Geologists may have solved the mystery of the Green River’s ‘uphill’ route

Scientists have long sought an answer to the question of how the Green River, the largest tributary of the Colorado River, managed to create a 700‑metre‑deep canyon through Utah’s 4km-high Uinta Mountains instead of simply flowing around them.
 
The question is particularly confounding because, while the Uinta Mountains are 50 million years old, the Green River has been following this route for less than eight million years.
 
Now, researchers from universities in the UK and the USA have gathered persuasive evidence that a phenomenon called ‘lithospheric dripping’, which causes mountains to subside and rebound over millions of years, is likely to be the cause of the Green River’s unusual route.
 
In order to cut its surprising path through the Uinta Mountains, the Green River ran over land that was temporarily lowered when a lithospheric drip developed beneath the mountains several million years ago. During that time, the river eroded the mountain rock and established the channel it flows through today, including the famous Canyon of Lodore, which eventually linked it to the Colorado River.

 
Seismic imaging is a process similar to a CT scan which helps scientists ‘see’ below the planet’s surface by collecting data on how seismic waves move and are reflected during earthquakes. By looking at previously published seismic imaging studies of the mountains, the team identified a cold, round anomaly about 200 km below the surface. This mass, which is between 50 and 100km across, is likely to be the broken-off section of the drip, the researchers say.
 
By estimating how far the drip had fallen and calculating the speed of its descent, the researchers estimate that the drip broke off between two and five million years ago. Their estimates match well with previous research which estimated the likely period of time during which the Green River cut through the mountains and integrated with the Colorado system.
 
Using modelling of the river networks, they identified and measured the bullseye pattern of uplift around the mountains – the telltale ‘fingerprint’ of a lithospheric drip. They also found that the crust beneath the Uinta Mountains is several kilometres thinner than expected for a mountain range of its height, which the team say is consistent with dense lower‑crustal material having dripped away. When they calculated the surface uplift expected from this missing material, it matched the roughly 400‑plus‑metre elevation change they had inferred from the river networks.
 
Dr Smith added: “This is a long paper, because we wanted not just to lay out the case for a lithospheric drip creating the route of the Green River but also to acknowledge some previous theories. The evidence we’ve collected strongly contradicts the idea that the river predated the mountains, or that sediment deposits might have built up enough for the river to overtop the range, or that erosion from the south of the mountains captured the Green River.
 
“We hope that this paper will help resolve a longstanding debate about one of North America’s most significant river systems, and help build the growing body of evidence that lithospheric drips may be the hidden answer to more tectonic mysteries than we’ve previously realised.”
 
Researchers from University College London, the University of Utah, and the Utah Geological Survey contributed to the research and co‑authored the paper. The team’s paper, titled ‘A lithospheric drip triggered Green and Colorado River integration’, is published in the Journal of Geophysical Research: Earth Surface.