Geology of the National Parks

Video Lecture


The Unit 6 lecture features Dr. Sridhar Anandakrishnan and is 1 hour and 4 minutes long.

Click for a transcript of the Unit 6 Lecture

Good morning. Welcome to G-SCI 10, Geology of the National Parks. We're going to be looking this time at tearing down mountains some more. Last time, we broke them apart using freeze-thaw action, and by chemical action, and made soils. This time we're going to talk a little bit more about the transport of them-- streams and rivers.

My name is Sridhar Anandankrishan, and I'll be your guide through some of the lovely places. And we're going to take a little bit of a detour to look at some of the damage that water can do. So, let's take a quick look at some lovely pictures and then we'll start on the section.

Redwood National Park on the coast of California. You can go fly into San Francisco and drive up the coast from there. And it's just magnificent trees. These trees, there's a famous one that they cut a hole in it and you can actually drive a car through it. These trees are monsters. They're as big around as a small house. And they tower up in the sky. And you can imagine you need a lot of water to keep a critter like that alive, and you do.

Just go over the mountain. Not very far. Maybe a couple hundred miles. Head inland from Redwood and it's dry as a bone. It's Death Valley National Park. Hottest, driest, lowest place on the planet. Why is it that these two places just very close together, a couple hundred miles apart. Redwood National Park over here with these gorgeous, enormous, towering Redwoods that suck up all this water, this huge root system that needs lots of water to keep it alive, cool climate, rainy climate. 200 miles to the east, you go over the mountain, and you're at Death Valley National Park, it's dry as a bone. What's going on? So that's the first thing we're going to look at.

Let's take a look at some pictures over here. Sequoia sempervirens, the ever-living Sequoia. They live for a long time-- couple thousand years, and then they'll fall over-- kawoomph. And then the new trees will sprout from the trunk. And so when you go there you'll see these lines of trees. They weren't planted that way. It's just that when an old tree falls down, a new one comes up from the trunk, and so they'll be a whole line of them where the old one had fallen down. It's quite an extraordinary sight.

All these ferns and rhododendrons, all of that, surrounding the base of them. The Redwood needles help produce these very acid soils that the rhododendrons really like around here. We have that as well up on the tops of the mountains, these beautiful rhododendron forests. They can tower way up, 200, 300, almost 400 feet up into the sky. They are the world's tallest trees. They are the world's most massive organism. The heaviest single living thing.

The Redwood Coast is rainy and foggy. These trees couldn't make it otherwise. And you go inland a little ways and at the same latitude, you've got this amazingly dry landscape. This is a picture pulled straight out of our very first section a long, long time ago on Death Valley National Park.

Now we're going to look at the surface. Before, we were looking at Death Valley to understand something about pull-apart zones, about spreading, all these other tectonic forces. This time, we're going to look at the surface. Why is it dry at the surface?

Here's a map. The Pacific Ocean is to the bottom, North is to the left on this map, not at the top as it normally is. And you can see this coastal Redwood range all up and down the coast. And then, inland you have Death Valley. And, in between, you have the Sierra Nevada mountains, and that's the key. You have these tall mountains in between, and they intervene and somehow prevent the water from getting from the coast inland to Death Valley. So, let's take a look at that.

The first thing we're going to do is look at winds, and I'll come back to this map in a second. Let's go to the drawing tablet here, and we'll take a look at why the Redwood Coast is wet and cool, and the Death Valley is dry and hot.

We're talking about wind and water this time. We have the Pacific Ocean. We have the Sierra Nevada mountain range. And we have, on the left, these towering Redwoods. And on the east, we have Death Valley. Hot, dry, cool, wet. What's going on? Why is the case?

When the winds blow in from the Pacific, they carry water. Water evaporates in the Pacific Ocean, it blows on shore onto land in California, and it hits the Sierra Nevada mountain range. So, these winds have lots of water in them. The winds are heavily laden with water. They blow in from the ocean, they blow onto the coast, they start heading inland, and they hit the Sierra Nevada ranges.

When they do that they have to start climbing up the mountainside to get over. They can't just tunnel right through the Sierras, and there aren't enough passes for all of that air to go through these low passes. So, the winds as they come along have to rise up the mountain, have to go up to get down on the other side.

As they do that, as the air rises, it cools off. So, as this air rises up to the top of the mountain, it cools off. Cool air can hold less water. You just can't have as much water in the air as that air starts to cool off. What's the result? Rain. As that air is climbing up the side of the mountain, it's cooling off, it can't hold all that water. That water's got to go somewhere, and it falls out as rain. And so, you have all of this rainfall on the windward side of the mountain.

