Running water-Geomorhology Chapter

Running water is the most important erosional
agent modifying the Earth's surface. Despite
the importance of running water as an agent
of erosion, sediment transports, and
deposition, its role is limited in some areas. In
areas covered by glacial ice, such as
Greenland and Antarctica, running water is
currently not important. Some parts of
deserts are also little affected by running
water.

• Stream Erosion
• Streams possess two kinds of energy,
potential and kinetic. Potential energy is the
energy of position, such as that possessed by
water behind a dam or at a high elevation. In
stream flow, potential energy is converted to
kinetic energy, the energy of motion. This
kinetic energy is dissipated as heat within
streams by fluid turbulence, but a small amount,
perhaps 5%, is available to erode and transport
sediment. Erosion involves the physical removal
of dissolved substances and loose particles of
soil and rock from a source area. Thus, the
sediment transported in a stream consists of
both dissolved materials and solid particles.

Development of gully erosion
Erosive activity at different stream gradient

• River erosion is the gradual removal of rock material from
the river banks and bed.
• A river erodes in the following ways:
• Corrasion (abrasion): - The river uses its load to grind
against the bed and sides The action would dislodge the
materials and carry them away This process operates in 2
ways (verticle--> depth; lateral--> width) Potholes are the
product of corrasion
• Attrition: The loosened materials that are being carried
away collide against the river sides and bed and against
one another. Over time, they would become smaller and
eventually reduced to fine particles called silt.
• Solution (corrosion): - The solvent action of water
dissolves soluble materials and carry them away in solution
Hydraulic action: - The breaking down of rocks.
Removing and dragging rocks from the bed and banks of
the river by the force of the running water (like a water jet)
Usually there are lines of weakness like joints and cracks
in the river. The work of hydraulic action forms plunge
pools (small lakes).

• Potholes: Circular depressions on the river bed. Formed by
corrosion (abrasion). Most effective in flood conditions.
Pebbles which are trapped in hollows on the river bed are
swirled about in turbulent/ fast flowing water.
• Plunge pools: A large depression at the foot of a waterfall.
This depression is deepened by hydraulic action of the
plunging water. Condition: likely to form when the water
plunges into less resistant rocks. It may be further enlarged
by rock debris swirling about by turbulent water at the base.
• Rapids: A series of very short and fast falls. Condition: an
area of alternating bands of resistant and less resistant rocks
• Waterfalls: - Vertical flows of fast moving water flowing from
great heights Formed by 2 ways: - Due to unequal
resistance of rocks or faulting .
Unequal resistance- less resistant rocks are eroded more
rapidly than resistant rocks As a result, there is a change in
gradient . By faulting- displacement of rocks results in a
difference in height between 2 rocks, water plunges
downwards.
(to cause to penetrate or enter quickly and forcibly into
something)

Gorges: Valleys which are steep sided, deep and narrow. The churning
(turbulent) water at the base of the falls eats away the soft underlying
rock layers of the gorge. The unsupported hard top rock then breaks off.

• The critical entrainment velocity curve suggests that particles below a
certain size are just as resistant to entrainment as particles with larger
sizes and masses. Fine silt and clay particles tend to have higher
resistance to entrainment because of the strong cohesive bonds between
particles. These forces are far stronger than the forces of friction and
gravity.

• A river transport its load (mud, sand, silt,
boulders,dissolved materials)in the following ways:
• Suspension- movement of fine eroded materials
(silt and clay) floating in water.
• Solution-movement of minerals dissolved in water.
• Saltation-a process by which rock fragments like
gravel and pebbles are lifted briefly and then
dropped on the river bed. They are thus
transported in a series of hops and jumps along
the river bed.
• Traction- movement of large rocks such as
boulders by being rolled along the river bed.

