Stream Load
Erosion/Settling Velocity
Importance
Equilibrium / Rejuvenation
Geomorphological Features
Stream Transport: Load
(This slide was introduced in The Dynamics of Moving Water)
The ability of a stream to carry load of solid material varies with
discharge, velocity and particle size.
discharge
particle
size
velocity
load
load
load
Question: Using these graphs, explain in words the relationship
between discharge, velocity, particle size and load (the amount
of material carried within the stream).
Stream Transport: Load
 Solid matter carried by a river is called its STREAM LOAD.
A river’s ability to do work (pick and move material) depends
upon its energy.
Stream Load can be carried in three different forms:
1. Dissolved load
2. Bed load
3. Suspended load
The varying dynamics of a rivers’ stream flow affect how these loads move.
(velocity, discharge rate, depth etc.)
The geology of the underlying bedrock and surrounding geomorphology
effect the amount of each of the above materials in a particular river.
Physical Geography, Strahler and Strahler
Dissolved Load
The dissolved load is not usually visible; it is carried in
solution. This is material carried in the form of dissolved ions
resulting from mineral alteration.
3.9 million tonnes annually are removed from the land in
dissolved load.
Amount depends on rock type (bedrock) and climate.
Bed Load
 Bed load is material that rolls, bounces, slides along the
bottom of the river and is not supported by the water.
Sand, gravel and larger particles move along as bed load.
Moves slowly and it is hard to measure
The largest grain a stream can move by bed load is called
its competence
A stream can move larger particles at times of flood
Capacity is the maximum amount of debris a given stream
can carry as bed load
Depends on the stream gradient, discharge, and size of the
load.
Suspended Load
Finer particles (clay and silt) are carried in suspension.
Held within the water by turbulence and flow of the stream.
Once these particles are picked up, very little energy is
needed to transport them, less than the energy needed to
pick them up.
Highest concentration is near the bottom
Larger particles are near the bottom; smaller ones evenly
distributed
Greater the stream discharge the greater the suspended
load.
Critical Erosion Velocity
The lowest velocity at which grains of a given size will move.
The smaller the particles, the lower the velocity to move them.
Sand is easily eroded and carried; silt and clay size particles
less so because the particles are “bound” or stuck together.
Gravel is hard to move simply because of its size.
Settling Velocity
The velocity at which grains are deposited
Varies according to grain size
Importance
Knowing the load of a river is important for planning
storage dams
Sediment trapped behind the dam will eventually fill it in and
end the useful life of the reservoir
At the same time it deprives the river of sediment further down
the river past the reservoir. This can upset the river’s regime
Eg., if the sediment load is reduced, when the same amount of
water is put back into the river below the dam it can erode a lot
more material, since the river is not carrying anything – this
can scour the river bottom.
Physical Geography, Strahler and Strahler
Importance
Knowing the load of a river is important when predicting the
changes in river dynamics due to FLOODS.
Floods increase the capacity of a stream to carry load (both
suspended and bedload) because the volume of water flowing
downstream increases.
Floods are usually associated with increased velocity and this
also increases a river’s capacity to move material.
Eg., capacity to move bedload goes up about 3rd to 4th power
of the velocity
If a stream’s velocity is doubled in a flood, its capacity to carry
bedload goes up 8 to 16 times, and the overall load will
increase even more due to increased volume.
Rivers, therefore, can change dramatically in floods.
Physical Geography, Strahler and Strahler
Stream Equilibrium
A stage of river maturity, when down cutting is completed
and there is a smoothly graded river course
In equilibrium the supply of load to a stream (from
tributaries and from overland flow) equals the average rate of
stream transport
In other words, capacity is satisfied by the load supplied.
The river cannot cut downwards without upsetting its
equilibrium and therefore it tends to meander, cutting
sideways at the banks.
As it erodes material from one bank (on the outside - Pool) it
deposits material on the other bank (the inside - Riffle) and
the load remains constant.
Physical Geography, Strahler and Strahler
Stream Equilibrium: Rejuvenation
Rejuvenation: The stream equilibrium changes due to a
change in the base level or increase in either the amount of
water a stream carries or its load.
 The re-establishment of a youthful stream when it starts to
downcut.
Can occur in three ways:
Eustatic rejuvenation – caused by the lowering of the sea
level during an ice age
Dynamic rejuvenation – caused by the uplift of the land
during tectonic activity
Static rejuvenation – caused when either the load is reduced
therefore increasing the capacity or when rainfall increases
thereby increasing capacity.
Physical Geography, Strahler and Strahler
Physical Geography, Strahler and Strahler
More Geomorphological Features!
FLOODPLAINS
Valley area through which a river flows in the mature (old age)
stage
Built by river depositing material on the insides of the bends as it
meanders and when finer materials are deposited on the valley
floor when the river floods.
Finer materials traditionally added nutrient value to the soil and
thus helped to make floodplains very fertile areas, supporting
large populations
Unfortunately, large floods also usually meant a loss of life as
well.
A few floodplains support a large portion of the world’s
population: Hwang Ho, Yangtze, Indus, Ganges and the Nile.
A Landsat 5 image taken Sept.
1992 shows a section of the
Missouri River. The oblique
perspective of this image is
looking upstream. This image
has been color enhanced and
modified to show an
exaggerated topographic relief.
Bare soil and plowed land
appears red, vegetation
appears green, and water is
dark blue. A flat river flood
plain can be seen in the center
of the image. Because of the
season, most of the farmland
located on the rich and fertile
soils of the floodplain is plowed
and devoid of vegetation.
http://www.geog.ouc.bc.ca/physgeog/contents/11j.html
More Geomorphological Features!
LEVEES
Form on floodplains when rivers flood.
Water quickly loses its velocity as it spreads out from the
channel and deposition occurs
Heavier sediments tend to settle out immediately close to the
channel and an area of slightly higher ground is built up on
both sides of the river
Because of levees, the river channel is often built up higher
than the floodplains, which is then very susceptible to flooding
Yazoo streams are tributary streams that can’t reach the main
river because of levees, and therefore flow parallel to the main
channel in the river valley
Natural levees
have been built
up by man for
flood control.
http://www.spk.usace.army.mil/what/envir/fldpln/fldpln.html
Meanders and levees, Red River flood, Manitoba, 1997
http://sts.gsc.nrcan.gc.ca/page1/landf/smanitoba/levees.htm