And by the time the air gets to the top of the mountain, all of the water has been squeezed out of it. You take that air, you cool it off, cool it off, cool it off as it rises up, and as it cools off, the water rains out of it, rains out of it, rains out of it. It gets to the top, it's cold and it's dry. It doesn't have any water left in it. It gets to the top, starts to sink, but it is dry. There's no more water left in it. No rainfall.

So, you get no rainfall on the backside of these mountains or what's known as the leeward side of these mountains because all of that water has fallen out on the windward side. This is the rain shadow effect. And this explains why it's so dry in Death Valley. Very simple process. It's you've got this nice, wet air coming in from the Pacific, but all that water gets dumped on the windward side of the mountain, and you end up with it being really dry on the leeward side or on the rain shadow side. That still doesn't explain why it's so hot over there in Death Valley, and that's the next thing we'll look at.

Same picture. Pacific. Sierra Nevadas. Redwoods. Sequoias. Here's that same picture. You've got air coming in off the Pacific, it's heading up the slope of the Sierra Nevadas, its cooling off. We talked about how all the water gets squeezed out. It's dry when it gets to the top of the mountain. And now it's sinking down the other side, and as it sinks it'll just heat up naturally. As you get to lower and lower elevations it gets warmer and warmer-- you know that. When you're at the base of the mountain, it's warm. As you go on a hike and you start climbing up the side of this mountain, it'll start to get cooler and cooler and cooler. Until finally you get to the top, and you've got put on your jacket.

Same process as you go down. As you head down the slope of the mountain, you start shedding your jacket, and then your sweater, and then you're in shorts and tee shirt by the time you get to the bottom of the mountain. So, this very natural process. It just gets cooler as you rise, and warmer as you drop. The one difference, though, is that the cooling off as you go up the hill is about three degrees Fahrenheit for every 1,000 feet that you rise up.

So, if you were to take a thermometer and go for a hike and go up Mount Nittany, and as you head up there, you would find that if you climb 1,000 feet, the temperature has dropped about three degrees Fahrenheit if the air is moist. If you have really wet air, if you have really moist air, lots of water in the air, very humid day, and you start to walk up, it'll cool off about three degrees Fahrenheit per 1,000 feet of rise.

On the other hand, as you head down on the rain shadow side, the leeward side, the Death Valley side, the air is heating up. Same way it was cooling off as you went up, now it's heating up. But now it heats up by five degrees for every 1,000 feet of drop. So, every time on the rain shadow side you have 1,000 feet of loss of elevation, the temperature's now gone up by five degrees. And you walk down another 1,000 feet, the temperature's gone up another five degrees if the air is really dry.

So now, we have this very dry air on the rain shadow side and it's heating up faster. And so, you drop down the same distance that you climbed up on the other side of the mountain, and you've heated up way more than you cooled off. And that's all the difference. When you take a packet of air and you raise it up to the top of the mountain, and then you remove all the water from it in that process, and then you bring it down on the other side, you've heated up way, way more than when you started out.

And so, this is why Death Valley is hot. You start out cool on the coast, you end up hot inland. There's an animation online, and you can take a look at that and review that again. Let's go back to the PowerPoint.

Where do these winds come from? We kept talking about these winds blowing in from the Pacific Coast. Why do you have winds coming in from the Pacific Coast? It's a fairly straightforward system. The sun heats the Earth, and hot air rises. That's the starting point for this whole process.

If you took one thing away from the first third of this class, it was the heat deep inside the Earth drives these convection cells, drives all of the mountain building. Take one thing away from this section, it's that the heat of the sun pounds down on the Earth, heats up the air, and that's what makes the winds, and then the winds do everything else. So let's take a look at that process.

Here we have a map of the Earth showing the places where you have these convection cells in the atmosphere. We're not talking about convection in the Earth anymore, but now convection in the atmosphere in very much the same way. You heat up the air, it rises up, cools off, and there's more heated air that's coming up from underneath it. And so this cooled off air gets shoved off to the side.

Let's take a look at this. We're going to go back to the drawing tablet. This is just so much better diagram, so you can keep this map in your head as I'm drawing my very crude pictures, that would be helpful, and then we'll come back to this.

Air circulation is driven by temperature, by heat-- same as convection in the Earth was. But the heat of the sun drives air movement. The equator is warm. The pole is cold. And the reason for that-- this is the Earth, this is the equator, and this is the North Pole where Santa has his workshop. The Earth is spinning around and around. And the Sun is somewhere way over here sending sunlight to the Earth.

If I were to go and look at a chunk, a square of sunlight, as it flies from the Sun to the Earth, I would find that it would smack straight into the equator, head on. The square, I just took a square piece of sunlight from the Sun and I carried it to the equator, and it went straight over there, and it ran straight into the equator.

