Geography 140
Introduction to Physical Geography
Lecture: Stream Processes
5. Stream processes
a. Stream processes entail the erosion, transportation, and
deposition of earth materials from the floor and bed of a
stream's channel by channelized water.
b. Means of erosion:
i. Streams can erode through hydraulic action or hydraulicking
a. This is erosion through the force exerted by moving
water alone.
1. Earth materials, obeying Newton's First Law of
Motion, would tend to just stay wherever they are,
unless acted on by a force.
2. Refresher: Newton's First Law states "every object
continues in its state of rest or of uniform motion
in a straight line if no net force acts upon it."
3. So, what's a force? It's any influence capable of
producing a change in the motion of a body (such as a
soil particle).
4. In this situation, water, responding to gravity,
transmits a force that imparts motion to earth
materials in its channel. Erosion could be thought
of as the act of imparting motion to otherwise
stationary earth materials.
b. Acceleration (the change in motion per second each
second) is equal to force divided by mass.
Alternatively, force is equal to mass times
acceleration.
1. This follows from Newton's Second Law of Motion:
"The acceleration that a net force gives an object is
directly proportional to the magnitude of the force
and inversely proportional to the mass of the object,
and the acceleration is in the direction of the
applied force."
2. So, the force exerted on earth materials in the
stream's bed or sides is directly proportional to the
mass of the water in motion and the acceleration of
the water created by the steepness of the water's
descent. That is, the force exerted on the stream's
cross-section is larger for faster-moving streams and
for higher discharges.
3. The velocity of a stream is not the same everywhere
in the stream, because frictional resistance to
motion varies from one place to another within a
stream.
A. The fastest-moving water is found where the
frictional resistance is least, that is, where
water is far from a surface or interface with
another material. In other words, where water
molecules flow over other water molecules with the
least shearing interference.
I. This is normally just below the surface of
the stream (its interface with air) and well
above the bottom and sides of the stream.
II. This current of especially fast-moving water,
however, is distorted by any bends or
meanders in the stream's channel. In a
straight stretch, the fastest current would
be in the middle just under the water's
surface. In a meandering stretch, the
fastest (and most erosive) current bounces
from one side of the stream to the other. It
"wants" to flow in a straight line, per
Newton's First Law of Motion, but that
straight path means that it is thrown into
the outside bank of a meander, concentrating
its erosive force there. It is reflected off
that bank and its new straight path is soon
rudely interrupted by the outside bend of the
next meander on the opposite side of the
stream.
B. The slowest net forward motion is normally found
along the sides and the bottom of the channel,
where frictional resistance is enhanced by the
irregularity of the channel bed and by the
interference produced by the turbulent flow that
results.
I. This is modified in meandering streams, when
the fastest current reflects from side to
side.
II. The outside bend, then, may see high flow
velocity, but the inside bend will show very
slow flow, perhaps only 10 or 20 percent of
the forward motion you'd find in the center
of the fastest current.
C. Plot complication: Velocity affects turbulence
and turbulence affects velocity.
I. When water is flowing slowly in a smooth
channel, it engages in a kind of aligned flow
called "laminar flow." The water can be
differentiated into layers, or laminæ.
The lamina closest to the channel flows the
most slowly, hindered by friction with the
channel itself. The next one up slides more
readily over the lamina below it, water
molecules flowing over other molecules, but
there is some shear resistance produced by
contact with the water slowed by frictional
resistance with the bottom. Each lamina,
then, moves a bit faster than the one below,
such that the top lamina moves the fastest.
II. When water speeds up, however, the flow
destabilizes, breaking into all sorts of
turbulent filaments which interfere with one
another. The interference is worst near the
channel, where irregularities in the channel
and friction are breaking the flow most
actively. Higher up, the shear force of the
filaments' interfering with one another
begins to even out their net velocity, so,
despite the turbulence, the fastest net flow
is just below the surface.
D. The normal position of the fastest-moving current
in a given stretch is marked by the thalweg, or
the line of maximum channel depth. This makes
sense, because the most erosion work (leading to
the excavation of the deepest part of the stream
channel) is accomplished by the water with the
greatest force, itself a function of velocity.
