A new study points to lithospheric dripping as the cause of the unusual path of the Green River, which runs through the Uinta Mountains.
Rivers are known for taking the easiest route, bypassing obstacles on their way to the sea. The Green River in the western United States, however, appears to break this rule. Instead of skirting Utah’s towering Uinta Mountains, the river directly crosses the mountain rangecarving a canyon up to 700 meters deep.
A new paper published in JGR Earth Surface suggests that this seemingly illogical path can be explained by the fact that the Earth’s crust behaves More like soft wax than rock solid.
The Green River stretches approximately 750 miles through Wyoming, Utah and Colorado before emptying into the Colorado River. The mystery lies in northeastern Utah, where the river runs through the heart of the Uinta Mountainswhose peaks rise to around 4 kilometers high. This route was formed tens of millions of years after the mountains themselves formed, which raises the question: why didn’t the river simply bypass them?
Researchers at the University of Glasgow believe the answer lies in a process known as “lithospheric drip“. This phenomenon occurs when dense portions of the Earth’s crust slowly sink, or “drip,” into the mantle beneath, causing the surface above to sag before recovering over millions of years. Using seismic imaging and advanced computer modeling, the team reconstructed how the region’s crust changed over time.
The analysis revealed a distinct elevation pattern in the shape of a target around the Uinta Mountains, along with evidence that the crust beneath them is several kilometers thinner than expected. This missing material closely matches the approximately 400 meter elevation drop needed to allow the Green River to establish its course through the mountain range, explains the .
In this scenario, the ground beneath the mountains temporarily sank as the crust gave way, creating a path low enough for the river to flow. As the land recovered and the mountains rose again, the river was already established, gradually eroding the rock and preserving its unusual course.
The consequences of this process go far beyond a single river canyon. When the Green River finally joined the Colorado River, reshaped the geography of the west of North America, influencing the continental divide and altering habitats that guided the evolution of regional wildlife.
“During about 150 yearsgeologists have debated how these rivers came together,” said lead author Adam Smith of the University of Glasgow. “We believe we now have convincing evidence that lithospheric dripping pulled the ground down enough to make it possible.”
