br adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
A stream is a body of water that carries rock particles and dissolved ions
and flows down slope along a clearly defined path, called a channel. Thus
streams may vary in width from a few centimeters to several kilometers.
Streams are important for several reasons:
Streams carry most of the water that goes from the land to the sea,
and thus are an important part of the water cycle.
Streams carry billions of tons of sediment to lower elevations, and
thus are one of the main transporting mediums in the production of
Streams carry dissolved ions, the products of chemical weathering,
into the oceans and thus make the sea salty.
Streams are a major part of the erosional process, working in
conjunction with weathering and mass wasting. Much of the surface
landscape is controlled by stream erosion, evident to anyone looking
out of an airplane window.
Streams are a major source of water and transportation for the
world's human population. Most population centers are located next to
Geometry and Dynamics of Stream Channels
The stream channel is the conduit for water being carried by the stream.
The stream can continually adjust its channel shape and path as the amount
of water passing through the channel changes. The volume of
water passing any point on a stream is called the discharge. Discharge is
measured in units of volume/time (m3/sec).
Cross Sectional Shape - varies with position in the stream, and discharge.
The deepest part of channel occurs where the stream velocity is the
highest. Both width and depth increase downstream because discharge
increases downstream. As discharge increases the cross sectional shape
will change, with the stream becoming deeper and wider.
Long Profile - a plot of elevation versus distance. Usually shows a steep
gradient near the source of the stream and a gentle gradient as the stream
approaches its mouth.
Base Level - base level is defined as the limiting level below which a
stream cannot erode its channel. For streams that empty into the oceans,
base level is sea level. Local base levels can occur where the stream
meets a resistant body of rock, where a natural or artificial dam impedes
further channel erosion, or where the stream empties into a lake.
When a natural or artificial dam impedes stream flow, the stream adjusts
to the new base level by adjusting its long profile. In the example here,
the long profile above and below the dam are adjusted. Erosion takes place
downstream from the dam (especially if it is a natural dam and water can
flow over the top). Just upstream from the dam the velocity of the stream
is lowered so that deposition of sediment occurs causing the gradient to
Velocity - A stream's velocity depends on position in the stream channel,
irregularities in the stream channel caused by resistant rock, and stream
gradient. The average velocity is the time it takes a given particle of
water to traverse a given distance. Stream flow can be either laminar, in
which all water molecules travel along similar parallel paths, or
turbulent, in which individual particles take irregular paths. Turbulent
flow can keep sediment in suspension longer than laminar flow and aids in
erosion of the stream bottom. Average linear velocity is generally greater
in laminar flow than in turbulent flow.
Discharge - The discharge of a stream is the amount of water passing any
point in a given time.
Q = A x V Discharge (m3/sec) = Cross-sectional Area (width x average
depth) (m2) x Average Velocity (m/sec)
As the amount of water in a stream increases, the stream must adjust
its velocity and cross sectional area in order to form a balance.
Discharge increases as more water is added through rainfall, tributary
streams, or from groundwater seeping into the stream. As discharge
increases, generally width, depth, and velocity of the stream also
Load - The rock particles and dissolved ions carried by the stream
are the called the stream's load. Stream load is divided into three
Suspended Load - particles that are carried along with the water in
the main part of the streams. The size of these particles depends on
their density and the velocity of the stream. Higher velocity currents
in the stream can carry larger and denser particles.
Bed Load - coarser and denser particles that remain on the bed of
the stream most of the time but move by a process of saltation
(jumping) as a result of collisions between particles, and turbulent
eddies. Note that sediment can move between bed load and suspended
load as the velocity of the stream changes.
Dissolved Load - ions that have been introduced into the water by
chemical weathering of rocks. This load is invisible because the ions
are dissolved in the water. The dissolved load consists mainly of
HCO3- (bicarbonate ions), Ca+2, SO4-2, Cl-, Na+2, Mg+2, and K+. These
ions are eventually carried to the oceans and give the oceans their
salty character. Streams that have a deep underground source generally
have higher dissolved load than those whose source is on the Earth's
As one moves along a stream in the downstream direction:
Discharge increases, as noted above, because water is added to the
stream from tributary streams and groundwater.
As discharge increases, the width, depth, and average velocity of
the stream increase.
The gradient of the stream, however, will decrease.
It may seem to be counter to your observations that velocity increases in
the downstream direction, since when one observes a mountain stream near
the headwaters where the gradient is high, it appears to have a higher
velocity than a stream flowing along a gentle gradient. But, the water in
the mountain stream is likely flowing in a turbulent manner, due to the
large boulders and cobbles which make up the streambed. If the flow is
turbulent, then it takes longer for the water to travel the same linear
distance, and thus the average velocity is lower. Also as one moves in the
The size of particles that make up the bed load of the stream tends
to decrease. Even though the velocity of the stream increases
downstream, the bed load particle size decreases mainly because the
larger particles are left in the bed load at higher elevations and
abrasion of particles tends to reduce their size.
The composition of the particles in the bed load tends to change
along the stream as different bedrock is eroded and added to the
Floods occur when the discharge of the stream becomes too high to be
accommodated in the normal stream channel. When the discharge becomes too
high, the stream widens its channel by overtopping its banks and flooding
the low-lying areas surrounding the stream. The areas that become flooded
are called floodplains.
Straight Channels - Straight stream channels are rare. Where they do
occur, the channel is usually controlled by a linear zone of weakness
in the underlying rock, like a fault or joint system. Even in straight
channel segments water flows in a sinuous fashion, with the deepest
part of the channel changing from near one bank to near the other.
Velocity is highest in the zone overlying the deepest part of the
stream. In these areas, sediment is transported readily resulting in
pools. Where the velocity of the stream is low, sediment is deposited
to form bars. The bank closest to the zone of highest velocity is
usually eroded and results in a cutbank.
