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 humanriver 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.
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