If I did the same thing at the pole, I took a square of sunlight and ran it towards the pole, I would find that it would be spread out over a huge chunk of the pole. It would be a small part of the equator. All of that Sun is beating right into that small part of the equator because it's head on. But at the pole, that same chunk of sunlight is smeared across a huge chunk of it simply because of the curvature of the Earth. The Earth is curved in such a way that when the sunlight hits it, it hits the equator head on, but it smears across the pole, and so the poles get colder. They get less sunlight.

It's as if you were to take a piece of toast and take a spoonful of peanut butter and spread it across that. Or if you were to take that same spoonful of peanut butter and try and spread it across a huge sheet of plywood, you wouldn't want to eat that plywood. But there wouldn't be very much peanut butter on that sheet of plywood because you've taken the same spoonful of peanut butter and tried to smear right across this huge sheet of area, and there's very little peanut butter on any given spot.

Same thing happens on the Earth. The Sun is beating down on the equator head on, and so, all of the Sun's energy is concentrated in a relatively small area. It's nice and hot. Up at the pole, the same amount of energy's spread across this huge area, so every spot is cold and shivering. That's why the poles are cold and why the equator is warm.

What happens? We'll go back to the Earth, we'll make it bigger now, we have the equator, and we have this heat along the equator. The equator's being heated up because the sun is beating down straight on it. All of the energy is being jammed into a relatively small area.

The air gets warm. And what happens when things get warm? You know that. We've seen it over and over again. When things get warm, air gets less dense. What happens when things get less dense? We've seen that over and over again. Rises. Air rises up, hot air rises. And so all around the equator you have hot air rising upwards into the atmosphere.

As it rises up, somewhere up here, it will cool off. As it rises up it will cool off, but there's more hot air coming up from below.

And so, this cool air can't just sink straight down. This cool air gets shoved off to the side. And eventually it sinks down. When it gets to the bottom, it's heated up again. As it sinks down, it gets hotter and hotter and hotter. And then, it returns to the equator, and the whole cycle begins again. It heats up, goes back to the equator, rises back up, and you have a convection cell.

This whole process is a convection cell in the atmosphere. And this is wind. It's movement of air over the surface. What's that? That's wind. So, this wind is driven by rising air in the equator. It goes up, cools off, sinks back down, and then blows back to the equator. Rises up, goes out, sinks back down, blows back to the equator. In the process of blowing back to the equator, that's wind. Air transport by that.

Let's go back to the map that I showed you on the PowerPoint because that illustrates it so much better than this rather crude diagram. But I hope you get the idea from this.

You're seeing the same thing here, but just more fancily drawn out. This is an image from the USGS. Rising warm, moist air produces what, in this diagram is called a Hadley cell. It's just a convection cell. It was discovered by a scientist named Hadley, so they called it a Hadley cell. Rising warm, moist air. Descending cool, dry air, it gets cooled off, it falls down, and then it rotates back around, blows through the mid-latitudes back to the equator.

And so you have what are called the trade winds. They were called the trade winds because they blew so steadily that sailing ships could use them for trading purposes. They always knew these winds would be there when you needed them, and you could count on them. And so, you could fill up your ship with whatever you wanted, and your sailing ship with these big sails would catch the trade winds, and would be blown from Europe to the new world without any trouble at all. That's why they were called the trade winds.

The North Pole is cold, and you have these secondary Hadley cells, or secondary convection cells, called the Ferrel cell and the polar cell. But really, the idea is the same.

This is a map of the world's deserts. And if you look at them, what you see is you have these deserts at a few degrees north and south of the equator where that convection cell brings that air down again. The air goes up and then it comes down. Where it comes down, you get these deserts. Why? Because there's no moisture in that descending air. All of it is gone. So, rising moist air. You have all this rainfall at the equator because all of that air is rising up, and the rain's all falling out of it.

As it gets colder and colder and colder, the rain's just falling out of it. It goes up there, it goes over, and it comes down in the Sahara. It comes down in Australia. And there's no water left in it. It's all gone.

And so, you get these huge deserts, the Saharan Desert, the great Sonoran and Chihuahuan deserts in the US and Mexico. You get the great Australian deserts, and the Kalahari, and the Namibian deserts in Australia and Africa. It's because of these convection cells.

So now, we've talked about wind, and we've talked about a rain shadows. Now, let's take a look at what happens with that water when it falls out of the air and it makes rivers. You get rain, you get rivers. Let's take a look at those.

Water falls from the sky. It's called rain. Very straightforward. What happens to it when it gets down is far more complicated. About two thirds of it goes straight back up into the air. It mostly is taken up by plants and trees through their root systems.

So, here in State College we get about 40 inches of rain. That's about something like that. Every year. Some of it comes as snow, some of it comes as rain. If I were being pedantic, I'd say we get 40 inches of precipitation, but think of as rain. Two thirds of it, about that much of it goes straight back up into the air. The plants drink it, it goes straight up. The trees drink it, it goes up through their roots, goes out through their leaves, straight up into the air again. Or it just falls down and just collects in puddles and then the Sun comes out and those puddles evaporate and it goes straight up into the air again.

