Answer: World's largest waterfall?

We've talked about this before...

An actual waterfall (not synthetic). P/C Pexels.

Whenever you search for something, you need to understand what you're searching for!

I know that seems obvious, but this is a very common mistake people make when searching. Let's unpack what this week's Challenge was really all about!

1. What is the world's largest waterfall? (This is pretty simple.)
2. Where is the world's second largest waterfall? (This isn't so simple.)

If you remember, we've talked about waterfalls and the question of understanding your search before in our discussion of the "elevation of Vernal Falls" (in Yosemite). The key question in that discussion was "what do you mean by elevation?" As in much of search, definitions matter.

When I did this search, my first query was the obvious one:

[ largest waterfall ]

which took me to the surprising underwater waterfall of the Denmark Strait.

My first thought (and probably yours too), "there are waterfalls underwater?" What would that mean?

A quick review of articles about the Denmark Strait taught me that there's a kind of sill (an area of relatively shallow depth) in the Denmark strait that's next to a fairly deep part of the north Atlantic. Water from the Arctic runs south, through the strait, then over the sill and falls into deep sea. Here are two figures for you:

Adapted from Figure 1 in Perner, Kerstin, et al. "Interaction between warm Atlantic sourced waters and the East Greenland Current in northern Denmark Strait (68 N) during the last 10 600 cal a BP." Journal of Quaternary Science 31.5 (2016): 472-483.

The currents from up north flow down south through the strait and over the sill. Here's a simpler diagram I made from Google Earth showing the hydrography a bit more clearly.

Figure by Dan, background image from Google Earth.

Notice the darker regions to the left of the red, dashed line? That's the dropoff--or, perhaps it's better described as a cascade. But the difference in water depth between the red line and the tip of the blue line is around 3,505 meters (11,500 feet).

In what sense is this a waterfall? Great question: What IS a waterfall?

Normally, we think of a waterfall as being a volume of water dropping over the face of a cliff or steep mountain slope. It's an interesting question: when does a steep enough mountain make a waterfall vs. a cascade? Usually, a cascade is a small waterfall, typically one of several that fall in several stages down a steep rocky slope. Here's a pic I took in the Geiranger fjord in Norway that illustrates the difference:

Geiranger Fjord, Norway. P/C Dan.

Are these all waterfalls, or is the one on the right side a "cascade"? Now... how do you measure the height of these waterfalls?

For our purposes, it doesn't matter much, except that it points out the issue of asking "what's the largest X?" Largest waterfall? Largest dinosaur? Largest fish? These sound like simple questions, but behind simple questions often lies a depth of subtlety.

For all such questions, it boils down to what are you specifically asking! Does largest mean "by height" "by volume" "by width"?

And then there's the question of time--are we going to look for largest waterfall ever? If so, should we include famous historic waterfalls like Guaíra Falls on the border of Brazil and Paraguay, which was once thought to be the most voluminous waterfall on Earth, and which itself was 12 times more voluminous than Victoria Falls (Zambia / Zimbabwe)? Or how about the waterfall that sprang from the time when the Mediterranean Sea was refilled by the Atlantic, causing the Zanclean Flood? (Water drop of 1km, water flow of 100M cubic meters / second. But the drop was fairly shallow, making it a cascade, rather than a waterfall. And it was several million years ago--should it count?)

In the case of the Denmark Strait waterfall, the "falling water" is created by the density difference of the water masses on either side of the Denmark Strait. Water from the eastern side is much colder than water on the western side. Because of this difference, when the two masses meet along the top ridge of the strait, the colder, denser water flows downwards and underneath the warmer, less dense water, forming a very distinct current, and hence, a waterfall. The drop is around 3,505 meters (11,500 feet), making it easily the tallest drop of water in the world.

At the same time, the Denmark Strait cataract has a flow rate exceeding 175 million cubic feet (5.0 million cubic meters) per second, making it much larger than the next several terrestrial waterfalls combined.

On the other hand... if we consider above water waterfalls (a concept I never thought I'd have to explain), it's relatively straightforward to find tables of waterfalls with height, width, and volume. I used this query:

[ list of waterfalls ]

and then looked at different lists of waterfalls. Here are a few:

WorldWaterfallDatabase.com

Civitas list of waterfalls (worth looking at for the remarkable photos of waterfalls)

Wikipedia list of waterfalls

This last list (Wikipedia) is especially insightful. As the article says

"... There is no standard way to measure the height or width of a waterfall. No ranking of waterfalls should be assumed because of the heights or widths provided in the list. Many numbers are estimated and measurements may be imprecise."

These are wise words that should be a cautionary note for us. If you compare the same waterfalls across different lists, you'll find a variety of numbers for their waterfall dimensions. Here's an example, note the differences from source-to-source:

Wikipedia list of waterfalls:

Angel Falls 979m
Tugela Falls 948m
Mattenbachfälle (not listed!)

Wikipedia list of waterfalls by height:

Tugela Falls 983m
Angel Falls 979m
Mattenbachfälle 840m

Civitas:

Angel Falls 800m
Tugela Falls 948m
Mattenbachfälle (not listed!)

WorldWaterfallDatabase:

Tugela Falls 948
Angel Falls (listed as Kerepakupai Merú) 807m
Mattenbachfälle 930m

Wikipedia individual articles on waterfalls:

Angel Falls 979m
Mattenbachfälle 930m (or 840, depending on which number you trust in the article)
Tugela Falls 948m (or 983, again, depending on which part of the article you believe)

You see the problem! Even something as simple as "height of the waterfall" is complicated. Angel Falls is either 979m, or 800m, or 807m. Tugela Falls is either 983m or 948m. Mattenbachfälle is either 930m or 840m.

We shouldn't just take the average of the measurements! So, the best answer is that the tallest waterfall by height from lip to the end of the cataracts at the base is ONE OF Angel Falls, Tugela Falls, or Mattenbachfälle!

Don't get me started on waterfall width or volume--both have even less standarized ways to measure than height!

SearchResearch Lessons

The biggest three lessons that we should take to heart:

1. Simple questions don't mean simple answers. You might think that the opening Challenge would be simple--there's an obviously bigger waterfall, isn't there? But when you start to ask about the details, you quickly find out that there's a lot more to the question. We should be teaching this as part of SearchResearch literacy! Do you know what your research question really means?

2. Expect the unexpected. When I posed this Challenge, I really didn't expect to learn about the Denmark Strait or the massive outflow of cold water from the Arctic into the Atlantic--I spent a lot of time learning about currents, salt water density, and the giant eddies that form as a result of the pulsing nature of the Denmark Strait current. Fascinating stuff, and utterly unexpected.

3. Compare sources. I was surprised to see that the numbers of estimated waterfall heights would vary so much... sometimes even within a single article! I know we always say "get multiple sources," but that's deeply true, even for something as straightforward as the height of a waterfall!

Search on!