4. So, while erosive force increases with the mass and
the acceleration of the water in motion, the
acceleration produced by a given level of force is
less the heavier the objects in the stream's channel
are: So, small objects, such as sand particles, will
move more readily and farther than will large
objects, such as pebbles or boulders.
c. The specific ways that hydraulicking applies force to
earth materials in the stream's channel is through
impact (pushing) and through dragging (pulling or
shearing).
d. Hydraulicking is especially effective at eroding poorly
consolidated alluvial materials (gravel, sand, silt, and
clay).
e. It typically undermines stream's banks, especially on
the outside of any bend in the stream's channel, causing
them to collapse into the stream and add their materials
to the stream's sediment load.
ii. A second mechanism of stream erosion is abrasion.
a. This entails the stream's use of suspended materials to
smack the sides and bottom of its channel or the other
materials already lying in its channel: Larger rock
materials are hurled against the channel, chipping other
material loose, which is then added to the stream load.
b. The rolling of larger clasts (e.g., pebbles, rocks, and
boulders) abrades them as they impact other objects or
the channel itself. They are eventually ground into
various smaller and mostly smoothly rounded pieces.
c. Again, as with hydraulicking, abrasion works more
efficiently the faster the water is flowing.
d. Abrasion especially dominates a stream's erosive work in
areas of bedrock too strong to be worn down by simple
hydraulicking.
e. A neat feature sometimes produced by abrasion is a
pothole in a bedrock channel: If there is a standing,
long-lasting swirling flow because of channel geometry,
sometimes a rock caught in it will bore a hole in the
bedrock.
iii. A third mechanism of stream erosion is corrosion or
chemical action.
a. Acids dissolved in stream water interact with
susceptible minerals in certain kinds of rock,
dissolving or crumbling them.
b. This is especially conspicuous in limestone, of course,
the calcium carbonate in which is attacked by carbonic
acid to become calcium bicarbonate, which is carried
away. The results are lovely curving and fluted
surfaces in the limestone bedrock.
c. Transport is the second geomorphic task accomplished by streams,
after erosion has yielded materials to move.
i. Stream load is the material carried by a stream, and it can
be divided into three types:
a. Dissolved load, which is the individual chemical ions
produced by corrosion, is carried invisibly (it doesn't
usually affect the clarity of water, though it does
affect the taste). It makes up about half of a stream's
overall load, more in the tropics where chemical
weathering prevails (to produce those ions) and less in
cold climates where mechanical weathering dominates.
b. Suspended load, which is made up of clay and silt
particles (that is, really tiny particles, generally
less than 0.05 mm) that is carried in turbulent
suspension. During flood stage, it can also include
sand (which settles out fast). This material really
affects the clarity of the water -- a stream that has a
high suspended load content (such as most streams after
a big storm) are turbid or muddy-looking (well, wet clay
and silt are mud!).
c. Bed load is made up of the larger materials, such as
sand, gravel, pebbles, rocks, and boulders, that are
stationary most of the time, but which will be moved
along during high water episodes. High discharge phases
increase the mass of the water in motion and usually
also its velocity, both of which are directly related to
the force of the moving water. These larger clasts are
generally moved by rolling, sliding, or saltation
(jumping or bouncing).
ii. Stream capacity is the maximum stream load that a given
stream can move at a given time. It varies greatly, even
for the same stretch of stream, through time.
a. Stream capacity is a function of velocity of stream flow
1. Speed is a directionless concept; velocity is the
vector version of speed: It means speed in a
particular direction. Water moves downward, so I'll
usually refer to the rate of that movement as
velocity.
2. If water is moving very quickly, it can transport
more sediment in relation to its own discharge than
if it is moving more slowly.
3. This goes right back to ol' Newton and his Second Law
of Motion: Force equals mass times acceleration.
The faster water flows, the more force it can impart
to accelerate objects in its path, by increasing the
impact, dragging, and turbulence forces on the
stream's load. The total force exerted on a particle
sitting there, minding its own business, is called
the tractive force.
4. An increase in velocity, thus, obviously increases
the stream's capacity to move suspended load and it
also allows the stream to pick up or roll or slide
larger and larger clasts. That is, the faster the
water flows, the larger the objects that the stream
can move and the larger becomes the bed load share of
its total load. The largest size of object that a
stream can move is called its competence. The faster
water flows, the larger the objects it can transport
(that is, the more competent it is).