About a third of it, about this much, either sinks down into the Earth and turns into groundwater, or it runs off across a surface, falls into Spring Creek, falls into some other creek, eventually makes its way to the Juniata, the Susquehanna, and then down to Delaware Bay, and then out to the ocean, or to Chesapeake Bay and out to the ocean.

So, about a third of it, about that much actually remains on the surface of the Earth, two thirds of it goes straight back up again. This is known as the hydrologic cycle. Fancy name. All it means is you get evaporation in the ocean, it blows onto land, it rains out because of the processes that we talked about, rains out of it because it cools off as it goes up a mountainside. It can't hold all that water, and you get rainfall. Some of it goes straight back up again, and then maybe rains out again and it goes straight back up again and rains again. And some of it gets trapped in the Earth as groundwater, and some of it flows into streams and rivers and goes to the ocean.

Water is unique to Earth, we think. Everybody's always looking for water in all the outer planets. They go to Mars, they want to look for water. They go to the moons of Jupiter, they want to look for water. They go to the Moon and they want to look for water. It's so cold on the Moon, the water probably is almost certainly in the form of ice.

The key is life needs water. If you're going to have critters, if you're going to have any kind of organisms alive, we think they've got to have water. Water is so important to everything, for life to exist, that people don't look for living organisms on Mars, they just look for water. And they say if we find water, I'll bet you there'll be some critters alive there. And that's probably the case. Anywhere on this planet you go and you look for life, you look in the places where the water is and you'll find something alive there.

The human population of this planet is exploding. We're up to six billion or so, and we're heading towards 10 billion in the next 100 years. We all need water. And all the great wars that will come, a lot of the wars that are going on now are probably, to some extent, about water. People need water, and they'll fight each other for it. So, we need to be careful about watching what we do with it.

Here is Johnstown. Here is Johnstown. The great Johnstown flood. It came through, and it smashed in and destroyed-- there was a dam up river from Johnstown. It was a private dam that collapsed, and all the water behind it took Main Street here, nice, lovely place, some horses going down it, and all of these trees, enormous trees, rammed into the second story of this building. So, water is not a benign material. It can be quite destructive.

It's now the summer of 2006 when I'm filming this material, and just recently, just a few weeks ago, Wilkes-Barre, Scranton, the whole Northeast of Pennsylvania, Elmira, Binghamton were all flooded. These enormous rains came down and flooded out all of those rivers and they rose up over the levees. Luckily, the levees held and so people weren't killed, or not that many people were killed, but it was still quite disastrous.

These streams, when they make their way down, they modify the landscape. They dig holes, basically. Streams are very good at eroding their bed. Streams carry water. Duh. We all know that. But streams also carry sediments. Sediments are simply small pieces of rock, or sometimes bigger pieces of rock. And in the case of streams in the mountains, really big pieces of rock. So, sediments are simply pieces of rocks, and streams and rivers carry that along with the water. And they're really good at washing it away.

And so, all those processes that we talked about last time, mass wasting, a freeze-thaw breaking up all these rocks, you put a stream in there, and it'll just take that material and move it away. And then, these freeze-thaw actions and mass wasting actions will bring more material down to the river valley, and the river will wash that out. And then, more material will come down and that will get washed out.

And you do that for long enough and that mountain is gone. It takes a long time, but eventually it'll be gone. Without the river, you wouldn't have been able to wash all that material out, and so the valley would have filled up, and the mountain would still be there. With the river, you just wash the stuff out, more comes in.

Floods do most of the work. The normal flow of the river just isn't powerful enough to move much. The normal flow of the river will carry a little bit of sediment, but it certainly won't move big boulders. It certainly won't break things up. But then, every so often, just about every year usually, you get these floods, and every 10 years you get big floods, and every 100 years you get these enormous floods, and they're the ones that do the work. They're the ones that move enormous masses of material downstream. We use it. We need it. We've got to have it. We need it for agriculture, we need it for industry, we need it for moving material up and down the rivers, we need it for making hydroelectric power.

So, water is enormously important, and controlling streams has been probably one of the primary industrial motives or the things that we do most of all down through human history. If you look at human settlements, they were always on the banks of rivers because you can get water out of them to water your crops. You can use them to run mills to mill your grain. You can build boats and you can go up and down and trade with your friends up and down the river. So, all human settlements have always been associated with rivers.