Meandering Channels - Because of the velocity structure of a stream,
and especially in streams flowing over low gradients with easily
eroded banks, straight channels will eventually erode into meandering
channels. Erosion will take place on the outer parts of the meander
bends where the velocity of the stream is highest. Sediment deposition
will occur along the inner meander bends where the velocity is low.
Such deposition of sediment results in exposed bars, called point
bars. Because meandering streams are continually eroding on the outer
meander bends and depositing sediment along the inner meander bends,
meandering stream channels tend to migrate back and forth across their
If erosion on the outside meander bends continues to take place,
eventually a meander bend can become cut off from the rest of the
stream. When this occurs, the cutoff meander bend, because it is still
a depression, will collect water and form a type of lake called an
Braided Channels - In streams having highly variable discharge and
easily eroded banks, sediment gets deposited to form bars and islands
that are exposed during periods of low discharge. In such a stream the
water flows in a braided pattern around the islands and bars, dividing
and reuniting as it flows downstream. Such a channel is termed a
braided channel. During periods of high discharge, the entire stream
channel may contain water and the islands are covered to become
submerged bars. During such high discharge, some of the islands could
erode, but the sediment would be re-deposited as the discharge
decreases, forming new islands or submerged bars. Islands may become
resistant to erosion if they become inhabited by vegetation
Erosion by Streams
Streams erode because they have the ability to pick up rock fragments and
transport them to a new location. The size of the fragments that can be
transported depends on the velocity of the stream and whether the flow is
laminar or turbulent. Turbulent flow can keep fragments in suspension
longer than laminar flow. Streams can also eroded by undercutting their
banks resulting in mass-wasting processes like slumps or slides. When the
undercut material falls into the stream, the fragments can be transported
away by the stream. Streams can cut deeper into their channels if the
region is uplifted or if there is a local change in base level. As they
cut deeper into their channels the stream removes the material that once
made up the channel bottom and sides.
Sudden changes in velocity can result in deposition by streams. Within a
stream we have seen that the velocity varies with position, and, if
sediment gets moved to the lower velocity part of the stream the sediment
will come out of suspension and be deposited. Other sudden changes in
velocity that affect the whole stream can also occur. For example if the
discharge is suddenly increased, as it might be during a flood, the stream
will overtop its banks and flow onto the floodplain where the velocity
will then suddenly decrease. This results in deposition of such features
as levees and floodplains. If the gradient of the stream suddenly changes
by emptying into a flat-floored basin, an ocean basin, or a lake, the
velocity of the stream will suddenly decrease resulting in deposition of
sediment that can no longer be transported. This can result in deposition
of such features as alluvial fans and deltas.
Floodplains and Levees - As a stream overtops its banks during a
flood, the velocity of the flood will first be high, but will suddenly
decrease as the water flows out over the gentle gradient of the
floodplain. Because of the sudden decrease in velocity, the coarser
grained suspended sediment will be deposited along the riverbank,
eventually building up a natural levee. Natural levees provide some
protection from flooding because with each flood the levee is built
higher and therefore discharge must be higher for the next flood to
Terraces - Terraces are exposed former floodplain deposits that
result when the stream begins down cutting into its flood plain (this
is usually caused by regional uplift or by lowering the regional base
level, such as a drop in sea level).
Alluvial Fans - When a steep mountain stream enters a flat valley,
there is a sudden decrease in gradient and velocity. Sediment
transported in the stream will suddenly become deposited along the
valley walls in an alluvial fan. As the velocity of the mountain
stream slows it becomes choked with sediment and breaks up into
numerous distributary channels.
Deltas - When a stream enters a standing body of water such as a
lake or ocean, again there is a sudden decrease in velocity and the
stream deposits its sediment in a deposit called a delta. Deltas build
outward from the coastline, but will only survive if the ocean
currents are not strong enough to remove the sediment. As the velocity
of a stream decreases on entering the delta, the stream becomes choked
with sediment and conditions become favorable to those of a braided
stream channel, but instead of braiding, the stream breaks into many
smaller streams called distributary streams.
Drainage Basins and Divides - Drainage systems develop in such a way
as to efficiently move water off the land. Each stream in a drainage
system drains a certain area, called a drainage basin. In a single
drainage basin, all water falling in the basin drains into the same
stream. Drainage basins can range in size from a few km2, for small
streams, to extremely large areas, such as the Mississippi River
drainage basin which covers about 40% of the contiguous United States
. A divide separates each drainage basin from other drainage basins.
Stream Order - The smallest streams in a drainage network have no
tributary streams. These are called first order streams. Two first
order streams unite to form a second order stream. Second order
streams only have first-order streams as tributaries. Third order
streams only have second and first order streams as tributaries, etc.
As the order of the stream increases, the discharge increases, the
gradient decreases, the velocity increases, and the channel dimensions
(width and depth) increase to accommodate the increased discharge.
Drainage Patterns - Drainages tend to develop along zones where rock
type and structure are most easily eroded. Thus various types of
drainage patterns develop in a region and these drainage patterns
reflect the structure of the rock.
Continental Divides - Continents can be divided into large drainage basins
that empty into different ocean basins. For example: North America can be
divided into several basins west of the Rocky Mountains that empty into
the Pacific Ocean. Streams in the northern part of North America empty
into the Arctic Ocean, and streams East of the Rocky Mountains empty into
the Atlantic Ocean or Gulf of Mexico. Lines separating these major
drainage basins are termed Continental Divides. Such divides usually run
along high mountain crests that formed recently enough that they have not
been eroded. Thus major continental divides and the drainage patterns in
the major basins reflect the recent geologic history of the continents.