Wow, the Grand Canyon made news twice this week: First for a controlled flood to boost the Colorado River’s health; second for a new study that shows the canyon is three times older than we thought.
In the first case, a three-day controlled flood began March 6 when officials released water from the Glen Canyon Dam. Some 300,000 gallons of water per second flowed out; the water level of the Colorado River rose 15 feet in places. (Watch a video report.)
Flooding the Grand Canyon allows sediment blocked by the dam to flow downstream, as it would naturally if there was no dam. Sediment rebuilds sandbars, which are essential to native fish and plants. Controlled floods also occurred in 1996 and 2004. The superintendent of Grand Canyon National Park and environmentalists would like to see the flows more frequently, about every 1-2 years.
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In the second story, scientists tried to answer the question, ‘Just how old is the Grand Canyon?’ A study published March 7 in Science (by Victor Polyak, Carol Hill and Yemane Asmerom) says the canyon is 17 million years old — much more than the 5-6 million years old thought before. The geologists arrived at the new age by climbing up the cliff walls to various caves, taking samples of mineral deposits in those caves, and dating the samples by measuring the predictable decay of uranium to lead in the samples. The deposits form underwater near the tops of water tables; by measuing the age of the deposits, the geologists claim to be measuring how long ago the water table was at the height of each cave — and thus they can construct a timeline of how long it took the river to erode the canyon to its present state.
The study has sparked some controversy. The Washington Post cites geomorpholgist Joel Pederson of Utah State as saying that 17 million years can’t be correct, because there’s no evidence of lots of sediment flowing out of the canyon more than 6 million years ago. Basically, where’s the dirt? Also, he disputes the notion that water-table height would always correlate with canyon carving.
As noted in a “Perspectives” article in the same issue of Science (”Canyon Cutting on a Grand Time Scale“), the new canyon study also found a tectonic influence on when the canyon formed. In order for a canyon to form on a high plateau, there needs to be a “step” so the erosion can begin. Tectonic shifts may have created such a step, which could have then led to the formation of the canyon.
The study’s authors also posit that the western part of the Grand Canyon is what formed 17 million years ago, intially as a small canyon, and has been eroding eastward ever since. Faster, deep erosion in the eastern part of today’s canyon happened much later, over the last 6 million years.
It’s a fascinating study to contemplate, both in its method for making a new estimate of the canyon’s age, and for the fact that we don’t know for certain how old the canyon is.
What do you think of these two stories?
Interesting story above. Very pleased to hear they are flooding it occasionally. But I’ve also heard that the sandbars that form disappear rather quickly. I guess that would be the reason for the desire for every 1-2 years?
The flooding is done when there is enough sediment built up - this can be 1 to 2 years