Here is Glen Canyon Dam in Arizona. And the river is on the lower part there, the Colorado River is flowing down. And if you were to simply imagine that lower picture, that narrow deep gorge heading up, that's what it would have looked like 100 years ago. But back in the '30s and '40s, that dam was built, and when that dam was built, the flow of the river couldn't get past that dam. It got caught over there. And now, that lake that's above the dam is very useful. People use it for recreational purposes. But they also use that dam for hydroelectric, they generate electricity. That dam is used for irrigating crops all through that area.

So, the hydrologic cycle, as I said before, as water evaporates from the ocean it rains onto the land, and some of it goes into streams, some of it goes into groundwater, but most of it goes straight back up into the atmosphere. Let's follow one of those streams to the ocean, and let's take a look at what happens with it.

Streams are driven by gravity. Just like everything else, water wants to head downhill. And water can head downhill a lot more effectively than rock can. Because you get the soil or this rock up on a hill slope, and it can be nicely cemented in place. It can be happy where it is, especially if it's got lots of trees and grass that's stabilizing the slope. That slope can be fairly stable. You get rain, the rain falls on it, that water's going to head downhill. There's nothing that can hold water back, except things like dams.

Streams are recharged by direct rainfall, and by groundwater springs. So, rain can fall straight into the stream, or onto the banks of the stream and flow into the stream. Or the water can fall on the land far, far away, sink down into the Earth, and then slowly make its way through the earth as groundwater, and then these springs will come out and recharge the streams. And that's why rivers run even after long periods without rain.

Streams carry sediment, and mass wasting delivers sediment to the rivers, and then the rivers carry that sediment out usually in these big floods, but also during the normal processes. When those streams get down to the flat areas where the stream is falling more slowly, than that sediment gets deposited. So, up here in the mountains, mass wasting is bringing material into the stream, the streams are flowing through these very steep valleys, and so they carry lots and lots of water, lots and lots of sediment. And then, they flow out onto the plain out in front of the mountain. The streams slow down because they're going in more shallow slopes. As they slow down, the sediments can't be carried by these streams anymore, and they deposit them.

This is how a stream carries sediment. This is a cross section through a stream. The surface of the stream is at the bottom. And the top of it is where the stream is flowing most rapidly, and the bottom of the stream is where it's flowing less rapidly. And it can slowly move material either by what's known as suspended load, which are really tiny particles that are just floating up in the stream. And if you look at a stream and you see it's brown, it's because it's got all of this junk in it.

Water's blue, you know that. And normally a stream, if it's flowing slowly, is blue. But after a big storm, after it's churning and it's carried all this junk into it, all the sediment into it, it's flowing fast. You look at it, it'll be brown. And why? It's because of the suspended load. All of this dirt basically that's suspended in it-- soil that's suspended in it.

Down at the bottom of the stream, you have what's called bed load. This is the material that bounces along the bottom. The water isn't flowing quite so fast at the bottom, and so it can't carry all this material right up in it, and so it just picks it up and drops it, picks it up and drops it, picks it up and drops it, and stuff bounces along the bottom of the stream bed.

And then, you have bigger boulders, or gravel sized pieces of rock that are down at the bottom, and those usually don't move that much until you get one of those big floods. And then, all those pieces along the bottom will be washed along and moved downstream. Suspended load is the fine stuff, bed load is the bigger stuff that bounce along the bottom breaking up the rocks as they go making smaller rocks. So, this is another way that you have mechanical disintegration. These pieces down at the bottom, you have these rocks bouncing along, and every so often one will hit another rock and break it open. Or it'll just simply scrape off the bottom of the river and break off small pieces that will then be carried away with the river.

Two types of streams. Meandering streams-- this is a classic picture that you have, and we'll see some pictures here where the stream goes around one way and then comes back. Streams never go straight, they always have these big curves to them. If a river has mostly fine sediments, it'll make a deep channel, it will get clogged, and it will just meander. It will stay in its channel for the most part. It will keep breaking out or eroding along the outside of one of these curves and depositing on the inside of one of these curves. And so, the curves will get bigger and bigger, and we'll see a picture of that in a second. And then, it will make another curve and another curve. And here's a photograph of that.

So, you've got the stream that's coming down, and when it goes to the outside of the curve, it will have to speed up to make that curve. And then, it will keep eroding against what's known as a cut bank. And then, on the inner part is where the water's flowing more slowly and it will deposit material there. And so, the river over time will keep migrating inwards.

Eventually, eventually, that neck could get pinched off completely, and that ox bow will be cut off, and you get these ox bow lakes where the river has been cut off from its upper curve and now the river is just going straight. It used to meander around that curve, but over time it just cut across that curve and went straight. And that stranded piece of the curve is called an ox bow lake. It's named after these big yokes that the farmers used to put around oxen, and then that yoke would be hitched up to a cart, and then these oxen would plod along dragging the cart behind them.

