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Today we're going to talk about rivers and how these dynamic, delicate, yet powerful systems are able to constantly change the landscape. We'll focus on the Zambezi River in Southeast Africa following its main features from the tributaries that feed into it in the upper course, to the deafening wonder that is Mosi-oa-Tunya waterfall (also called as Victoria Falls) in the middle course, all the way to the delta as the river flows into the Indian Ocean. Along the way, we'll teach you all about why rivers meander, why some rivers have rapids and others are calm, and how we talk about classify and river systems more broadly.

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#CrashCourse #Geography #Rivers
As we journey down the Zambezi River, plumes of  mist rise in the distance.

And as we get closer,   we’re almost deafened by the sound of water  crashing down cliffs up to 108 meters tall.   We’ve reached the largest sheet  of falling water on the planet:  . The Smoke That Thunders, or Mosi-oa-Tunya,  the name given by the Kololo people.   Others know it as Victoria Falls, the name  given by the British colonizers of the area.

And though the river before the  falls looks tame in comparison,   this crashing sheet of water forms because  of the Zambezi River's erosion power,   which is the power to move the  rock and sediment of the river bed.   Here erosion causes soft rock to fall hundreds of  meters away to create a dramatic fall of water. We don’t always see their erosional force  on a daily basis, but over centuries the   steady flow of water does the hard work  of transporting energy and nutrients,   and carving rock, creating impressive landforms. I’m Alizé Carrère, and this  is Crash Course Geography.

INTRO. Of all the erosional agents, water’s power  can be seen in every landscape on Earth.   Rivers of water move arctic and  antarctic glaciers and cut canyons,   carve monoliths, and create amazing valley  formations. They form flat floodplains that   hold seasonal flood waters and move the  nutrients and silt that make deltas and   estuaries rich, fertile lands -- from the  Mississippi, to the Amazon, to the Zambezi.

With so many physical and economic ways  that water shapes our surroundings,   there’s a lot to cover! So for  the next three episodes we're   going to first talk about how the flow  of rivers shapes the physical landscape,   and then how human use can shape the  environments that rivers and groundwater provide. Let’s start here, where the Zambezi River  crosses Southeast Africa.

In general,   a river or stream is any body of water that flows  in a channel and uses gravity to move downhill. Textbooks like to say “stream” because it  includes large and small flows of water,   while “river” usually refers to the  main stream or an entire river system.   In the real world we tend to say “river,”  so I’ll use them interchangeably. To form the river, the waters of the  Zambezi come from a variety of places.   Water already on Earth’s  surface from rain, springs,   and lakes will flow from higher ground into  the nearest channel, or bed where water flows.

But stream water also comes from underground  sources. Like from water stored in the   soil either near the surface, or from deeper  underground areas that hold water, like aquifers. Near the headwaters, all of the tributaries,  or small streams that feed into the Zambezi,   are supplied by this groundwater, which  seeps into the river channel through sandy,   permeable soils or burbles up as a spring.

And we know this because even in dry months, these  streams have a fairly steady and very clean water   flow, indicating that the water comes up through  the soil, functioning as a natural filter. Further downstream, as the  soils and conditions change,   the river will go through distinct dry and wet  periods that match the dry and rainy seasons.   Those segments of the river depend  more on precipitation than groundwater. The whole area that a stream drains and all  its tributaries is called a stream system.   But when talking about stream systems, we’ll often  call it a drainage basin or watershed instead,   because those phrases help  us picture the land and all   the water that flows together  to a single drainage point.

Like the continent of Africa has these major  drainage basins that shift the continent’s   water to the oceans. Some drain into the Atlantic  ocean, the Nile drains into the Mediterranean Sea,   and a few flow into the Indian Ocean.  In general, water flows from high   points in the continental interior to low  points, which are usually oceans or seas. But there are some rare exceptions, like the  interior highlands near present-day Chad.   These highlands drain to the lowest point  on the land, which happens to be Lake Chad.

