Fluvial System:
The Drainage Basin Concept
River/streams:A long narrow channel of water that flows as a function of gravity and elevation
across the Earth's surface. Many rivers empty into lakes, seas, or oceans.
Hydraulics, Erosion and Sediment Transport in Rivers
There is a relationship between the amount of energy a river has (a product of its volume, speed and
gradient) and the amount of work it can do. Rivers achieve their work (effects) by using the energy
available to them.
The potential energy, or stored energy, that is available is proportional to the height above seal level.
As water moves down slope, potential energy is converted to kinetic energy which is used to carry out
the river's tasks.
In the transfer from potential to kinetic most of the energy is lost as heat owing to friction encountered as
the water move.
Some 95% of a river's energy is used in this way, leaving just 5% to carry out erosion and transport of
debris.
You should be familiar with the terms turbulent flow, and laminar flow,
where friction is low.
Geomorphologists and hydrologists often view streams as being part of drainage basins.
Running Water: How It Erodes & Deposits
http://www.youtube.com/watch?v=_HFmxRicX4o&feature=related
A drainage basin is the topographic region from which a stream receives runoff, throughflow,
and groundwater flow.
Drainage basins are divided from each other by topographic barriers called a watershed (Figure
1). A watershed represents all of the stream tributaries that flow to some location along the
stream channel.
Figure -1: The following image shows the nested nature of drainage basins as determined from a
topographic map sheet. The red lines describe the watersheds for the drainage basins of first
order streams. The yellow lines define the watersheds for two drainage basins from locations
further upstream. Note that the first order basins are components of these much large drainage
basins.
Drainage basins are commonly viewed by scientists as being open systems.
Inputs to these systems include precipitation, snow melt, and sediment.
Drainage basins lose water and sediment through evaporation, deposition, and streamflow.
A number of factors influence input, output, and transport of sediment and water in a
drainage basin. Such factors include topography, soil type, bedrock type, climate, and
vegetation cover. These factors also influence the nature of the pattern of stream channels
(Figure -2).
Figure 10aa-2: Common drainage pattern types.
Trellised drainage patterns: channels align themselves parallel to structures in the bedrock with
minor tributaries coming in at right angles.
Areas with tectonic faults or bedrock joints can cause streams to take on a grid-like or
rectangular pattern.
Parallel drainage patterns are often found in areas with steep relief or where flow is over noncohesive materials.
Dendritic patterns are typical of adjusted systems on erodable sediments and uniformly dipping
bedrock.
Deranged drainage patterns are found in areas recently disturbed by events like glacial activity
or volcanic deposition.
Over time, the stream will adjust the topography of such regions by transporting sediment
to improve flow and channel pattern.
The movement of water upstream carries with it dissolved and solid materials eroded from the
surrounding landscape. Sediments carried by streams to lower elevations are occasionally
deposited and stored at numerous locations for various periods of time within the stream system
before they reach their final resting place.
Losses of material carried by the stream system occurs through a number of processes. Water is
lost by evaporation, seepage, and flooding. Streamflow ends when the water carried by stream
enters a receiving basin like a lake or an ocean. Sediment is lost by various types of deposition.
The Long Profile of Streams
The topographic long profile or grade of an average stream is concave-upwards (Figure 10y-2).
At their headwaters, the grade of a stream is usually steep. As streams get closer to sea-level,
the angle of the grade becomes more gently sloping. Near the mouth of the stream, the grade
becomes almost flat.
The grade of a stream develops over thousands/ millions of years. It is an equilibrium process to
reduce topographic bumps in the long profile through erosion and deposition.
The following diagrams and photographs illustrate the nature of the stream channel at three
different locations along the stream profile.
1. The first illustration describes the channel at upstream point A-B (Figure 10y-3). This
channel is located in the stream's headwaters. The stream gradient and surrounding
topography is quite steep. Stream velocity is at a maximum and cuts a narrow deep
channel. The floodplain is minimal as high velocities carry all sizes of load down stream.
Stream load is high as the extreme relief provides the energy for extensive erosion
producing a V-shaped river valley.
II. Further down the profile, at midstream, changes in relief causes the nature of the stream
channel to modify (Figure diagram C-D).

As you move midstream the channel is now leaving the mountainous terrain and
entering a landscape that is more gently sloping.

This change causes a sudden reduction in the stream's velocity. The stream adjusts to
this change, by depositing most of its coarse stream load onto the floodplain.
The stream also takes on a braided channel form. These channels are always changing in
size, number, and location because of temporal variations in stream discharge.
The amount of sediment in the floodplain also varies significantly over time because of
temporal fluctuations in flow.


