A River Stole Another River 89,000 Years Ago — and It's Still Making Everest Taller Today
Scientists have finally explained why Mount Everest is taller than it should be: an ancient river heist that's still lifting the mountain by up to 2mm a year.
At a glance
- Published
- TopicRecords & People
- Verified againstUniversity College London / Nature Geoscience
- Treks coveredEverest Base Camp Trek — 14 Days, Makalu Base Camp Trek — 18 Days
Mount Everest should not be quite this tall. Erosion wears mountains down faster than tectonic collision alone can build them back up, geologists have long known — yet Everest keeps growing. In September 2024, a team led by University College London finally worked out why: roughly 89,000 years ago, one Himalayan river swallowed another, and the ground beneath Everest, Lhotse and Makalu has been springing upward ever since.
Key facts
- Everest's internationally recognised height is 8,848.86 metres, jointly confirmed by Nepal and China in December 2020.
- A 2024 study in Nature Geoscience found the peak has gained an extra 15–50 metres over the past 89,000 years from this one event alone.
- The mountain is still rising by up to 2mm a year — a change measurable within a human lifetime.
- Neighbouring giants Lhotse (world's 4th-highest) and Makalu (5th-highest) were lifted by the same process.
The mechanism is something geologists call river piracy. Around 89,000 years ago, the Kosi River system, cutting north into the Himalaya from the Ganges plain, worked its way headward until it broke into the course of the Arun River and diverted its flow. The Arun had been meandering gently below Everest; suddenly it was carrying far more water, with far more erosive force, through the Arun-Kosi gorge system that drains the very valleys that today's Makalu Base Camp trekkers walk through.
More water cutting a steeper, faster river means more rock scoured away and carried downstream, year after year. Removing that much mass from the Earth's crust has a predictable effect: the crust, no longer weighed down, floats upward to compensate — the same principle that makes a cargo ship ride higher in the water once you unload it. Geologists call it isostatic rebound. The UCL-led team modelled the volume of rock the Arun-Kosi system has since carried away and found it was enough to explain Everest's unexplained extra height almost exactly.
The rock that now forms Everest's summit pyramid was limestone on the bed of the ancient Tethys Ocean, packed with marine fossils.
The collision starts thrusting that seabed skyward, building the Himalaya.
River piracy accelerates erosion below Everest, triggering isostatic rebound.
Nepal and China jointly re-measure the summit by GPS and trigonometric survey, ending a decades-long dispute.
Everest gains up to 2mm a year — outpacing the erosion wearing its slopes down.
What this means for trekkers
This is not abstract geology confined to a journal. The Arun and Kosi valleys the study is built on are the same ground the Makalu Base Camp Trek follows, climbing from subtropical rice terraces at under 1,000 metres to the foot of the world's fifth-highest peak at over 4,800 metres — one of the steepest climate gradients on the planet, carved by exactly the river system now quietly lifting the summits above it. Everest itself keeps its precise, contested height only because Nepal and China now re-measure it with GPS receivers carried to the true summit, work that grew directly out of questions like the ones this study answers.
What this means for you
The mountain under your boots on any Everest or Makalu trek is not a fixed monument — it is actively rising, right now, for reasons scientists only fully explained in 2024. Few landscapes on Earth are still this alive.
Trekkers wanting to see the process firsthand — the deep Arun gorge, the rebound-lifted peaks, the dramatic climate zones — can walk it directly on the Makalu Base Camp Trek, or take in Everest's confirmed 8,848.86m summit from the classic Everest Base Camp Trek.
Cover photo: Abdul Kayum via Pexels (Pexels License).
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