Drainage basins are open systems, which means  water can enter the system from a variety   of sources, like precipitation, snowmelt, and  underground springs -- basically all the places   we said rivers got their water. And water leaves,  or is discharged, a variety of ways too -- like   draining to the ocean, evaporation, or even humans  withdrawing water and moving it out of the basin. As for the Zambezi basin, it covers land across  eight countries throughout Southern Africa.   It gets water from the slow release of groundwater  throughout the year and additional spikes of   groundwater discharge from precipitation  before draining into the Indian Ocean.

Basically rivers and all their tributaries are a   way of moving water from all parts  of the basin to the main stream   as part of a drainage network. And then we can  talk about them in a kinda stream hierarchy.   Like take a look at a drainage network for the  Luangwa tributary of the Zambezi. First order streams are streams with no  tributaries.

They’re often the headwaters   which are spring-fed streams at a higher  elevation than the rest of the drainage basin.   A first order stream may be very small, seasonal  or ephemeral and not have a formal name. Then when two first order streams join, they  create a second order stream. And on and on.   The larger the number, the more tributaries  and branches that have fed into the stream.   And by the time the branches meet the main trunk,   there might be hundreds of streams feeding  into it.

The Zambezi can be a different   order at different parts, depending on  how many streams feed into that section. And as the river flows, the  water rushing or meandering   or just trickling from higher  elevations down to sea level   carves the Earth into channels and  plains, changing the land as it moves. But not all streams carve the Earth the  same way.

The shape and material in the   drainage basin impacts how water flows and  what type of channel the water can carve,   along with whether that stream is able to  rush or meander or trickle in the first place. Like the elevation the stream drops over a  given distance is called the stream gradient.   And it influences the stream energy,  or how much work the stream can do,   like eroding through rock or picking up  sediment -- all ways rivers shape landscapes. Basically, stream energy comes from potential  energy turning into kinetic energy as gravity   pulls the river from high elevation to low  elevation.

A steeper gradient will give the   river more velocity, or speed, but sediments  throughout the river’s course can change the   friction and slow the river down. So gradient,  sediments, and even how much water the river   has all influence how much kinetic energy is  available and how much work the river can do. If we followed the Zambezi as it  journeyed through Southeast Africa   we could see -- like most major rivers  -- it has three main parts and shapes   the landscape differently depending on  the composition of the Earth’s crust.

The upper course of the Zambezi, where the  headwaters are, begins in an equatorial   forest with a peat swamp in Northwest  Zambia. From here the river flows in   fits and starts through the swamps  about 1500 meters above sea level. The river moves along the low gradient  of the upper course and can carry only   tiny fine grained sediments.

Then, as  it transitions into the middle course,   the stream energy picks up as the stream gradient  becomes steeper. Here the rock changes from   sandstone to a plateau of igneous and metamorphic  rock, which allows for dramatic erosion. As it moves, the water is applying  force to the channel’s bottom and sides,   breaking off and picking up any particle  that’s light enough to be lifted by the water,   from grains of sand to massive boulders.  This part of the river has a high stream   load, or how much sediment the  stream can carry and dissolve.

The Zambezi dumps those sediments in a flat area,   creating the Barotse Floodplain, or flat  area that holds seasonal flood waters.   This leads up to the dramatic Mosi-oa-Tunya  waterfall, which formed because the igneous   rock of the channel erodes more quickly  than the surrounding metamorphic rock. After the sudden 108 meter drop at  Mosi-oa-Tunya, stream energy is still   high and the Zambezi continues a turbulent  course as it cuts a channel through the   easier-to-erode sedimentary rocks filtered  in with hard basalt for about 145 kilometers. Then it’s slowed by a series of dams that supply  electricity for much of Zambia and Zimbabwe.   As the Zambezi reaches its lower course,  gaining new tributaries along the way,   the stream reaches its lowest point and exits the  drainage basin, slowing down even more.

