Monday, November 15, 2021

Salvage Geology in Cataract Canyon

Aerial view of the confluence of the Green (light brown) and the Grand (darker brown)
that formed the Colorado River (left) prior to 1922 . (Image taken May 25, 2015) 

I've run the rapids through Cataract Canyon in Canyonlands National Park and Glen Canyon National Recreation Area a few times and marveled at the exquisite geology exposed there. In fact, I believe that the country surrounding the confluence of the Green and Grand rivers is the quintessential location of the heart of the Colorado Plateau. Names like the Orange Cliffs, the Land of Standing Rocks, and Island in the Sky evoke images of stratified terrain that may be equaled elsewhere nearby but never surpassed. You can see some of my past blog postings of trips through the canyon here, here, and here.  

Image taken May 25, 2015

The ongoing drought in the American Southwest has caused the level of the Powell reservoir behind Glen Canyon Dam to drop 156.5 feet from its full pool elevation of 3,700 ft. (above mean sea level). This has caused the once drowned channel of the Colorado River in Cataract Canyon to slice through over 35 years of sediment within a narrow, confined channel. In the photo above, you can see part of this great sediment pile directly above the boat and in the shadowed bank on the right. The Powell reservoir once inundated  these terraces but the lower level of the reservoir causes the river to slice through them.

The Powell formation. Image taken May 25, 2015

Now, the Returning Rapids Project is documenting the scientific and social significance of this sediment excavation event. You can read an article from the Salt Lake Tribune here about some of the results. 

For the Edward Abbey that resides in many Southwestern souls (and I proudly proclaim my fondest affection for the anticipation of the demise of the unneeded reservoir), this is an exciting moment in time. It is an opportunity for scientists to monitor both the deposition of the Powell beds for the mid-1960s to about 2000, and the erosion of this sediment pile as the level of the reservoir drops. 

In the linked article, the author refers to the sediment body as the Dominy formation, after Bureau of Reclamation Director Floyd Dominy, who essentially spearheaded the drive to construct Glen Canyon Dam in the 1950s. However, all rock units in geology must be named after a geographic location where they are first studied and described (called the type section). Until someone names a feature after Dominy, a better name for unit might be the Powell reservoir formation, or simply the Powell formation (both names in a rock unit are capitalized if the name has been formalized with a scientific description; if it has not yet been described then the suffix name is lower case, signifying that it is an informal name).

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