Â鶹ÒùÔº

Colorado Plateau's dramatic canyons: How rivers' ancient pauses and rapid cuts shaped them

Long after tectonic plate movement has created mountains, forces such as weather and river erosion can continue to shape a landscape. One such landscape is the Colorado Plateau, which spans more than 336,000 square kilometers (83 million acres) across four states and encompasses iconic sites such as the Grand Canyon and Arches National Park.

Dramatic gorges such as these are created when a river incises into the rock below over millions of years. The rate of river incision in the Colorado Plateau has varied over time and is not well understood.

Natalie Tanski and colleagues looked at two reaches of the Colorado River to determine how and why incision varied during the Pleistocene, dating fluvial gravel deposits that form river terraces, which are steplike features that mark the former floors of river valleys. Their study is in the journal AGU Advances.

To constrain the ages of the river terraces, they used luminescence dating, which measures a mineral's release of photons to determine the last time it was exposed to sunlight or intense heat, and isochron dating, a form of radioactive dating that, in this case, measured the changing relative abundance of beryllium and aluminum isotopes.

They then simulated how and when a wave of incision may have migrated up through a drainage basin in response to a change in baselevel, the lowest point that a river can erode. They compared the results to their terrace ages to determine whether drops in baselevel could have contributed to unsteady incision rates.

The researchers found that in the Moab region of the central Colorado Plateau, river incision paused from 1.8 million years ago to about 350,000 years ago. This pause was followed by a period of rapid incision lasting to the present, during which the river systems carved 200 meters deeper into the plateau. These shifts can be linked to base-level changes as the Colorado River established its path, eventually contributing to the shape and iconic chasm of the Grand Canyon and its tributaries.

However, this particular period of rapid erosion represents only about a quarter of the total exhumation that took place in the central Colorado Plateau over a much longer time period, suggesting that yet-unknown events occurring millions of years earlier may have also shaped the landscape.