What rivers can tell us about the earth's history | Liz Hajek

76,723 views ・ 2017-07-28

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All right, let's get up our picture of the earth.
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The earth is pretty awesome.
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I'm a geologist, so I get pretty psyched about this,
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but the earth is great.
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It's powerful, it's dynamic, it's constantly changing.
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It's a pretty exciting place to live.
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But I want to share with you guys today my perspective as a geologist
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in how understanding earth's past
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can help inform and guide decisions that we make today
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about how to sustainably live on earth's surface.
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So there's a lot of exciting things that go on on the surface of the earth.
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If we zoom in here a little bit,
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I want to talk to you guys a little bit about one of the things that happens.
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Material get shuffled around earth's surface all the time,
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and one of the big thing that happens is material from high mountains
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gets eroded and transported and deposited in the sea.
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And this process is ongoing all the time,
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and it has huge effects on how the landscape works.
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So this example here in south India --
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we have some of the biggest mountains in the world,
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and you can see in this satellite photo
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rivers transporting material from those mountains out to the sea.
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You can think of these rivers like bulldozers.
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They're basically taking these mountains and pushing them down towards the sea.
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We'll give you guys an example here.
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So we zoom in a little bit.
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I want to talk to you guys specifically about a river.
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We can see these beautiful patterns that the rivers make
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as they're pushing material down to the sea,
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but these patterns aren't static.
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These rivers are wiggling and jumping around quite a bit,
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and it can have big impacts on our lives.
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So an example of this is this is the Kosi River.
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So the Kosi River has this nice c-shaped pathway,
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and it exits the big mountains of Nepal
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carrying with it a ton of material,
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a lot of sediments that's being eroded from the high mountains,
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and it spreads out across India
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and moves this material.
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So we're going to zoom in to this area
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and I'm going to tell you a little bit about what happened with the Kosi.
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It's an example of how dynamic these systems can be.
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So this is a satellite image from August of 2008,
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and this satellite image is colored
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so that vegetations or plants show up as green
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and water shows up as blue.
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So here again you can see that c-shaped pathway
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that this river takes as it exits Nepal.
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And now this is monsoon season.
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August is monsoon season in this region of the world,
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and anyone that lives near a river is no stranger to flooding
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and the hazards and inconveniences at minimum that are associated with that.
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But something interesting happened in 2008,
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and this river moved in a way that's very different.
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It flooded in a way that's very different than it normally does.
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So the Kosi River is flowing down here,
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but sometimes as these rivers are bulldozing sediment,
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they kind of get clogged,
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and these clogs can actually cause the rivers
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to shift their course dramatically.
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So this satellite image is from just two weeks later.
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Here's the previous pathway,
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that c-shaped pathway,
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and you notice it's not blue anymore.
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But now what we have is this blue pathway
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that cuts down the middle of the field of view here.
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What happened is the Kosi River jumped its banks,
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and for reference, the scale bar here is 40 miles.
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This river moved over 30 miles very abruptly.
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So this river got clogged and it jumped its banks.
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Here's an image from about a week later,
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and you can see these are the previous pathways,
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and you can see this process of river-jumping continues
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as this river moves farther away from its major course.
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So you can imagine in landscapes like this,
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where rivers move around frequently,
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it's really important to understand when, where and how they're going to jump.
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But these kinds of processes also happen a lot closer to home as well.
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So in the United States,
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we have the Mississippi River that drains most of the continental US.
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It pushes material from the Rocky Mountains
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and from the Great Plains.
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It drains it and moves it all the way across America
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and dumps it out in the Gulf of Mexico.
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So this is the course of the Mississippi that we're familiar with today,
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but it didn't always flow in this direction.
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If we use the geologic record,
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we can reconstruct where it went in the past.
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So for example, this red area here
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is where we know the Mississippi River flowed and deposited material
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about 4,600 years ago.
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Then about 3,500 years ago it moved
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to follow the course outlined here in orange.
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And it kept moving and it keeps moving.
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So here's about 2,000 years ago,
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a thousand years ago,
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700 years ago.
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And it was only as recently as 500 years ago
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that it occupied the pathway that we're familiar with today.
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So these processes are really important,
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and especially here, this delta area,
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where these river-jumping events in the Mississippi
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are building land at the interface of the land and the sea.
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This is really valuable real estate,
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and deltas like this are some of the most densely populated areas on our planet.
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So understanding the dynamics of these landscapes,
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how they formed and how they will continue to change in the future
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is really important for the people that live there.
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So rivers also wiggle.
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These are sort of bigger jumps that we've been talking about.
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I want to show you guys some river wiggles here.
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So we're going to fly down to the Amazon River basin,
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and here again we have a big river system
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that is draining and moving and plowing material from the Andean Mountains,
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transporting it across South America
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and dumping it out into the Atlantic Ocean.
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So if we zoom in here, you guys can see these nice, curvy river pathways.
