Influential Prose

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A River Runs Through It

 

 

What you see here is over 300 million years of geological history at Goosenecks State Park in Utah. You’ve seen similar vistas before, such as looking into the Grand Canyon, and there’s a reason for that.

At the bottom of these chasms is the San Juan river, and at a glance you might imagine the river slowly carving its way down through rock over eons. It didn’t happen that way. What actually happened is much more interesting.

You’ve seen photos of rivers that meander back and forth in a form called oxbow tails, such as this one in the Innoko National Wildlife Refuge in Alaska:

 

These develop in areas that have low gradients, i.e., the upriver areas are very slightly higher than downriver, just enough for the water to slowly flow downward. The San Juan developed in that way, over 300 million years ago.

But rivers like these don’t cut into rock. They actually tend to collect and carry sediments, then drop them at turns, which is why it ends up winding back and forth.

The continental crust underlying the San Juan river broke off from the supercontinent Pangea and wandered around the globe. It’s still moving west, at about the speed fingernails grow; if you’re living in North America, you’re along for the ride right now.

And so through eons, about 280 million years, the river placidly plodded along, providing dinosaurs and other animals with drinking and bathing water.

Then the mountain building began, in fits and starts, over millions of years. Today that process is called the Laramide and Sevier orogenies. A thin dense layer of underwater basalt rock that makes up the Pacific plate began diving under North America. The angle of the collision was very shallow, but still powerful enough to squeeze and deform the rocks that make up the continental crust.

That compression created the column of mountains that stretch from Canada down through the US and into Mexico. The average thickness of continental crust runs between 22-25 miles. Imagine, if you can, the epic scale at work here; much of the western basin, an area that was once the bottom of a shallow inland sea, being slowly pried upwards. You can duplicate the pattern on a smaller scale by pushing a piece of tablecloth.

The new mountains transformed the gradient of the surface water’s flow. Upriver was now significantly higher than downriver, which in turn generates faster water flow.

Normally when water runs fast, it flows relatively straight, creating long classic valleys. But this river was already formed. The new, faster flow was strong enough to carry sediments out and further downhill, but not strong enough to alter the shape of the river. So it carved down through the rock…in 20 million years.

So what you see here, in a sense, is a region that has been power washed by an incessant high-flow river running 24/7 for 20 million years, steadily working its way down through 300 million years of layers. This all happened in the last 15% of the river’s existence.

So it doesn’t take 300 million years to carve this deep channel. Twenty million years and fast water is enough to make it happen.

Written by Influential Prose

May 10, 2015 at 10:58 am

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