A braided stream is a second kind. This is where you have lots and lots of channels that are all intermingled, and you don't know which is the main channel. They all are carrying a little bit. And what happens is you have all these big chunks of rock. They fall into the river and they block a channel, and so the river picks a different one. And then a big chunk of rock falls into that one, it gets blocked, and the river picks a different one.

So, you need big chunks of rock. Where you find big chunks of rock? In the mountains. So, it's mainly in the mountains that you have these braided streams, and it looks like a braid of hair from above, which is where the name comes from. And here's a photograph. You have all of these different channels that you don't know which one is the main one. And we'll go to another picture here. Mainly in mountainous areas where there's lots of big boulders. And one of these channels could get blocked very easily, and the river will pick a different way to go.

Another way to stop the flow of river is to put up a dam. Beavers build dams, natural landslides can create dams, or you can go out with a bulldozer and a great pile of concrete and you can simply stop the river. And that will stop the water temporarily until you get a lake built up. But, eventually, the lake will rise to the height of the dam, and then the water will start to flow again. The engineers will open the valves and the water will flow, but you'll still have a lake behind the dam.

So, the water stoppage is temporary, the sediments get stopped forever. Engineers don't like to have sediments going through their machinery, it clogs them up. And so, they pull water from the upper part of the dam. And all these sediments that we've been talking about, they come down with the river, they don't make it past the dam. They get deposited behind the dam, more sediments come along, deposited, more sediments come along and get deposited. The water keeps going through, but eventually the sediments are going to fill up that dam.

Here's a picture showing that. You have a dam, and over time, the water is filling up behind it, but along with the sediments are filling up. And that deposition is going to slowly, over time, completely fill up that lake, and then you're left with a useless dam, and you're going to have to build another one. And it's happening to many dams around the world. Dams have been-- some of these big dams have been in place for 100 years or more, or 50 to 100 years, and they're starting to fill up. People go out there, they dredge them. They have big machines that try and suck up these sediments. But in the end, these dams are going to fill up.

The river below the dam is clean of sediments. All the sediments have been dumped into the lake above the dam. And so, the river below the dam doesn't have any of those sediments left. Lots of water-- remember, the water's going right through. But that water that's coming out is clean. It doesn't have any sediments in it. What's it going to do? It's going to pick up sediments.

And so, you get extra erosion below the dam that you didn't used to have before. Before the dam was built, the river would come down, sure, there was lots of water coming, but there was also sediment coming. And so, if there was any erosion, there would also be deposition. And then, that was that most of those places would continue to have these gravel and sand bars. You build a dam, all the sediment's up there. You get this clean water rushing out past the dam, and it will enhance the erosion.

So, let's go to the drawing tablet and take a look at that process. This is a cross section of a slope, of a hill slope. Then you have a river that's happily slowing down. So, this is water that is flowing down. And remember, in addition to that, it has sediment that's caught up, and that's being transported down along with the river.

This is the case before I come along and I build a dam. It's a big old concrete wall. As soon as I do that, I put a lake here. What I've also done is I've done away with all the sediments downstream of that dam. All these sediments still coming down, all those red particles still coming down, but they're now filling up the lake upstream of the dam.

Downstream of the dam, I have just this lovely clean, blue water rushing along, and what's it going to do? It's going to start picking up sediment from the riverbed that it would not have picked up before. Before, it had all these sediments it was already carrying. It couldn't carry any more sediments. And so, it left the riverbed alone. But now, it's bright, blue, clean water, no sediments in it, it has lots of ability to carry sediments, and so it does.

Over time, you will erode downwards. Erode below the dam. This would not have happened until you put the dam in. So, you get deposition above the dam, and erosion below the dam.

Back to the slides. These gravel and sand bars below the dam get washed away. Lot of critters live on those bars. They don't have a place to live anymore. All of the beautiful, blue, clean water that's rushing out, now there's all these fish that used to like having this warm, dirty water-- dirty because it had soil in it to live in, and now they're living in this bright, blue, clean water, and they don't know what to do anymore.

They've evolved to hide in this from their predators, and they can't do that anymore. So, there's a lot of fish that go extinct because of that. Eventually, the reservoir above the dam will fill up and you need to build a new one anyway.

Let's follow that river past the damn, past the mountains, out on to the Gulf Coast. So, this river has just flowed from the mountain down onto the plains, and across the plains to the ocean into New Orleans. And we're going to talk about Hurricane Katrina. There are all these levees protecting New Orleans. Why does New Orleans need levees? It does. We'll find out why.

And one of those levees broke, the 17th Street levee, and all this water came rushing into New Orleans because New Orleans is below sea level. Why is New Orleans below sea level? We'll find out. It's below sea level, the levee broke that was protecting the city, and the water rushed in, and you know the disastrous consequences of that.

