Geology of the National Parks

GeoMations and GeoClips


This week, we feature three GeoMations, and two GeoClips from the CAUSE trip out west, featuring Dr. Alley and Dr. Anandakrishnan in the Spring of 2004.

The three GeoMations could appropriately be called 1) How Deltas work, 2) How Dams work, and 3) How New Orleans Doesn't work, and in the video, you'll get to see Dr. Alley and Dr. Anandakrishnan "argue" over the Glen Canyon dam and its effect on Lake Powell above the dam and the Colorado River below.

As before, we hope you enjoy these, and find them to be useful complements to the readings, class notes, and slide shows of Unit 6.


How Deltas Work

How Deltas Work
Click Here for Transcript of How Deltas Work Video

We're flying along in our helicopter out over the ocean, and we're looking towards shore at a beautiful beach we just saw there. And we're seeing the pretty waves of the ocean underneath us. And we see a great river coming down to the shore, something like this, meandering along the way some of these great rivers do.

Now if we could somehow hang along in our helicopter and wait for a flood, what we'd notice is that sitting next to the river, there's a bunch of trees that are growing. And when the flood happens, the water would start trucking out of the river into both directions. And as the water came trucking out of the river, it would slow down when it got into the trees.

And as it did so, it would start depositing a layer of mud. And that layer of mud would be thickest very close to the river, and then it would thin on out across the flood plain of the river. And this would be happening on both sides of the river. And so after a while, we would see that the river is contributing to building a natural levee that sits along the river and runs all the way out to the mouth.

Now, if we could keep watching this happen over very long times, over hundreds or thousands of years, we would see that in many places, the ocean is not strong enough to get rid of all the mud that the river delivers. And so the river would start building out into the ocean, and it would just extend its way out there, building a levee. And as it did so-- not a very high one-- but as it did so, the river itself would lengthen. And so it would be coming out in there, and the water would be flowing down to the sea.

And when this is going on, it has to keep going downhill. So it builds up, as we saw earlier, as well as building out. And so you start getting higher walls that hold the river in, on up here, to allow it to flow downhill and run out like that.

And so as the bed of the river is raised, and it gets higher walls, it's like being on the log flume at the amusement park. And at some point, there's a flood, and that wall breaks, and the river takes the short way down to the sea. And then the whole process will start over again. It'll start building a new wall, and building out that way. And eventually, sometime in the future, this one will become big, and then it will do it over again. And so rivers build deltas. They switch from one place to the other as they build out.

Credit: Dr. Richard Alley, Sridhar Anandakrishnan

How Dams Work

How Dams Work
Click Here for Transcript of How Dams Work Video

So let's take a look, a strange look, at a river. This is going along the river towards the sea. And this river happens to have trees that grow up on the riverbank. So you can get an idea how we're looking at this.

And what we want to do is ask what happens when we build a dam on this river? And we're concerned about the future of a couple of houses that used to be along the river, one just below the dam right down here, and the other one up above the dam up here, sitting along the riverbank. Well, they build the dam, and the dam fills with water, and it doesn't quite take out the house up above.

But the river's carrying mud. It's carrying sediment. And the sediment starts to deposit out into the lake to fill it with a delta.

Now, rivers have to go downhill. So as the lake is replaced with the mud, if the river were to hit a perfectly flat spot like this, you know what it has to do. It has to build up so that it's headed downhill. And so as the river builds the delta out into the lake to fill the lake, why, you have to bury the house, and that makes the person who lives there mad.

Now, at the other side, it's even more interesting. There's no floods anymore, so the river loses the ability to carry gimongous rocks, which might make the homeowner happy. But the water coming out from the dam is clean. It has no sediment in it. And if there's sand below the dam, why, the water will start washing that sand away.

And that will do a number of things. Your house, now, rather than sitting there right next to the riverbank-- you'll look out some morning and you're ready to fall into a giant hole that's been cut. And you get out there and you fall and then you say, oh, no, and you're very unhappy.

You are not nearly as unhappy as the salmon that are trying to come upstream, because as you may know, salmon like to get really amorous around sand and gravel bars. And if the river has washed away the sand, then you can't have fun with your honey and then leave your eggs there to do well. And so you have a big loss in salmon, as well as getting people's houses unhappy. And so putting a dam on a river makes a big difference.

Credit: Dr. Richard Alley, Sridhar Anandakrishnan

How New Orleans Doesn't Work

How New Orleans Doesn't Work
Click Here for Transcript of How New Orleans Doesn't Work Video

Here's a great tree. And the great tree is standing up on a bluff looking down on the Mississippi River, which sits down in a valley. And the river has a little natural levee-- this, we haven't gotten to human built ones yet-- and it sits down in a valley.

And then, over on the other side, there's another natural levee, and then you go up another bluff to the top. And the river itself sits down in its river valley like this. And it is flowing towards you, which is what the head of this arrow shows.

And the river is sitting on a few miles thickness of mud that have accumulated over the years as the delta has built out into the Gulf of Mexico. And whenever you get a few miles of mud, everything is sinking under its own weight. And that, in turn, means that the surface is sinking, as well.

