see large quantities of precipitation.

This bountiful fief makes the Amazon a leviathan. It single-riverly supplies 20% of all the fresh water that drains into oceans on Earth. Its average output — of 209,000 cubic meters (7,381,000 cubic ft) per second — exceeds the average output of the next seven rivers combined.

All that water helps make the Amazon the widest river in the world. During the dry season, it is about 11 kilometers (6.8 miles) wide at its widest points. During the wet season, it explodes to some 40 kilometers (24.8 miles) in width. In terms of surface (width times length), the Amazon covers around 110,000 sq km (42,400 sq miles) during the dry season, and it more than triples during the wet season to 350,000 sq km (135,000 sq miles), according to Extreme Science.

The Amazon currently holds the Guinness World Record for the widest river in the world — 11 kilometers (7 miles) at its widest point, measured on the 18th of March, 2005.

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