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Scientists may have cracked the mystery behind one of North America’s most significant river systems, understanding how a river managed to cut through a Utah mountain range that’s been around for millions of years.
The question of how the Green River, the largest tributary of the Colorado River, formed a canyon around 2,296 feet-deep through Utah’s 13,123 foot-high Uinta Mountains, instead of flowing around them, has perplexed geologists for more than a century and a half.
What’s even more baffling is that while the Uinta Mountains are 50 million years old, the Green River has been following this "uphill" route for less than eight million years, noted researchers led from the University of Glasgow, Scotland.
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A phenomenon known as “lithospheric dripping,” which causes mountains to subside and rebound over millions of years, is likely to be the reason for the Green River’s surprising route, the team now believe.
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“For about 150 years now, geologists have debated over exactly how the rivers merged, which is a particularly challenging question for a tectonically inactive area where major geological events are rarer,” said paper author and geologist Adam Smith of the University of Glasgow in a statement.
Smith and his team believe they have “gathered enough evidence to show that lithospheric drip, which is still a relatively new concept in geology, is responsible for pulling the land down enough to enable the rivers to link and merge.”
The study shows that the Green River carved out its path through the Uinta Mountains by running over land that had been temporarily lowered when a lithospheric drip occurred beneath the mountains several million years ago.
In this period, the river eroded the mountain rock and formed the channel it flows through today, which includes the famed Canyon of Lodore, which later linked it to the Colorado River.
Lithospheric drips occur when dense, mineral‑rich material forms at the base of the crust of the Earth, later becoming heavy enough to sink into the mantle below, which is a thick layer of rock between the crust and core of the Earth.
“As they sink, they can drag down the land above them, pulling parts of mountain ranges downward,” the researchers explained. “When the drip breaks off and continues to sink on its own, the mountain range rebounds, leaving behind a distinctive ‘bullseye’‑patterned zone of uplift across the landscape above the drip’s point of origin.”
Based on how far the drip had fallen and the speed of its descent, the geologists believe that the drip broke off between two and five million years ago. Their estimates match that of previous studies that approximate the likely period of time during which the Green River carved through the mountains and joined the Colorado river system.
“The merging of the Green and Colorado Rivers millions of years ago altered the continental divide of North America. It created the line that separates the rivers that flow into the Pacific from those that flow into the Atlantic, and created new habitat boundaries for wildlife that influenced their evolution. It’s an enormously significant area of the continent,” Smith said:
Smith hopes that the latest research will “help build the growing body of evidence that lithospheric drips may be the hidden answer to more tectonic mysteries than we’ve previously realized.”
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Reference
Smith, A. G. G., Fox, M., Miller, S. R., Morriss, M. C., & Anderson, L. S. (2026). A Lithospheric Drip Triggered Green and Colorado River Integration. Journal of Geophysical Research: Earth Surface, 131(2). https://doi.org/10.1029/2025JF008733
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