5. A stream's capacity to transport bed load increases
between 8 and 16 times for every doubling of its
velocity!
C = V3-4
You might want to convince yourself of this. Invent
a velocity (English or metric, makes no difference),
ideally rather a small one to avoid taxing your
calculator. Take its cube (basically, multiplying it
by itself and then itself again), or look for the
yx button on your calculator, where
y=velocity and x=3. Now, double that (small) speed.
Take ITS cube. Divide the answer by the first
answer. Eight times. If you did it with the 4th
power, it would be 16 times.
6. So, velocity governs both capacity and competence,
but what governs velocity? There are five governors
of stream velocity, and they interact with and
interfere with one another in complex ways to govern
stream velocity.
A. Gradient: The steeper the slope, the faster the
water flows (everything else here being equal,
which, of course, it rarely is). Gradient is
generally given as centimeters or meters of change
in elevation with kilometers of horizontal
distance (or inches or feet per mile).
B. Channel shape: The most efficient stream is the
one with the smallest length of bed perimeter for
a given cross-sectional area ("wetted perimeter").
To visualize the concept of wetted perimeter,
imagine crossing a stream in your rubber boots,
laying a measuring tape down from one bank through
the stream bottom to the other bank. That
measurement would be the wetted perimeter.
![[ wetted perimeter, WaterWare online
reference manual, Environmental Software and
Services, Austria]](http://www.ess.co.at/MANUALS/WATERWARE/GIF/perimeter.gif)
I. The efficiency of the channel shape is
optimized by a semi-circular stream , as
opposed to a long, flat channel or a V-shaped
or rectangular one.
II. The larger the cross-sectional area, the
smaller the length of the wetted perimeter
per unit of area. So, a stream at high
elevation in mountainous terrain is flowing
over a steep gradient, but the velocity of
water flow won't be as high as you'd expect
from the gradient, because mountain streams
are small, which increases the length of
their wetted perimeters per cross-sectional
area, and they often have V-shaped channels,
which increase the frictional resistance.
C. Channel size: The larger the channel size, the
faster the water flows. This is related to channel
shape, because the larger streams generally have
lower ratios of wetted perimeter per unit of
cross-sectional area. Larger streams, then,
experience less frictional resistance to their
overall flow.
D. Channel roughness: The more larger objects
(rocks, boulders) are in the stream channel, the
more frictional resistance and the more turbulence
there will be in the stream flow, which slows
things down. In a way, it can be viewed as
increasing the wetted perimeter (if you laid that
tape measure right into all those nooks and
crannies among all the large clasts, you'd need a
longer tape measure!). Again, this is another
reason why mountain streams don't flow as fast as
you would expect from the steep gradients.
E. Discharge: The more water is flowing by, the
faster it can flow. This is related to channel
size and through that to channel shape.
Turbulence, with all its contradictory small
individual filaments of water flow seen near the
bed, eventually produces mixed flow that averages
out at higher velocity as the discharge increases.
The upshot of that is that large streams flow
faster than you'd expect from their low slope
gradients because of the compensating effects of
discharge and the ratio of wetted-perimeter to
cross-sectional area.
b. Stream capacity is also a function of discharge.
1. Discharge reflects the volume of water flowing
through a given cross-section.
A. You can draw a cross-section of a stream channel
by measuring its width and then taking a whole
bunch of depth readings along a line as you cross
the stream. When you plot the depth readings by
distance along the line, you have created a graph
of the stream's cross-section at that point. You
can then use the graph to calculate the area of
the cross-section in square meters or square feet.
B. When you bring in the third dimension, you have a
measure of discharge. That is, if you observed
how long a floating object in the stream takes to
travel a given distance, such as a meter or a
foot, and you took readings at several points
along your line and at various depths (by
attaching weights to the object), you would be in
a position to describe the average rate of flow
for that stretch. You could then say that this
stream has a discharge of 20 cubic meters per
second ("cumec") or 9,000 cubic feet per second
("cfs").
