Download Introduction to Rivers

Introduction to Rivers
Where the river begins…

The start of a river is called its source.
Large portions of the world's largest
rivers begin in hollows in the land. Water
drips in from the surrounding soil to allow
the formation of a tiny flow of water
called a seep.
As the river flows…

From its source, a river flows downhill.
This is because of the Earth's gravity.
Small trickles of water join up to form a
stream. The amount of water in the
stream increases steadily as more
streams, called tributaries, join it.
Eventually the stream becomes big
enough to be a river.
Drainage Patterns
A drainage basin is all the land that
supplies a river and its tributaries with
water.
 If you could look down on a river from
above, you would see that it branches.
This is called a drainage pattern. The
shape of the pattern depends on rocks,
soil, climate and the changes made to
the river.

Radial and Dendritic Drainage
Radial drainage happens when streams
flow down from a central high point, such
as a mountain top.
 Other rivers, such as the Amazon, form a
pattern like the branches of a tree. This
is called dendritic drainage.

River Erosion

Corrasion is the grinding of rock fragments
carried by the river against the bed and banks
of the river. This action both widens and
deepens the channel.
 Attrition is the knocking of rock fragments in
the water against one another. The fragments
are broken into smaller pieces and become
smoother along the process.
River Erosion

Solution is the process by which river water
reacts chemically with soluble minerals in the
rocks and dissolves them.
 Hydraulic action is the breaking down of rocks
and dragging them away from the bed and
banks by the force of the running water itself.
When water from a fast moving stream enters
cracks in a rock, the force breaks up the force
into pieces.
Transporting along the channel…

A river uses its energy to carry or
transport eroded materials such as mud,
sand, boulders and dissolved materials.
These materials are called its load.
Rivers transport their load by four
processes. These processes are
traction, saltation, suspension and
solution.
Saltation

Large particles such as gravel and
coarse sand are lifted and dropped along
the river, so they bounce along the river
bed in a series.
Traction

Larger particles like pebbles and
boulders roll and slide along the river
bed.
Solution

Dissolved materials containing minerals
like calcium and sodium are carried in
the water.
Suspension

Smaller particles such as clay, silt and
fine sand are carried along without
contact with the river bed. Materials
carried in suspension usually forms the
greatest part of the total river bed.
Waterfalls

A waterfall is where a river falls steeply over a
band of hard rock.
 It is formed through a thousand-year process
in which a river flows over hard rock followed
by soft rock, leaving a small step of hard rock
sticking up into the river.
 Over thousands of years, the soft rock is worn
away until the river falls from a great height.
The middle river

The amount of water and material in the river
increases as more tributaries join in. The bed
widens, its slope becomes gentler, and the
water flows much more slowly as the river
flows out of the mountains.
 The path of the middle river is always
changing as it cuts sideways into the land and
starts to deposit its load.
Meanders

Loops, called meanders, and oxbow lakes are
typical features of the middle river.
 During a storm, meanders stop river water
flowing easily. This causes water to build up in
places, and may lead to flooding.
As it reaches the sea…

Most rivers end their journey when they flow
into a sea. The river slows down and deposits
the sediment it is carrying. Heavy grains of
sand and gravel drop to the bottom while
lighter particles of fine silt and clay are carried
further out to sea.
 Eventually, the sediment spreads out to form a
new piece of land with gently sloping sides.
This is called a delta.