“It’s such a weird path,” said Adam Smith, a researcher in numerical modeling at the University of Glasgow, describing a river that seems to ignore a mountain range that should have turned it aside. The Green is the greatest tributary of the Colorado River, and the most picturesque challenge to the topography is the Canyon of Lodore a passage flanked with walls of rock where the sides of the canyon are about 2,300 feet over the torrents. The opening to the canyon is like a warning sign on a map, called Gates of Lodore, but the greater mystery lies deeper in: rivers usually simply go around the obstacle, and the Uinta Mountains are a long, high wall to cross the course of the Green. The Uintas are between 50 million years old, whilst the contemporary path of the Green across them is only between 8 million and 10 million years of age, with major integration usually being concentrated in the final several million years. That imbalance has left the history of drainage in the region unresolved over the generations.
The same question was raised in 1800s when John Wesley Powell explored the Green and Colorado rivers trying to find some logic that would fit the scenery. He rejected an easy “cracks in the rock” solution and wrote that “very little examinations show that this explanation is unsatisfactory. The proof is abundant that the river cut its own channel; that the cañons are gorges of corrasion.” Powell had a greater concept in mind, which was beautiful the idea of an older river which stood its ground when mountains arose but subsequent geologic dating could not support such a plot. The Uintas already being old when the Green took its modern course, the canyon was now to appear to be more of a problem of rerouting which required a missing mechanism.
Two common explanations have been floating around decades long; a seizure by the Yampa system to the south, or a seasonal elevation of the river level due to sedimentary deposition which allowed the water to run across the range. They both are difficult to match with the size of the incision and the traces that are left on deposits and valley shapes. The focus has narrowed in – below the walls of the canyon, below the crust, to processes that are capable of providing continuous reworking of relief without leaving a single disastrous mark.
In a more recent synthesis it is maintained that the Uintas did not stand waiting there to be cut. Rather, the range probably sunk and then recovered by a process of lithosphere connection known as “lithospheric drip.” When mountains accumulate the crust, high pressure may cause dense minerals to be formed at the depth; with time, a heavy root may start peeling off and sliding into the mantle. As the thick material is contracting and lengthening toward the ground the surface over may sink. Once dislodged, the lowered load causes uplift, a rebound that leaves a typical, bullseye mark on the surrounding topography and drainage.
Researchers found such a bullseye uplift signal in models which were tuned to river profiles around the Uintas which were then compared with deep-Earth imagery. Through seismic tomography, which is sometimes compared to a CT scan of the planet they aimed at a cold, rounded object some 200 kilometers below the range, which is in line with a detached drip. The estimated age of detachment (2-5 million years) coincides with free estimates of the time of establishment of the cross-mountain path by the Green and the time when it began to incise features such as Lodore actively.
The suggestion is a landscape which had been temporarily made simpler to traverse. At a period of low elevation the Green might have discovered a passage over the Uintas; when established thus, it continued to make inroads as the range rose. In that perception, the canyon is not merely an indication of the tenacity of water, but a record on the surface of profound shifts in buoyancy, an event in which the mountains themselves might have dipped, then shot up, as a river snatched the moment and would not relinquish it.