Figure c-d: Stream channel near the middle of a typical stream profile.
III.
The final image and illustration describes the channel near the mouth of the stream
(Figure E-F).
Extensive flat floodplain is composed of flooding deposits or point bar deposits from channel
meandering. The channel is quite large and U-shaped. Stream discharge is at a maximum
and sediment load is generally composed of finer materials.
The Mighty Amazon
http://www.youtube.com/watch?v=ta35C488dnE
Figure: E-F Stream channel near the mouth of a stream.
Stream Discharge
The flow of water through a stream channel is called stream discharge. In most countries, it is
measured in cubic meters per second.
Discharge normally increases downstream. As discharge increases corresponding changes in
velocity, channel depth and width are made within the stream system.
Sediment Transport
All streams carry sediment. Most of the sediment found in a stream has been washed into the
channel from surface runoff. Sediment is also added from the erosion of the stream channel bed
and banks.
The quantity of sediment in a stream varies temporally due to changes in discharge. Normally, as
discharge and velocity increase, the amount of sediment being carried by the stream rises
correspondingly.
River Processes
http://www.youtube.com/watch?v=73S5X2BhhVA&feature=related
Meander
A meander
is a bend in a stream.
However during later stages as base level is achieved and channel equilibrium is approached,
lateral migration of the stream channel is more prevalent.
http://www.earthscienceworld.org/images/search/results.html?Keyword=Oxbows#null
Meanders grow both laterally and in the down stream direction. As water flows into a meander it
takes on a helical or spiral flow which determines where erosion and deposition is concentrated.
Centrifugal force draws water toward the outside bank (cut bank) causing erosion. Sediment
eroded from the outside bank is deposited on the inside bank and transported downstream.
http://www.wwnorton.com/college/geo/egeo/animations/ch14.htm#
As water rounds a meander, the water swings toward the outside bank where erosion is
concentrated and then spirals toward the inside banks. As the water spirals toward the inside of
the meander it is slowed by frictional drag imposed by the bed of the channel. This causes
deposition of alluvium on the inside bank to form a bar. A point bar forms on the inside bank of
a meander and rising from the channel as an accumulation of alluvium. As the channel meander
continues to erode laterally, a succession of bars with intervening swales form called bar and
swale topography.
Neck & Cutoff
A neck is the upland between opposing meanders of a stream. A cutoff occurs when the neck
between river meanders is eroded away and the meanders join to shorten the length of the
channel. The slope of the channel increases as well when the river shortens its length.
Mark Twain aptly described the process and effect of river cutoffs when he wrote:
"The Mississippi is remarkable in another way--its disposition to make prodigious jumps by
cutting through narrow necks of land, and thus straightening and shortening itself. More than
once it has shortened itself thirty miles at a single jump! These cut-offs have curious effects: they
have thrown several river towns into the rural districts, and built up sand bars and forests in front
of them. The town of Delta used to be three miles below Vicksburg: a recent cutoff has radically
changed the position, and Delta is two miles above Vicksburg."
~ Life on the Mississippi ~
Oxbow lake & meander
scarOxbow lake (Courtesy USGS)
A river cut-off results in a portion of the river
isolated from the new channel called an oxbow
lake.
Oxbow lakes are typically crescent
shaped - like that of an oxbow. Deltas
Streams flowing into standing water normally
create a delta (Figure 9).
A delta is body of sediment that contains numerous
horizontal and vertical layers.
Deltas are created when the sediment load carried by a stream is deposited because of a
sudden reduction in stream velocity.
The surface of most deltas is marked by small shifting channels that carry water and sediments
away from the main river channel.
These small channels also act to distribute the stream's sediment load over the surface of the
delta. Some deltas, like the Nile, have a triangular shape.
Streams, like the Mississippi, that have a high sediment content and empty into relatively calm
waters cause the formation of a birdfoot shaped delta.
Figure 10z-8: Nile Delta (Source: NASA).
Figure 10z-9: Mississippi Birdfoot Delta (Source: NASA).
Deltas
Large deposit of alluvial sediment located at the mouth of a stream where it enters a
body of standing water.
Formation of the Mississippi Delta
http://www.youtube.com/watch?v=t_IIA2o6-lk&feature=related
An alluvial fan is a large fan-shaped deposit of sediment on which a braided stream flows over
(10z-10). Alluvial fans develop when streams carrying a heavy load reduce their velocity as they
emerge from mountainous terrain to a nearly horizontal plain. The fan is created as braided
streams shift across the surface of this feature depositing sediment and adjusting their course.
The image below shows several alluvial fans that formed because of a sudden change in
elevation.
Figure 10z-10: Alluvial Fans -
The Ganges Delta
http://earthasart.gsfc.nasa.gov/ganges.html
Summary: Rivers and pollution
http://www.youtube.com/watch?v=eSpKdz5xD9A