Optional Enrichment Articles and Videos

Optional Enrichment Articles and Videos

Incised Meanders and Geologic History

Canyonlands poses a special puzzle. The rivers in Canyonlands meander, making big, sweeping curves through the red rocks. Earlier in this module, we discussed how meandering rivers typically occur in flat, lowland regions lacking supply of big rocks to the river. This is decidedly not the situation in Canyonlands. The rivers are somewhat steep, and in places (although not just where the picture below was taken) are laden with rapids, challenging for whitewater rafters. Landslides and rockfalls from the canyon walls deliver large blocks to the streams.

This meander in the Colorado River along the edge of Canyonlands National Park was photographed from Dead Horse Point State Park. Potash “Road” crosses the point in the foreground, blocking the view of the river, which flows in from the lower left, loops around, and flows out to the upper left. This meander sits in a deep canyon, carved by the river. Note also that there are large sand bars along the river, covered with small trees and other vegetation and friendly to wildlife, and that the river is muddy. Below the Glen Canyon Dam, the sand bars are washing away in the clean water released by the dam, and wildlife is much less happy. Notice, also, where rocks have fallen down the cliffs on the inside of the meander.

Credit: Annie Scott, USGS (Public Domain)

The likely story is that the streams once meandered across nearly flat lowlands. Then, uplift of the rocks began, raising the Earth’s surface to a higher elevation in what is now the US West, giving the streams a steeper slope to the sea and so speeding their flow and causing them to erode. But, the uplift was gradual enough that the streams held their old courses. The streams cut downward without a change in pattern, which is called incision. Canyonlands contains clear examples of incised meanders. Similar features are preserved throughout the Colorado Plateau, documenting widespread uplift of the Plateau. The Goosenecks of the San Juan River, shown below, are also beautiful incised meanders.

Goosenecks of the San Juan River, as seen from the Goosenecks Parking Lot (top), and from space (bottom).  These are incised meanders.  More than 5 miles along the river is only 1.3 miles in a straight line.  

Credit: GeoSights: The Goosenecks of the San Juan River, San Juan County, Utah, By Marshall Robinson, Utah Geological Survey.  

Much of geology involves study such as this; the history of a region produced the modern features. One can often learn much about that history by understanding the modern features and how they were formed. Clearly, if we did not know what conditions produce meandering streams today, we would not know what conditions likely occurred in the past when the meandering streams developed. The geological saying that captures this idea is “The present is the key to the past.”

More About Sinking Mud

Down at the Mississippi Delta, we saw that the mud under New Orleans is compacting under its own weight, contributing to sinking of the city. A couple of other things also contribute to the sinking.

As the delta grows, the weight of the mud pushes down the rock beneath, with soft, hot rock much farther below flowing away laterally, like air flowing out from beneath you if you sit on an air mattress. The sinking of the Earth under the weight of more mud can take thousands of years. And, just as the strength of the air-mattress cover spreads the dimple around you when you sit down, a fairly large region around the delta is pushed down by the weight of the delta. So, adding some mud anywhere near New Orleans causes a little sinking in New Orleans for a while.

Perhaps more importantly, as we saw with mass movements at the Grand Tetons (and as you can see in the picture of Canyonlands just above, by the cliff on the inside of the meander), rocks tend to fall off steep slopes. Sometimes a single rock falls, or a thin layer of rocks slides. Other times, large thicknesses of material move. The Mississippi Delta is a giant pile of mud towering above the deep waters of the Gulf of Mexico, and that “cliff” is subject to downward motion of its materials. Some of this occurs along faults that are something like pull-apart Death-Valley-type faults—the fault intersects the Earth’s surface well inland, sloping down toward the Gulf, and the mud on top slides down and toward the Gulf.

After Hurricane Katrina devastated New Orleans, geologists renewed their efforts to understand what is going on geologically—such understanding should help in planning how to slow down the next disaster. An argument has erupted about the relative importance of the various reasons for sinking in and near New Orleans, with faulting probably more important (and mud compaction less important) than previously believed, and with sinking of the rock beneath the delta small but not zero. All of these are almost certainly contributing, but with a little work remaining to figure out just how much to blame on each one. Naturally, all of them contributed to lowering of the surface, and sediment deposited from the river's floods filled that space—the river doesn't really care why the surface dropped, and deposits sediment in low places formed by any process.

Video: Erosion at Bryce Canyon National Park (1:04 minutes)

We can measure the uplift of mountains, which may occur slowly, or suddenly in earthquakes, and we can watch volcanoes erupt. But overall, nature tears down mountains about as rapidly as they form, and we can watch and measure the tearing-down, too. The slow disappearance of names from old tombstones, the hubcap-rattling holes in late-winter city streets, and the maintenance budget for university buildings all attest to the effects of nature on human-made things. Here, Dave Witmer takes you to Bryce Canyon, one of the many, many places where you can see nature removing natural things.

Video: Chain of Events / Zion National Park (1:18 minutes)

Geologists observe the wear-and-tear of nature on human-made and natural things, gaining clues to help understand how mountains are torn down. When climbing the sheer cliffs of Zion National Park into the mysterious crevice of Hidden Canyon, the intrepid hiker clings to a rather precarious-looking chain to avoid falling into the stream-carved potholes just beside the trail, and on down to the Virgin River, in the Canyon a hair-raising drop below. In these two clips, Dave Witmer and Dr. Anandakrishnan show how rocks are worn away, a little at a time, and what this has to do with south-Indian cuisine. You might begin thinking about what this wearing-away of rocks has to do with the Virgin River in the Canyon far below.

Video: Pothole Grinding / Zion National Park (1:27 minutes)