So, what's happened? Every year, snow melt comes along, extra rains come along in the spring, the rivers can't carry all of this. The rivers come along, and the water goes over the bank and on to the land on either side of the river and it floods. It happens. It happens every year. And it carries water and it carries sediment out onto the fields on either side of the river, and it deposits it there. Why? The river's rushing along, carrying sediment, it floods out onto the sides, the water spreads out, slows down, sediments can't be carried if the water's moving slowly.

Sediments will slowly sink down and be deposited onto the field. The farmers love it. The Nile Delta famously, pyramids, the Sphinx. All that civilization 3,000 years ago depended on the Nile flooding every year bringing this lovely sediment up into the farmer's field and depositing it, and then they could plant their corn and wheat and-- probably not corn. I don't think they had corn. Wheat, and grow it and store it.

And the lands and the farmers fields didn't get played out, they didn't get old because every year there was new material. When the Nile flooded, it would bring new material onto their lands. People built houses by these rivers, and they don't like the river coming into their basement. They don't like it coming into their first floor. They don't like it coming into their second floor sometimes if the flood is big enough. And so, they build levees. They build walls that keep the river from flooding.

"The Big Easy," New Orleans was built on mud from long ago. From when the river used to flood every year. Every year, the river would flood and put all this mud out onto the plains around the Mississippi, and New Orleans was built on that mud. And then, we came along, we put the levees in, and now we don't get anymore mud. But New Orleans was built on this mud. The water is slowly being squeezed out of that. And as it's squeezed out, New Orleans sinks down. It sinks lower and lower and lower. The levees are still there. And now, New Orleans is below the level of the river.

Let's go to the drawing tablet here and take a look at that sequence. I'm going to try and draw a river in cross section. And it's flowing down like this. And on the banks of the river, you've got trees and houses and tractors-- that's supposed to be a tractor-- farm fields. And so, you have everybody happily living their lives over here.

Once a year or a a few times a year, this river will flood. And water and sediments will spread out all along the banks. The sediment will get deposited as a layer on either side. So, you've got this layer of sediments on either side. And the water will slowly seep away or evaporate or whatever happens.

And the next year, you'll get a second layer on top of that. And the next year, you'll get another layer of sediments on top of that. And the next year, you'll get another layer. Now you don't build up to the sky because as you add each new layer, where the older layers sink down. So, the level actually remains the same. So, older layers sink down, new layers are dumped on top of them, they sink down, new layers are dumped on top of them, they sink down. The reason they're sinking is the water is being pulled out of them. Water is being squeezed out. And the soil is compacting.

And we go to a new page here. So, we're going to take a look at that same picture again. You've got a river, and there's the river flowing down. And on the side here, we have layers and layers of sediment. If I build a wall here, build a levee, and drive my Chevy to it, if I build a wall there and prevent the river from flooding-- so we've got this nice, solid wall here, and the river can never get out of its banks anymore. Over time, these sediments sink down because of water squeezing out.

So, they're sinking down because the water's squeezing out, but nothing is replacing it. So, over time, the surface sinks below the river level. And this is what happened in New Orleans. This is what has happened in the Mississippi River and all of Louisiana and in New Orleans. New Orleans has had these wonderful levees built to protect the city from the flooding of the river.

But at the same time, they prevent all this new mud from being spread over the landscape year after year to keep the surface up high, and so the surface has sunk down. And it sunk quite deep. In some places 20, 30, 40 feet below the level of the river.

If you go to New Orleans, you're walking along Poydras Street, or one of those streets in the French Quarter. And up there, somewhere, you'll see the top of the levee and you'll see a barge floating by above your head. It's a very, very strange feeling to be walking around and to see a ship floating by above your head way up there somewhere. But that's what New Orleans has. It's sinking down, and the river's up high. Now, that wall breaks, that water's going to come down at you.

Let's go back to the slide show. "The Big Easy" is built on mud from long ago floods. The water's slowly squishing out, the mud is compacting, New Orleans is sinking, the river's up high. Eventually, "The Big Easy" is lower than "The Big Muddy," which is what the Mississippi River is known as. The stage is set for disaster, and that disaster struck when the big Hurricane Katrina hit last summer.

It's actually even worse than that. Usually rivers do build natural levees, but then they'll break through those levees themselves and take a different path to the sea. And they'll always build these big deltas around where they come out into the ocean because you'll have one path and then a different one and a different one, and you'll just spread everything out over. But in the case of New Orleans, the Mississippi's been in the same place for the last 100 years.

Here's that picture that I drew so crudely with my pen over here, but that's the same thing that we looked at before.

Here is a satellite image of the Mississippi River coming down, and that sort of rounded peninsula is the Mississippi Delta. And then, you can see that big brown smear going out from it. That's all of the mud coming down the Mississippi and spreading out in the Gulf of Mexico. That's what that big smear of mud is. And that's how much mud "The Big Muddy" carries.