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Again, they're really beautiful, but again, they're not static.
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These rivers wiggle around.
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We can use satellite imagery over the last 30 or so years
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to actually monitor how these change.
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So take a minute and just watch any bend or curve in this river,
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and you'll see it doesn't stay in the same place for very long.
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It changes and evolves and warps its pattern.
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If you look in this area in particular,
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I want you guys to notice there's a sort of a loop in the river
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that gets completely cut off.
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It's almost like a whip cracking
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and snaps off the pathway of the river at a certain spot.
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So just for reference, again,
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in this location, that river changed its course over four miles
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over the course of a season or two.
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So the landscapes that we live in on earth,
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as this material is being eroded from the mountains
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and transported to the sea,
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are wiggling around all the time.
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They're changing all the time,
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and we need to be able to understand these processes
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so we can manage and live sustainably on these landscapes.
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But it's hard to do if the only information we have
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is what's going on today at earth's surface.
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Right? We don't have a lot of observations.
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We only have 30 years' worth of satellite photos, for example.
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We need more observations to understand these processes more.
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And additionally, we need to know
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how these landscapes are going to respond to changing climate
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and to changing land use
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as we continue to occupy and modify earth's surface.
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So this is where the rocks come in.
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So as rivers flow,
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as they're bulldozing material from the mountains to the sea,
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sometimes bits of sand and clay and rock get stuck in the ground.
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And that stuff that gets stuck in the ground gets buried,
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and through time, we get big, thick accumulations of sediments
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that eventually turn into rocks.
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What this means is that we can go to places like this,
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where we see big, thick stacks of sedimentary rocks,
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and go back in time
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and see what the landscapes looked like in the past.
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We can do this to help reconstruct
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and understand how earth landscapes evolve.
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This is pretty convenient, too,
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because the earth has had sort of an epic history. Right?
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So this video here is a reconstruction of paleogeography
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for just the first 600 million years of earth's history.
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So just a little bit of time here.
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So as the plates move around,
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we know climate has changed, sea level has changed,
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we have a lot of different types of landscapes
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and different types of environments that we can go back --
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if we have a time machine --
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we can go back and look at,
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and we do indeed have a time machine
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because we can look at the rocks that were deposited at these times.
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So I'm going to give you an example of this
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and take you to a special time in earth's past.
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About 55 million years ago, there was a really abrupt warming event,
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and what happened was a whole bunch of carbon dioxide
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was released into earth's atmosphere,
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and it caused a rapid and pretty extreme global warming event.
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And when I say warm, I mean pretty warm,
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that there were things like crocodiles and palm trees
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as far north as Canada and as far south as Patagonia.
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So this was a pretty warm time and it happened really abruptly.
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So what we can do
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is we can go back and find rocks that were deposited at this time
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and reconstruct how the landscape changed in response to this warming event.
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So here, yay, rocks.
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(Laughter)
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Here's a pile of rocks.
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This yellow blob here,
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this is actually a fossil river,
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so just like this cartoon I showed,
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these are deposits that were laid down 55 million years ago.
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As geologists, we can go and look at these up close
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and reconstruct the landscape.
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So here's another example.
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The yellow blob here is a fossil river.
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Here's another one above it.
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We can go and look in detail and make measurements and observations,
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and we can measure features.
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For example, the features I just highlighted there
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tell us that this particular river was probably about three feet deep.
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You could wade across this cute little stream
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if you were walking around 55 million years ago.
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The reddish stuff that's above and below those channels,
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those are ancient soil deposits.
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So we can look at those to tell us what lived and grew on the landscape
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and to understand how these rivers were interacting with their floodplains.
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So we can look in detail and reconstruct with some specificity
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how these rivers flowed and what the landscapes looked like.
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So when we do this for this particular place
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at this time,
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if we look what happened before this abrupt warming event,
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the rivers kind of carved their way down from the mountains to the sea,
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and they looked maybe similar to what I showed you in the Amazon River basin.
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But right at the onset of this climate change event,
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the rivers change dramatically.
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All of a sudden they got much broader,
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and they started to slide back and forth across the landscape more readily.
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Eventually, the rivers reverted back to a state that was more similar
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to what they would have looked like before this climate event,
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but it took a long, long time.
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So we can go back in earth's time and do these kinds of reconstructions
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and understand how earth's landscape has changed
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in response to a climate event like this or a land use event.
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So some of the ways that rivers change
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or the reasons that rivers change their pattern and their movements
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is because of things like with extra water falling on the land's surface
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when climate is hotter,
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we can move more sediment and erode more sediment,
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and that changes how rivers behave.
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So ultimately,
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as long as earth's surface is our home,
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we need to carefully manage the resources and risks
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associated with living in dynamic environments.
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And I think the only way we can really do that sustainably
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is if we include information
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about how landscapes evolved and behaved in earth's past.
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Thank you.
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(Applause)
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