The stream   gradient is flatter, and the river becomes  very shallow as it enters a broad valley. The bed is also very sandy, which increases  the stream load and slows down the river.   So just before reaching its base level,  or the lowest level a stream can erode to,   the Zambezi begins splitting into  numerous channels or distributaries   and slows so much that it starts  dropping its finest sediments. Like all good things, a river must come to an  end, which it does by flowing into another river,   ocean, lake, or sea.

And as it ends,  it basically has a “going out of   business sediment sale” and deposits lots of  sediments, shaping the land one last time.  If this deposition happens where two rivers  meet, we might see what’s called an alluvial fan,   which often appears at the base of a  mountain. Or we may see floodplains,   where sediments are deposited along the  banks of a channel. Or it might form deltas,   like at the mouth or end of the Zambezi  River as it flows out into the Indian Ocean.

Even with all that happens from the  headwaters to the mouth, the shape   of a stream channel is determined  by what happens at the base level.   In places where sea levels are rising,  the velocity of the river changes because   it reaches the sea sooner, changing  the whole flow of the lower course. But overall we’d say most rivers have  a concave profile, meaning that there’s   a steep part near the headwaters that gives  way to a gentle slope towards the base level. Middle courses are flatter after the initial  steep gradient from the higher elevations of   the upper course, and soils and rocks are  often easier for the river to cut through.   This makes the channel bed smoother, so all the  energy that usually goes towards eroding the bed   goes into carving the sides and creating  sinuosity, the meanders, or the bends in a river.

Think of all the naturally-formed  rivers you’ve ever seen in real life   or on maps -- a river rarely goes from the  headwater to the sea in a straight line.   They twist and turn, creating oxbow  lakes, terraces, and floodplains. We get distinct landforms partly because curves  change how fast the water moves in the channel.   Like in mostly straight segments, the current will  be faster in the center and slower on the banks. The Ayeyarwady River in Myanmar is an example of  a stream with faster water eroding the center,   and the parts along the banks are slow  enough for sediment to get deposited.   So overall the Ayeyarwady  does different erosional work.

But when the stream is sinuous, like the Luangwa  tributary of the Zambezi , the inside of the bend   is shallower and the water is slower, so more  sediment gets deposited. The outside of the bend   is deeper, and water moves faster and erodes more. Along with shape, the characteristics of the river   can also change as the moving water erodes  more in some places and less in others.

Rivers over areas that are hard to erode,   like the hard metamorphic rock of  the Upper Luangwa or middle Zambezi,   end up forming steeper slopes with cliffs,  waterfalls, steep rapids, and narrow canyons. And a river’s work is never truly done. Rivers  will adjust their course to maintain the most   efficient path through the basin, which is called  equilibrium.

Whenever there’s a flood, rivers   have an opportunity to dump enough sediments  to create natural levees and deltas -- which   we’ll explore more next time as we look at  floods, wetlands, and human-river relationships. So the shape and roughness of the water; the  stream load that’s eroded, carried, and deposited;   and the velocity of the flow, all  contribute to the equilibrium -- and   ultimately the river’s path over time.  Which makes each river a dynamic, delicate,   powerful system that’s constantly changing  and shaping the landscape, even as we speak. Many maps and borders represent modern  geopolitical divisions that have often   been decided without the consultation, permission,  or recognition of the land's original inhabitants.   Many geographical place names also don't reflect  the Indigenous or Aboriginal peoples languages.   So we at Crash Course want to acknowledge these  peoples’ traditional and ongoing relationship   with that land and all the physical  and human geographical elements of it.

We encourage you to learn about  the history of the place you call   home through resources like native-land.ca  and by engaging with your local Indigenous   and Aboriginal nations through the  websites and resources they provide. Thanks for watching this episode of Crash  Course Geography which is filmed at the Team  . Sandoval Pierce Studio and was made with the  help of all these nice people.

If you want to   help keep all Crash Course free for everyone,  forever, you can join our community on Patreon.