• RIVER DEPOSITION
• A river deposits its load of eroded materials when it is
unable to transport it. This occurs when :
• there is not enough water to transport the load during a
dry season
• a river flows across a desert where there is a high rate of
evaporation.
• a river flows across permeable rocks which allow water
to infiltrate into the underlying rocks.
• a river carries a larger load than it can transport.
• there is a sudden change in gradient (e.g. river leaves
the mountain and flows onto a plain)
• a river flows into a lake or sea.
• one side of the river is shallower than the other (e.g. the
convex bank of a meander)
• there are aquatic plants or rocks obstructing the flow

• They are:
• Floodplains and Levees : A floodplain
is a low-lying plain on both sides of a river
that has repeatedly overflowed its banks
and flooded the surrounding areas. When
the floods subside, alluvium is deposited
on the floodplain. The larger materials,
being heavier, are deposited at the river
banks while the finer materials are carried
and deposited further away from the river.
The larger materials at the river banks
build up into embankment called levees.

• The floodwaters spilling from a main channel carry large quantities of
silt- and clay-sized sediment beyond the natural levees and onto the
floodplain. During the waning stages of a flood, the floodwaters may
flow very slowly or not at all, and suspended silt and clay eventually
settle as layers of mad that build upward by deposition during
successive floods.

• Meanders
• Meanders are loop-like bends in a river. The water flows
round the meander in a spiral manner. This causes erosion
to take place on the outer bank and deposition on the inner
bank. Gradually, a steep river cliff is formed on the outer
bank, making it concave in shape. On the inner bank,
deposition of alluvial materials produces a gentle slip-off
slope and the bank takes on a convex shape.
• OXBOW LAKES
• An oxbow lake is a crescent -shaped lake formed on a river
when a meander has been cut through and abandoned.
When a river meanders in very big loops, the outer bank is
so rapidly eroded that the river cuts through the narrow
neck of the meander. The river then flows straight through
the channel. When deposition seals off the cut-off from the
river channel, an oxbow lake is formed. It may silt up and
eventually dry up

Helicoidal flow
Meanders are formed mainly
because of the riffles and pools in
rivers. Riffles and pools are the
shallower and deeper part of a
river’s bed. Water will never flow
straight originally. When water
flow through riffles and pools, a
Helicoidal flow will occur because
of the differences in velocity of
river flow.

• Meandering Streams and Their Deposits
• Meandering streams have a single, sinuous channel with broadly
looping curves called meanders. Such stream channels are semicircular in
cross section along straight reaches, but at meanders they are markedly
asymmetric, being deepest near the outer bank, which commonly descends
vertically into the channel. The outer bank is called the cut bank because
flow velocity is at a minimum near the inner bank, which slopes gently into
the channel.

• DELTAS - A delta is a flat piece of land built-up
from layers of sediments deposited by a river
where it enters a lake or calm sea. The river may
have to branch into smaller distributaries to carry
the water to the sea.
• Types of deltas
• Arcuate delta - triangular in shape e.g. Nile Delta
• Bird's foot delta -e.g. Mississippi Delta.
Conditions that favour the formation of deltas-
• A large load of sediment ,
• Shallow sea at the river mouth,
• Sheltered coast with weak tides and currents,
• Absence of large lakes along the course of the
river which will siphon off the load.

• Subaqueous deltaic plain: typically composed of clay and silt
• Active delta plain: the area of active building dominated by fluvial and
marine processes
• Abandoned delta plain: No longer receives sediment from distributaries.
•

• Floods and Floodplain Deposits
• Most streams periodically receive more water than either channel
can carry, so they spread across low-lying, relatively flat areas called
floodplains adjacent to their channels. Even small streams
commonly have a floodplain, but this feature is usually proportional
to the size of the stream; thus, small streams have narrow
floodplains, whereas the lower Mississippi and other large streams
have floodplains many kilometers wide. Streams restricted to deep,
narrow valleys usually have little or no floodplain.
• Some floodplains are composed mostly of sand and gravel that
were deposited as point bars. When a meandering stream erodes its
cut bank and deposits on the opposite bank, it migrates laterally
across its floodplain. As lateral migration occurs, a succession of
point bars develops by lateral accretion. That is, the deposits build
laterally as a result of repeated episodes of sedimentation on the
inner banks of meanders.

