Victoria Falls in southern Africa is often regarded as the world’s largest waterfalls. “Regarded”, because there is no standard method to measure the largeness of a waterfall. A tall waterfalls might not have enough width, while an extremely wide one might have an insignificant drop. If we go by volume or flow rate, then the largest waterfalls should be Boyoma Falls—I bet you’ve never heard that name. But Boyoma Falls, in reality, are only rapids on the Lualaba river in central Africa.
The truest waterfalls are those that forms when a river falls over a gorge, and among them the one with the largest flow rate is Niagara in the border between the US and Canada, followed by Iguazú Falls in the border between Argentina and Brazil. Victoria falls, located between Zimbabwe and Zambia, come third with a flow rate less than half that of the Niagara. But geographers have decided that Victoria Falls shall be the largest, based on its combined width (1,708 meters) and height (108 meters). In height and width Victoria Falls is rivaled only by Iguazu Falls.
The Victoria Falls is considered to be the largest waterfalls on earth. Photo credit: zuhanna/Shutterstock.com
But that’s only on land. In the ocean, everything is bigger—bigger mountains, deeper gorges, bigger animals, even bigger and powerful waterfalls. These underwater cascades fall silently over immense gorges hidden from our view beneath the ocean.
Scientists have found that the biggest underwater waterfalls is located beneath the Denmark Strait, which separates Iceland and Greenland. The waterfall forms when the colder, denser water from the Nordic Seas meets the warmer, lighter water from the Irminger Sea. The cold water being heavier, flows down and underneath the warm water, and flows over the huge drop in the ocean floor, creating a downward flow estimated at 5 million cubic meters per second. For comparison, Niagara Falls’ flow rate is only 2,407 cubic meters per second. The Denmark Strait cataract is also 3,500 meters tall, more than three times the height of the tallest land-based waterfalls, Angel Falls.
Oceanographers knew about undersea waterfalls as early as the 1870s, but their great depth and limited area prevented their study. It was not until the 1960s that investigation of the phenomenon became possible with modern equipment. About half a dozen cataracts have been discovered in the Atlantic Ocean alone.
All of them are enormous. For instance, the Ceara cataract in the North Atlantic between the continents of South America and Africa has a flow rate between 1 and 2 million cubic meters per second. Similarly, the Rio Grande cataract at 20 degrees south latitude in the Atlantic Ocean has a flow rate of 4 million cubic meters per second, perhaps as large as the Denmark Strait cataract.
Temperature difference between basins of water, however, is not the only driving force of these enormous fluxes. For example, the cataract in the Strait of Gibraltar is driven by differences in salinity. The water in the Mediterranean is much saltier owing to evaporation, and therefore much denser than even the deepest water in the Atlantic, even though the Mediterranean sea is warmer. This causes water flowing out of the Mediterranean
through the Strait of Gibraltar to sink into the Atlantic as a cataract.
Major cataracts of the Atlantic ocean. The red line traces the part of the 1972-73 expedition to study these cataracts. Graphics courtesy John A. Whitehead.
Ocean cataracts play a big role in maintaining the salinity and climate of the oceans. They also have an effect on marine biology. Take for example, the krill, a small crustacean that gather near the South Shetland Islands, near Antarctica, to spawn. The spawning grounds lie near an immense Antarctic circumpolar current that sweeps eastward. Yet, the hatched larvae are found hundreds of kilometers to the west, even though the Antarctic circumpolar current flows to the east. Biologists now believe that the eggs sink after they are laid and then carried westward by an underwater cataract. After hatching, the larvae rise and are carried back eastward to the spawning ground by the Antarctic current.