That's why the Mississippi-- that's its nickname is "The Big Muddy." It's brown. It's beautiful water, it's beautiful, clean water, but it's just filled with sediments being carried down to the ocean. And normally, the river would go along different paths and make this nice delta at the bottom of the river. But for many, many years, the Mississippi's gone down the same path year after year.

We're going to switch now to talk about groundwater, and then we'll finish up this section. Groundwater is, as the words says, its water that's in the ground. Soil's kind of like a sponge. You can put water down into it and it'll just filter through it. It's not a solid rock where you pour water on and it'll just flow off of it. It's more like a sponge where the water will go down into the little holes.

If you were to take a shovel and start digging around here somewhere, the very top part of the soil would be more or less dry. I mean there might be some water in it, but there'd be water and air. Until you got to what's known as the water table, and below that, every little hole would be filled up with water. You'd have soil particles, grains of soil, and you'd have water and no air. Everything below that is groundwater. And then the surface, they're separating groundwater from the upper part, which has some water and some air is known as the water table.

The rain seeps down into the ground and it slowly filters its way down to the water table and down into the ground water. And because it does it so slowly, any organisms, any bacteria that are living in it will die over time. Plus the little holes that the water has to make its way through are so tiny that the water can make it, but larger organisms can't.

And so the water, eventually when it gets down to the bottom, is actually incredibly pure. You can just-- people used to do this. They would just drill a hole down to the groundwater, pump it out and they would drink it. And it didn't matter. If there was all this agriculture and sheep and cattle and pollutants and everything at the surface, because most of those organisms would be filtered out before the water got down to the groundwater. You could just pump it out and you could drink it. That's true in most places.

That's not true around here in State College because in this area we have these big holes in the ground, caves, all these big sink holes around here because it's a heavily limestone dominated landscape. And so we have these big holes in it, and the water goes in, and boom, it shoots straight down to the groundwater level. It shoots straight down to the water table, and so we have a little more careful around here that we don't pollute the surface and down it goes.

If you pour a chemical into the groundwater, the water is like a sponge and it will take it. And unlike natural organisms, like bacteria, and giardia, an amoeba, which are killed over time, these pollutants like motor oil or the famous case here in State College is dry cleaning fluid. I forget what it's called, but there's a dry cleaning store here right in the middle of State College, and they found some of these dry cleaning fluids in the soil underneath it.

Those things can go down into the groundwater, and there's no killing those. You can't kill motor oil. It will eventually make its way down into there. So, do not pour your motor oil onto the ground. Take it to the recycling center. Take it to a gas station and they'll recycle it for you. Once that stuff gets into the groundwater, it is really difficult to clean it up. Think of the sponge. You add soap to the sponge, you've got to rinse it out over and over again to get it clean again.

In State College, lots of limestone. And limestone is particularly sensitive to that carbonic acid that we were talking about last time. It really breaks it down, eats away at it really quickly, and makes these big caves and caverns around here.

Our high school football stadium is actually built in a sink hole. If you go to it, the roads are all up here, and the stands head down along the walls of the sink hole, and there it is, football stadium, sink hole. The new school, the new Mount Nittany Middle School that we built for a great deal of money recently was actually more expensive than expected because there were all these sink holes underneath it that had to be filled in first and stabilized before the thing goes up.

So, State College has this problem, it's called Karst terrain, named after the region in Yugoslavia that is very similar to this-- has lots and lots of holes. Water seeps down into these cracks, get bigger and bigger, and makes these sink holes. If you put this cave up near the surface, then you can get these big depressions in the surface, or you can put a football stadium in it if you're lucky enough to have it in just the right place.

In areas with lots of limestone, the water doesn't go slowly. It goes foosh, shooting straight down because these big caves make a wonderful conduit down to the bottom into the Earth. And so, you don't have time to kill off all these bacteria. You don't have that natural filtering where the water trickles through these tiny, tiny pores and keeps out the bigger critters.

That doesn't happen here. The pores aren't tiny, tiny around here. They're big, and so the water just goes straight down, and so we're much more sensitive to groundwater pollution around here. But we are sensitive to groundwater pollution everywhere in the world.

So, my conclusion to you is that water is one of the most important substances on this planet. It falls out of the sky, it seems like it's free, it ain't, because it has to be collected in some way, for example in a dam, but that has its problems. It has to be collected down in the ground in some way, and that happens in some places and not in others.

All of the conflicts that are going to be coming up are going to be about water. So, if you do nothing else, teach yourself about the politics of water, the realities of water, and try and conserve this amazingly precious and amazing material. Thank you.

Want another look?

Check out the Unit 6 Presentation used in the online lecture.