• Particles too large to be suspended even temporarily are
transported by rolling or sliding. Obviously, greater flow velocity is
required to move particles of these sizes. The maximum-sized
particles that a stream can carry define its competence, a factor
related to flow velocity.
• Capacity is a measure of the total load a stream can carry. It
varies as a function of discharge; with greater discharge, more
sediment can be carried. Capacity and competence may seem
quite similar, but they are actually related to different aspects of
stream transport. For instance, a small, swiftly flowing steam may
have the competence to move gravel-sized particles but not to
transport a large volume of sediment, so it has a low capacity. A
large, slow-flowing stream, on the other hand, has a low
competence, but may have a very large suspended load, and hence
a large capacity.

• Stream Deposition
• Streams can transport sediment a considerable distance from
the source area. Some of the sediments deposited in the Gulf of
Mexico by the Mississippi River came from such distant sources as
Pennsylvania, Minnesota, and southern Alberta, Canada. Along the
way, deposition may occur in a variety of environments, such as
stream channels, the floodplains adjacent to channels, and the
points where streams flow into lakes or the seas or flow from
mountain valleys onto adjacent lowlands.
• Streams do most of their erosion, sediment transport, and
deposition when they flood. Consequently, streams deposits,
collectively called alluvium, do not represent the continuous day-to-
day activity of streams, but rather those periodic, large-scale events
of sedimentation associated with flooding.

• Braided Streams and Their Deposits
• Braided streams possess an intricated network of dividing and rejoining
channels. Braiding develops when a stream is supplied with excessive
sediment, which over time is deposited as sand and gravel bars within its
channel. During high-water stages, these bars are submerged, but during
low-water stages, they are exposed and divide channel into multiple
channels. Braided streams have broad, shallow channels. They are
generally characterized as bed load transport streams, and their deposits
are composed mostly of sheets of sand and gravel.
• Braided streams are common in arid and semiarid regions where there is
little vegetation and erosion rates are high. Streams with easily eroded
banks are also likely to become braided. In fact, a stream that is braided
where its banks are easily eroded may have a single, sinuous or
meandering channel when it flows into an area of more resistant materials.
Streams fed by melting glaciers are also commonly braided because the
melting glacial ice yields so much sediment.

• As a consequence of the unequal distribution of flow velocity across
meanders, the cut bank is eroded, and deposition occurs along the
opposite side of the channel. The net effect is that a meander
migrates laterally, and the channel maintains a more or less constant
width because erosion on the cut bank is offset by an equal amount of
deposition on the opposite side of the channel. The deposit formed in
this manner is a point bar; it consists of cross-bedded sand or, in
some cases, gravel. Point bars are the characteristic deposits that
accumulate within meandering stream channels.
• It is not uncommon for meanders to become so sinuous that the
thin neck of land separating adjacent meanders is eventually cut off
during a flood. The valley floors of meandering streams are
commonly marked by crescent-shaped oxbow lakes, which are
actually cutoff meanders. These oxbow lakes may persist as lakes for
some time, but are eventually filled with organic matter and fine-
grained sediment carried by floods. Once filled, oxbow lakes are
called meander scars.
• One immediate effect of meander cutoff is an increase in flow
velocity; following the cutoff, the stream abandons part of its old
course and flows a shorter distance, thereby increasing its gradient.
Numerous cutoffs would, of course, significantly shorten a meandering
stream, but streams usually establish new meanders elsewhere when
old ones are cut off.

• Many floodplains are dominated by vertical accretion of
fine-grained sediments. When a stream overflows its
banks and floods, the velocity of the water spilling onto
the floodplain diminishes rapidly because of greater
frictional resistance to flow as the water spreads out as a
broad, shallow sheet. In response to the diminished
velocity, ridges of sandy alluvium called natural levees
are deposited along the margins of the stream channel.
Natural levees are built up by repeated deposition of
sediment during numerous floods. These natural levees
separate most of the floodplain from the stream channel,
so floodplains are commonly poorly drained and
swampy. In fact, tributary streams may parallel the
natural levee system.

• Deltas
• The fundamental process of delta formation is rather
simple: when a stream flows into another body of water,
its flow velocity decreases rapidly and deposition
occurs. As a result of such deposition, a delta forms,
causing the local shoreline to build out, or prograde.
•

Running water-Geomorhology Chapter

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