Everest is still growing, and a river that stole another river is pushing it up

Mount Everest is still getting taller, and part of the reason is a river 75 kilometres away that captured another river about 89,000 years ago. A 2024 study in Nature Geoscience found the erosion that followed is still lifting Everest by up to about 2 millimetres a year, on top of the tectonic collision that built the Himalaya.

The river that stole a river

The Kosi river system worked its way into the course of the Arun and absorbed it, a process geologists call river piracy or river capture. Suddenly carrying far more water, the merged system began cutting through the bedrock of the valleys below Everest much faster than before.

Removing that much rock lightens the crust, and a lighter crust rises — isostatic rebound. Study co-author Adam Smith of University College London compared it to a cargo ship losing its load: “The ship becomes lighter and so floats a little higher.” The rebound drags the neighbouring peaks up with it.

How much of Everest is a river’s doing

Somewhere between 15 and 50 metres, on the team’s modelling. The study, led by Xu Han of the China University of Geosciences with Jin-Gen Dai, Adam Smith and Matthew Fox, estimated the rebound’s total contribution over 89,000 years and its continuing rate.

So Everest has at least two engines pushing it up at once: the continental collision that began tens of millions of years ago, and a comparatively recent case of one river swallowing another. The official figure of 8,848.86 metres, itself 86 cm higher than Nepal’s previous number, is a snapshot of a mountain that has never stopped moving.

Not just Everest

Lhotse, the fourth-highest peak, and Makalu, the fifth, are being lifted by the same rebound. Makalu, closest to the Arun’s new course, appears to be rising slightly faster than its neighbours.

How the height is measured has its own history: the first calculation was made a century before GPS, by a mathematician working from more than 150 kilometres away. The summit rock itself is 450-million-year-old seafloor.

What this means for trekkers

Nothing changes underfoot — two millimetres a year is invisible on a two-week trek. What it does mean is that you can walk the very valleys the study is built on.

Sources: Han et al., “Recent uplift of Chomolungma enhanced by river drainage piracy”, Nature Geoscience (2024); University College London; Smithsonian Magazine; The Kathmandu Post (2020 height).

Cover photo: Shashank Brahmavar via Pexels (Pexels License). Section photos: Vijay Richhiya via Pexels (Pexels licence); Dipak Chettri via Pexels (Pexels licence); Chetna Mahajan via Pexels (Pexels licence).