Stages of River Development
• Young streams flow fast
through steep valleys.
• They are found in mountainous or
hilly regions and may have white
water rapids and waterfalls.
• They have high levels of energy
and erode the stream bottom more
than the sides.
Old-stage rivers flow very slowly
through a very broad, flat floodplain
that is curved.
• A river in this stage mostly
erodes its sides causing
changes in its meanders, like
• Meanders can eventually be
cut off forming Oxbow lakes.
What can cause the banks of the Colorado
River to be so steep?
shows that the
and the river is
relative to base
A fall in sea level or a rise in the level of the
land can have a dramatic effect on a river's
journey to the sea. The old age stage of the
river becomes perched above the sea and
therefore the river, with new-found energy, has
to erode through the already deposited material
to create a new route to the sea. The features
formed as a result of this rejuvenation will
deepen and straighten the river, have more
Causes of Rejuvenation
• Climatic change: Glaciations and
changes in rainfall.
• Tectonic change: Where land is uplifted
after plate movement or volcanic activity.
Changes can be positive sea level rises
in relation to the land, or negative sea
level fall in relation to the land.
Features of a Floodplain
Wide valleys often include
»Cut bank and point bar
»Cutoffs and oxbow lakes
Flood Plain – is a broad strip of land built up by sedimentation on either side of a
– During floods, flood plains may be covered with water carrying suspended silt and clay.
– When the flood water recedes, these fine-grained sediments are left behind as a horizontal
– The sudden decrease in velocity
of water leaving the channel
causes the river to deposit most
of its sediment near the main
• Progressively less sediment is
deposited away from the
• A series of floods may build up
natural levees – low ridges of
flood-deposited sediment that
form on either side of a stream
“Click to view animation”
Formation of natural levees by flooding
Flood over banks, sediment carrying water into wide area, greater drag, lower velocity, load drops
Erosion and deposition along
a meandering stream
Meanders get more extreme with time. Deposition at point bar keeps up with erosion at cutbank
River meanders across
Base level drops,
or region uplifts.
Terrace forms in floodplain
Further incision cuts a new
Why? Steeper gradient
causes cuttoffs, stream
straighter, cuts down
• Occur where bed (sediment) load is very
high. Often big boulders in the stream.
• Many channels because mid-channel bars
split the stream
• Adjacent to mountains in high rainfall
A man-made levee reinforces the bank and helps prevent
flooding. By confining the flow, levees can also increase
the speed of the water.
The simplest is just a mound of dirt bulldozed up when
there is an advancing flood. These levees are not very
good and can be breached by the power of the water.
Properly designed levees (that may include specially
compacted earthen mounds, reinforced concrete walls
or other methods of solid construction) provide
protection from river flooding to areas behind them.
However all levees are at risk of failure from overtopping
or breaching if the flood is bigger than what the levee
was designed for or if the levee hasn't been properly
Back Swamps & Floodplain
On a flood plain, a marshy area where floodwater
may be confined between the valley walls.
Back Swamp - poorly drained oval-shaped
depressions located between the natural levees
and the edge of the floodplain.
Small tributary channel that is prevented from joining
the main stream channel by the presence of levees.
Yazoo tributaries tend to flow on the floodplain parallel
to the main stream channel.
Flooding is a natural process that occur along
river flood plains in response to heavy rains
and/or the rapid melting of snow in the spring.
–Floods are described by recurrence interval
A 100 year flood is one that can occur, on
average, every 100 years.
–Flood erosion is caused by the high velocity
and large volume of water in a flood.
• Relatively harmless on an uninhabited
• Can be devastating to an inhabited area,
such as a city on the flood plain.
Flash Floods – are local, sudden floods of
large volume and short duration, often
triggered by heavy thunderstorms.
–Man-made structures can mitigate the
dangers of floodwaters and sedimentation to