GEOSC 10
Geology of the National Parks

Dams and Deltas

Dams and Deltas

 

The Glen Canyon Dam

The Glen Canyon Dam
Image by Brigitte Werner from Pixabay

Video: How Dams Work (2min, 27sec)

How Dams Work
Click Here for a Transcript of the 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: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Dams are generally built to influence water, but as noted on the prior page, dams always influence sediment, too. An important example of a dam influencing sediment as well as water, and sediment influencing many other things, occurs downstream of Canyonlands National Park and upstream of the Grand Canyon along the Colorado River. The Glen Canyon Dam was built on the river in the 1960s. The dam stopped floods coming down from the Rocky Mountains through Canyonlands. Water from floods that had raged through the Grand Canyon is now stored in the reservoir and then released gradually. Several things began happening to the Colorado River once the dam was completed. The dam traps the sediment carried by the river, and releases clean water, so the reservoir is filling with sediment, and in a few centuries or less will be full. Unique fish species that thrived in the muddy waters of the Grand Canyon suddenly are much more easily visible to predators and thus easier prey, often for introduced species, so many of the native species are endangered and disappearing. Evaporation from the large surface of the lake trapped by the dam removes water from the river, increasing the scarcity of water downstream.

In this “straight” river mostly flowing over bedrock, floods in the past had washed sand into corners along the river to make sand bars. This stopped when the floods stopped. However, the clean water released by the dam still flowed fast enough to remove some sand, so the existing sand bars below the dam were slowly washed away. But, many types of wildlife depend on sand bars, and thus were harmed—cottonwood trees had grown in the sand, birds lived in the cottonwoods, and deer could drink from the river by standing on the sand bars but not from the rocky cliffs. Floods on un-dammed side streams continued to dump large rocks into the Colorado, but the Colorado lacked the high flows to move this material onward, so the rapids in the main river at the mouths of the side canyons began to steepen.

In the spring of 1996, efforts began to rebalance the system by releasing artificial floods from the dam. The first flood was a partly successful experiment, rolling some of the big rocks out of the rapids at the mouths of the side streams, freeing sand trapped beneath, and putting some of that sand into bars. But, those bars weren’t very big and didn’t last very long. Additional human-made floods have been released more recently, timed to occur when natural floods coming from side streams were delivering additional sediment, to help make bigger sand bars. Additional attempts may be made because these artificial floods really have helped. Such human-caused floods cost money (lost hydroelectric power when extra water is routed around the hydroelectric plant in the dam to get water into the river in a hurry) and require lots of planning (you need to warn people camping or hiking in the Canyon before you suddenly flood them out!), and may be stopped during times of drought when other uses for the water are considered to be more important.

Video: How Deltas Work (2min, 23sec)

How Deltas Work
Click Here for a Transcript of the 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: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Meanwhile, as sediment fills the reservoir, sediment will also accumulate along the river upstream of the reservoir. For the tributaries to Lake Powell behind the Glen Canyon Dam on the Colorado River, there aren’t many people living in places where this sediment accumulates, but if there were, their fields and houses would be buried by mud. A river slows down as it enters a reservoir, or any other lake or the ocean, and sediment is dropped from the slowing water. Unless strong waves and currents in the reservoir or ocean take that sediment away, a pile called a delta forms. But the delta cannot be perfectly flat on top. If it were, then the stream would drop its load when it hit the flat spot and slowed down, and that would raise the flat spot. So, as the delta grows into the lake, the upstream end of the delta must build up so that the river still flows downhill, and that, in turn, will cause sediment to build up for some distance upriver (see the figure below).

Diagram showing elevations measured along a river. Explained in text
Diagram showing elevations measured along a river. The original stream bed is shown at (1). After building the dam and filling the reservoir (2), sediment begins to fill the lake, building a delta into the reservoir and raising the elevation of the stream bed upstream of the reservoir.
Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

When two dams were built between 1910 and 1926 to supply hydroelectric power from the Elwha River, which flows north from Olympic National Park in Washington state, the dams caused major problems because of their effects on sediment and water, and wildlife. The dams were built without ways to allow passage of salmon farther upstream to spawn. Most of the salmon had spawned upstream, but some had spawned in sand and gravel downstream of the dams, and the river washed that sand and gravel away after the supply of new sediment was blocked by the dams. The annual “flood” of more than 300,000 salmon that returned to the river before the dams were built turned into a trickle of barely 3,000 salmon. (Instead of building fish ladders to allow salmon to move around the first dam and continue upstream, a fish hatchery was built, but the hatchery was quickly abandoned.)

After the river washed away the sand and gravel in which the salmon had spawned, the river quit delivering sediment to the beaches of the Strait of Juan de Fuca (an arm of the Pacific Ocean). Those beaches then washed away. Without the beaches, the native peoples were no longer able to carry on their traditional shellfishing because the shellfish were lost after their sand-and-gravel beaches were lost. The nearby harbor of Port Angeles had been guarded by sediment-fed sandbars that also washed away, requiring more money to be spent on human-constructed protection for the harbor. The little bit of hydroelectric power being produced did not come close to covering the costs of all these damages.

In an ambitious plan to help the port, the people, the beaches, the river, the salmon, and the park, the federal government purchased the dams to remove them.

Beginning in 2011, both dams were removed. Within a few months after removal, salmon were returning to the river. The river, beaches, harbor and more will take a while to get back to "normal," because so much was changed by the dams, but there is much optimism about the recovery, which is proceeding rapidly. You can see the amazing changes at the mouth of the river in the pictures below.

Conflicts such as these between dam-builders and those who prefer free-flowing streams are not new. You don’t need to know these details, but it might interest you to learn that In 1731, a mill dam on the Conestoga River near modern Lancaster, Pennsylvania was torn down because it was ruining the fishing industry, which, in a petition in 1763 to remove other dams on that river, was said to include shad as well as salmon, rock fish, and trout in tributary streams. In 2018, a dam on the Neuse River in Raleigh, North Carolina was removed to allow natural fish runs of shad as well as striped bass and Atlantic sturgeon.

 

before and after pictures of the river post dam removal = more land visible where the river used to be
Change in the mouth of the Elwha River following dam removal.
Credit: National Park Service (NPS) (Public Domain).

Video: Dam It! (2min, 44sec)

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.

DamIt!
Click Here for a Transcript of the 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: S. Anandakrishnan and R. Alley © Penn State is licensed under CC BY-NC-SA 4.0