Now, nature has a way of handling this surface sinking problem, which is that during a big flood, the water spreads out over the flood plain. And mud falls out of the water, and the mud makes a new layer. And so as the surface sinks, more mud is added. And so there's no net change in the elevation. And that's just fine, except for one little problem.

Over here you have a city. And you've built this big city. And you really don't want that flood coming into your city. So you just call up the Corps of Engineers and you say, make a big wall, and make sure that that flood is not going to get into my city.

Well that's just fine, except that doesn't stop the sinking, because the thing is going down. And so if you come back later, what you'll find is that the surface has moved down to a place like this, and your city has moved down to a place like this. I'm using a darker line so you can see where it's gone to from where it was.

And so you tell them to make the wall bigger. But meanwhile, the city is sinking even deeper. And you've gotten up somewhere way down here, now.

And things are getting really nasty, because at this point, there's a really big storm. And it manages to get just over the top, and it fills your city with water. And then you're underwater and you're very, very unhappy. And this happened to New Orleans. And the sinking is not stoppable, so if it's rebuilt, it is likely that it will happen again.

Credit: Dr. Richard Alley, Sridhar Anandakrishnan


Dams cause huge changes on rivers, both upstream and downstream. In this film clip, Drs. Anandakrishnan and Alley discuss the Glen Canyon Dam and Lake Powell on the Colorado River. Huge changes were caused by this project, including in the Grand Canyon far downstream. The CAUSE 2004 class used some clever editing to manufacture a disagreement between the professors, who are much closer to being on the same wavelength than you might imagine by watching this.


Click Here for Transcript of DamIt! Video

Today we're going to go see one of the most extraordinary manifestations of man's desire to tame the wilderness.

We're looking, standing here above what was once a vast and deep and beautiful canyon and has turned into a wonderfully used lake that people like to go boating on and people like go swimming in. And so we've seen a great change in what happened here from a world that was used by very few who love solitude to a world that's used by many who love running around in motor boats.

Well, I think a lot of the early settlers really wanted to place their mark. They said, we are not going to be defeated by this land that has only a few inches of rain per year. We will live here. And the way to do it is to get lots of money from Washington that we collect out East and bring it out West here and build these dams. And these places just do not belong. They are magnificent creations. They are incredible engineering masterpieces. But they should not be here.

And so it obviously does a tremendous amount of good, and it's very clean. Once you have a dam, once the lake gets here, you don't dirty anything up. You're not running out smoke from your smokestack that will dim the air in the Grand Canyon.

They should not be here. And I think we've learned that over and over again. But you go to India, you go to China, and they're doing the exact same thing. The Three Gorges Dam in China, one of the most enormous, incredible engineering projects, displaced millions of people, is an absolute ecologic disaster.

If you've got hungry people, and you can save that water and ship it to them, it's food. If you've got people who need power to do things, and you can ship them the power then they can use it.

Dams are amazing human achievements, but they have incredible costs associated with them. So to me, it's very much of a bittersweet kind of a thing. As a former engineer myself, I can appreciate the artistry and the mastery that goes into building these. But as a practicing geologist, as a practicing resident of this planet, I find them a little depressing to be quite honest.

We're going to argue this one again on a lot of rivers in a lot of places for a lot of time.

Credit: Dr. Richard Alley, Sridhar Anandakrishnan

Human actions are more and more affecting the amount and quality of water and the ground, with effects that bounce back on us and on other living things. Here, Dr. Alley chats about groundwater issues of great concern to the National Park Service at the Grand Canyon.

Canyon Groundwater / Grand Canyon National Park

Canyon Groundwater/Grand Canyon National Park
Click Here for Transcript of Canyon Groundwater Video

So now we're on the rim of the canyon. The rim of the canyon does not have streams on the surface. When it rains, which is not all that often, but when it rains, what happens is that the water soaks in. So if you had a rainstorm, you see the water start to make a stream, but then it soaks in. What happens to that water? It goes down through the spaces in the rocks until it hits a rock that it can't get through, such as the Bright Angel Shale. Then it runs along laterally and it comes out at those beautiful springs that we see in the canyon.

Now what's happening here is there's more and more people want to live here, and they want to have water, so they drill wells. And after you drill a well, you suck the water out and you use it to water your grass, or your crops, or your golf course, or to drink or flush or what have you, and eventually it evaporates. And so what you're doing is removing the water that should be running to the springs. And there's worry-- it's not happening yet, but there is worry-- that someday, we may start to lose the springs in the Grand Canyon and the unique biota, the wonderful ecosystems, that live down there because of development outside of the park up here.

Credit: Dr. Richard Alley, Sridhar Anandakrishnan

Want to see more?

Here are some optional resources you might also want to explore! (No, these won't be on the quiz!)

Join Dr. Alley to learn about formation of Fossil Fuels, in Barataria Reserve, Jean Lafitte National Historical Park and Preserve, Louisiana, from the PBS television special Earth: The Operators’ Manual.

Deltas and Plumes
(An extensive collection of animations on this subject)

River Systems: Process and Form
(An extensive collection of animations on this subject)

Processes of River Erosion, Transport, and Deposition
(An extensive collection of animations on this subject)