 Good morning. Welcome to G-Site 10, Geology of the National Parks. We're going to be looking this time at carrying down mountains some more. Last time we broke them apart using freeze-thaw action and biochemical 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 Shridhar Anandakrishnan. I'll be your guide through some of the lovely places. We're going to take a little bit of a detour to look at some of the damage that water can do. Let's take a quick look at some lovely pictures and then we'll start in on the section. Redwood National Park on the coast of California. You can go flying to San Francisco and drive up the coast from there. 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. 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 stride is 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 west, to the east, sorry. You go over the mountain and you're at Death Valley National Park. It's dry as a bone. What's going on? All right, so that's the first thing we're going to look at. Let's take a look at some pictures over here. Sequoia Semper Virens, the ever-living Sequoia. They live for a long time, a couple thousand years and then they'll fall over. Then the new trees will sprout from the trunk. 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 there'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. 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. You go inland a little ways and at the same latitude. You've got this amazingly dry landscape. We're 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. The top, as it normally is. You can see this coastal redwood range all up and down the coast. In inland, you have Death Valley. In between, you have the Sierra Nevada Mountains. That's the key. You have these tall mountains in between. They intervene and somehow prevent the water from getting from the coast inland to Death Valley. Let's take a look at that. The first thing we're going to do is look at winds. 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's mountain range. We have on the left these towering redwoods. On the east, we have Death Valley, hot, cool, wet. What's going on? Why is this the case? When the winds blow in from the Pacific, they carry water. Water evaporates in the Pacific Ocean. It blows onshore onto land in California and it hits the Sierra Nevada mountain range. 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 mountain side to get over. They can't just tunnel right through the Sierras and there are enough passes for all of that air to go through these low passes. 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. 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 has got to go somewhere and it falls out as rain. You have all of this rainfall on the windward side of the mountain. 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. 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 back side 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. This explains why it's so dry in Death Valley. Very simple process. You've got this nice wet air coming in from the Pacific but all of 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. This 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 Nevada's, Redwood's, Sequoia's. Here's that same picture. You've got air coming in off the Pacific. It's heading up the slope of the Sierra Nevada's. It's cooling off. We talked about how all the water gets squeezed out. It's dry when it gets to the top of the mountains 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. 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 until finally you get to the top and you've got to put on your jacket. Same process as you get down. As you head down the slope of the mountain you start shedding your jacket and then your sweater and then your shorts and t-shirt. By the time you get to the bottom of the mountain for 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 3 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 is dropped about 3 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 3 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 5 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 has now gone up by 5 degrees and you walk down another 1,000 feet the temperature has gone up another 5 degrees if the air is really dry. So now you 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 heat it 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 you get 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 that heat deep inside the earth drives these convection cells drives all of the mountain building take one thing away from this section is 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 a 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 all right 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 poles 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 would want to eat that plywood but there wouldn't be very much peanut butter on that sheet of plywood because you've taken this same spoonful of peanut butter and tried to smear it 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 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 is 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 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 alright 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's 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 oops, sorry about that rising warm moist air produces what in this diagram is called a Hadley cell it's just a convection cell by a scientist named Hadley so they call 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 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 secondary Hadley cells or secondary convection cells called the ferrule 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 brings that 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 the rising moist air you have all this rainfall at the equator because all of that air is rising up and the rain is all falling out of it as it gets colder and colder and colder 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 desert and the Kalahari and Namibian deserts in Australia and Africa these convection cells so now we've talked about wind and we've talked about rain shadows now let's take a look at what happens with that water when it falls out of the 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 very straight forward but when it gets down it's 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 system so if you've got lives 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 it as rain two thirds of it about that much of it goes straight back up into the air the plants drink it goes straight up the trees drink it goes up through their roots goes out through their leaves straight up into the air again or it just falls down backs and 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 the surface falls into spring creek falls into some other creek eventually makes its way to the and then down to Delaware Bay and then out to the ocean or to Chesapeake Bay 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 on the land it rains out because of the processes that we talked about it 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 then goes straight back out 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 into the ocean water is unique to earth we think everybody is 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 and so cold on the moon the water probably is almost certainly in the form of ice he is he needs water if you are going to have critters if you are going to have any kind of organisms alive we think they 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 will be some critters alive there and that's probably the case anywhere on this planet 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 are up to 6 billion or so and we are 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 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 upriver from Johnstown there was a private dam 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 recently just a few weeks ago Wilkesbury's Scranton the whole northeast of Pennsylvania Elmira Binghamton 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 rock 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 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 all those 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 takes a long time but eventually it'll be gone without the river you wouldn't have been able to wash that thing all that material out and so the valley would have filled up and the mountain would still be there and that 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 you get these enormous floods and they're the ones that do the work they're the ones that move enormous mass of the material downstream we use it we need it, we gotta 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 the things that we do the most of all down through human history if you look at human settlements they're 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 all of human settlements have always been associated with rivers here is Glen Canyon Dam in in Arizona and the river is on the lower part there the Colorado River is flowing down and that's 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 to generate electricity that dam is used for irrigating crops all through that area so the hydrologic cycle as I said before is 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 those streams let's follow one of those streams