C. More elegantly:
Q = VA, where Q = discharge
V = volume
A = area
D. Specifically using the metric system, that would
be:
Q(m3/s) = A(m2) x V(m/s)
2. An increase in discharge increases both velocity and
turbulence, which increases the force with which
water can pick up and transport its bed load.
A. Going back to Newton's Second Law of Motion, an
increase in discharge is an increase in mass,
which co-determines the force with which a stream
can impart motion to particles in it (F = MA,
force is equal to mass times acceleration).
B. Discharge also increases velocity, as we saw
above, and so indirectly increases stream force
through acceleration (which is set by the
gravitational constant operating on water to pull
it downstream).
C. Capacity increases with the second or third power
of discharge (you double the discharge, and you
increase capacity some 4-8 times):
C = Q2-3
Again, you can try experimenting around with some
imaginary discharge numbers (cumecs or cfs). Take
the square of your first experiment. Then, double
the imaginary discharge and take ITS square.
Divide that answer by the first one and it should
come out 4. If you did the same experiment, but
you cubed Q, you'd wind up with 8.
D. An increase in discharge particularly affects
suspended load, so large streams are more turbid
than small streams, all else being equal.
iii. A change in velocity (that is, acceleration, whether
positive or negative) affects the stream's capacity to
transport its load.
a. An accelerating, fast-moving stream is capable of
transporting more sediment than it is, so it tries to
increase its sediment load by engaging in erosion, which
it can do, courtesy of the increased force given it by
acceleration.
b. A decelerating, slow-moving stream is less and less able
to erode and transport load, so it adjusts its load to
its diminished force by dropping some of it
(deposition).
1. It first drops the largest clasts and then drops
progressively smaller clasts until its load matches
its diminished capacity.
2. Stream deposits, then, are very typically texture-
sorted, with larger clasts dominating deposits
upstream near the point it first experienced
deceleration and finer clasts dominating deposits
farther downstream.
3. River deposits are called alluvium.
d. Deposition is the third geomorphic task accomplished by streams.
i. It occurs whenever the erosive and transport force of a
stream has been diminished by deceleration or by a decline
in discharge (for example, well after a storm).
ii. It can also occur when there is an increase in its solid
load (bed load and suspended load) due to denudation of
slopes in its watershed (see discussion of accelerated
erosion in the overland processes lecture).
iii. It can also occur if something raises its base level, the
elevation of the still water body into which it ultimately
flows. Streams flow downward to the ocean or, in some
desert areas, to playas or ephemeral lakes (e.g., the Dead
Sea or the Salton Sea or Soda Lake out by Baker,
California, on I-15 halfway from Barstow to Las Vegas). If
sea level changes, that will effectively raise or lower the
average elevation of the watershed with respect to the new
base level. Sea levels come up at the end of an ice age,
as the water stored in glaciers melts and fills the sea.
This triggers a loss in stream force all over the world,
and streams worldwide are dominated by deposition or
aggradation until a new graded equilibrium can be reached.
iv. Alluvial materials will increase on the stream bed (channel
deposition). You will see sand, pebble, and rock bars
forming in the stream, diverting its flow into many
smaller, braiding flows.
v. It seems almost counterintuitive, but this actually
increases stream gradient right downstream from the
deposits, which increases velocity right there, which
causes the stream to attack the bars and smear them
downstream to spread out the increase in elevation. So,
bars are unstable and migrate downstream.
vi. Aggradation also changes the stream cross-section from
narrow and deep or semi-circular to wide and shallow. This
makes it even easier for braided channels to form by
increasing the wetted perimeter to area ratio, which slows
a stream down and reduces its capacity. Here are two
stream channels with roughly 24 m of cross-sectional area.
The one on the left, being deeper, has a wetted perimeter
of only ~14 m. The one on the right, being very shallow,
has a wetted perimeter of roughly 26 m, 26 m for friction
to work to slow the stream down.
