What do you think of this fact?
Mount Everest is 8,849 meters tall, and it sticks out. According to UCL, it rises about 250 meters above the next tallest peak in the Himalayas, while K2, Kangchenjunga and Lhotse differ from each other by only about 120 meters. So why is Everest the odd one out? Part of the answer is a river.
A river heist in the mountains
In 2024, a team from University College London and the China University of Geosciences published a study in Nature Geoscience. Their idea: about 89,000 years ago, the Arun river merged with the Kosi river network. Geologists call this “drainage piracy”, because one river system effectively captures another.
The result was more water flowing through the Kosi and a lot more erosive power. The river network carved a deep gorge and carried away huge amounts of rock and soil. Scientific American points out that the Arun’s gorge has near-vertical walls compared with neighboring rivers, a hint that it is relatively young.
Lose weight, get taller
Here’s the twist. When the crust loses mass, it gets lighter, and the mantle below pushes it back up. This is called isostatic rebound. The eroding river network is about 75 kilometers from Everest, yet the effect reaches the summit. The team estimates that it has added 15 to 50 meters to Everest’s height.
And it’s not over. UCL researcher Matthew Fox says GPS instruments show Everest and its neighbors growing by about two millimeters a year. Lhotse and Makalu, the fourth and fifth highest peaks on Earth, get a similar boost, and Makalu, closest to the Arun, may rise slightly faster.
Bonus fact: Not every geologist is convinced by the timing. Peter van der Beek of the University of Potsdam told Scientific American that the date of the river capture is uncertain because the model is simple.
The highest point on Earth is partly a side effect of a river eating the landscape around it. Everest didn’t just get pushed up by colliding continents. It also got a lift from what the water took away.