to the ocean 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 and it's stabilizing the slope that slope can be fairly stable you get rain the rain falls on it that water is going to head downhill there's nothing that can hold water back except things like dams all right streams are recharged by direct rainfall and by groundwater springs so water rain can fall straight into the stream or onto the banks of 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 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 flowing more slowly then 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 plane 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 alright this is how a stream carries sediment this is a cross section through a stream the surface of the stream is at the bottom 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 is blue, you know that and normally a stream if it's flowing slowly it's blue but after a big storm after it's churning and it's carrying 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 a 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 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 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 then you 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 then comes back if a river has mostly fine sediments it'll make a deep channel it won't get clogged and it will just meander it'll 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 it will make another curve and another curve and here's a photograph of that so you've got a 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 is flowing more slowly and it will deposit material there and the river over time will keep will keep migrating inwards eventually eventually that neck could get pinched off completely and that oxbow will be cut off and you get these oxbow 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 oxbow lake it's named after these big yolks that the farmers used to put around oxen and then that yolk would be hitched up to a cart and then these oxen would plow 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 and it gets blocked and the river picks a different one so you need big chunks of rock where do 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 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 and get a closer 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 temporarily the sediments get stopped forever engineers don't like to have sediments going through their machinery to clog 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 get come along, deposit come along, 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 it 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 some of these big dams have been in place for 100 years or more or 50 or 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 that's a water, remember the water is going right through but that water that's coming out is clean 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 use 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 the net was that most of those places would continue to have these gravel and sand bars you build a dam all the sediments up there 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 and you have a river that's happily flowing down so this is water that is flowing down and remember in addition to that 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 have also done so 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 river bed that it would not have picked up before before it had all these sediments that it was already carrying it couldn't carry any more sediments and so it left the river bed 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 you 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 lots of critters live on those bars they don't have a place to live anymore all 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 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 alright let's follow that river past the dam past the mountains out onto the Gulf Coast this river has just flowed from the mountain down onto 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 levee broke that was protecting the city and the water rushed in and you know the disastrous consequences of that 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 onto the land on either side of the river and it floods 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 is rushing along carrying sediment it floods out onto the side the water spreads out and the sediments can't be carried if the water is moving slowly sediments will slowly sink down and be deposited onto the fields and 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 farmers fields and then they could plant their corn and wheat probably not corn I don't think they had corn, wheat and grow it and store it alright and the lands 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 build houses by these rivers and they don't like the river coming into their basement they don't like coming into their first floor they don't like 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 the river would flood and put all this mud 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 any more 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 alright 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 we've got trees and houses and tractors it's supposed to be a tractor farm field and so you have everybody happily living their lives over here once a year or a few times a year this river will flood alright 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 the older layers sink down and so the level actually remains the same 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 alright going to 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 a 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 I'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 of water 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 in all of Louisiana and in New Orleans has had these wonderful levees built that 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 it's 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 Poitre 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 but that's what New Orleans has it's sinking down and the river is up high now that wall breaks that water is going to come down at you alright let's go back to the slide show the big easy easy is built on mud from long go floods the water is slowly squishing out the mud is compacting New Orleans is sinking the river is up high eventually the big easy is lower than the big muddy but the Mississippi River is known as the stage is set for disaster and that disaster and that disaster struck when the big Hurricane Katrina hit in 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 the 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 has been in the same place for the last 100 years here's that picture that I drew so crudely with my pan over here but that's the same thing that we looked at before all right 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 its nickname it's the big muddy it's brown all right 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 river but for many many years Mississippi is going 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 all right groundwater is as the word says it's water that's in the ground soils 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 the water on it will 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 the 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 a water table and below that the soil 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 a water table is groundwater and then the surface there separating groundwater from the upper part which has some water and some air is known as a water table the rain seeps down into the ground and it slowly filters its way down to the water table and down into the groundwater 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 it's 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 it's not true around here in state college in this area we have these big holes in the caves all these big sinkholes 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 to be a little more careful around here that we don't pollute the surface and down it goes and 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 and 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 right in the middle of state college and they found some of these dry cleaning fluids in the in the soil underneath it those things can go down into the groundwater and there's no kill in 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 a sponge you add soap to the sponge you gotta 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 sinkhole if you go to it the roads are all up here and the stands head down along the walls of the sinkhole and there it is football stadium sinkhole the new school the new mountaineer middle school that we built for a great deal of money recently was actually more expensive than expected because there were all these sinkholes underneath it that had to be filled in first and stabilized before the sink 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 sinkholes 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 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 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 teach yourself about the politics of water the realities of water and try and conserve this amazingly precious and amazing material thank you