![[ wetted perimeter of ~14 m to 24 sq. m. of
cross-sectional area, South West Grid for Learning Rivers
project, England ]](http://www.swgfl.org.uk/rivers/images/flashimages/rsexam2.gif)
![[ wetted perimeter of ~26 m to 24 sq. m. of
cross-sectional area, South West Grid for Learning Rivers
project, England ]](http://www.swgfl.org.uk/rivers/images/flashimages/rsexam3.gif)
vii. Just the opposite would happen if there were a drop in sea
levels (as during a new ice age, which would raise the
elevation of the watershed above its base level). The
streams would become more erosive. Similarly making streams
more erosive would b an increase in elevation through
tectonic uplift, an increase in discharge due to a change
in climate, or a decrease in bed load (perhaps due to
reforestation on badly denuded slopes or due to a dam going
in upstream, trapping sediments).
a. The streams would become more erosive because their
capacity relative to their present loads would have
increased, and the streams would try to adjust to their
newly increased relative capacity.
b. Channel profiles would become narrower and deeper.
c. It would cut through its own alluvial deposits, creating
stepped valley surfaces called alluvial terraces.
e. Stream systems are highly dynamic, changing the balance of
erosion, transportation, and deposition tasks constantly and
rapidly, always trying to achieve a dynamic equilibrium among
inputs into the stream and capacity to move them through. This
equilibrium is called the graded stream. It refers to a stream
that, on average, exactly balances its actual sediment load to
its potential sediment load. We've seen how streams balance
their work on the longer time frames of ice ages and
reforestation. I'd like to look at shorter-term changes in a
stream's dynamics.
i. Storm hydrographs show discharge (Q) through time before,
during, and after a storm:
![[ idealized hydrograph, EPA ]](http://epa.gov/watertrain/stream/s10.jpg)
a. The histogram (like a bar chart) on the left shows a
storm event: The tallest bar is the most intense
precipitation, which for this storm took place pretty
early in the storm.
b. The base flow connects the discharge curve before and
after the water hit the stream gage: It was slowly
being depleted through time before the storm, and it
resumes slowly declining after the storm's discharge
blasts past the stream gage.
c. The bulge in the middle is the arrival of the storm
discharge at the location of the stream gage. Notice
that the crest is well to the right of the peak
precipitation -- it is normal for peak streamflow to
crest after a lag time, because it takes time for water
falling on the watershed to make its way to streams
overland or through groundwater.
d. The rising limb is the part before the crest hits when
water levels are increasing; the falling limb or
recession limb is the period of decline after the crest
hits.
e. We could draw a horizontal line through the whole graph
above the base flow and call it "bankful capacity," if
we knew the discharge capacity of the stream channel.
The crest of the storm flow could well be greater than
the channel's bankful capacity, which means a flood
would happen if the crest were taller than that bankful
capacity line. The stream would top its banks and flood
across the countryside beyond.
ii. There are predictable changes in stream channel cross-
sections as a storm's effects hit, related to the changes
in mass and velocity associated with the storm.
![[ discharge vs bed depth during storm, USGS ]](http://web.archive.org/web/19970707102437/http://water.wr.usgs.gov/gravel/bedscour.gif)
a. Water input is raised after the storm (duh), as shown by
the height of the red line in the graph above.
b. Early in the rising limb, bed load increases because of
the heavy runoff after the storm, as shown in the level
of the black line above.
c. But, as the water level continues to rise, the discharge
(and the erosive and transportation force) of the stream
outstrips the additional bed load. So, stream capacity
now exceeds its actual solid load, and the stream begins
to downcut its own channel, substantially deepening it.
d. So, right about the time of the highest water discharge,
the streambed is at its lowest elevation (the stream is
at its deepest).
e. As discharge then drops, the capacity of the stream
declines (loss of flowing mass and deceleration of
what's left), so the stream starts building the stream
bed back up towards its former level!
f. Landforms associated with stream action.
i. Erosional landforms in mountainous terrain include narrow,
steep-sided valleys (we call the especially steep ones out
here "canyons"), that have a sort of V shape to them. The
stream bed itself often is narrow, and it occupies all or
most of the valley bottom. If you go hiking in these
canyons, you often find yourself jumping from rock to rock
IN the stream channel to make any progress. The stream
primarily engages in downcutting of its valley in such a
location, because it has the erosive force to do so: High
gradients. Valley walls are undercut from time to time and
fall into the stream for transport downstream.
![[ V-shaped stream canyon, Arakawa River, The
University Forest in Chichibu, Japan ]](http://www.uf.a.u-tokyo.ac.jp/english2/chichibu/chichibu011.jpg)
ii. Lower down, closer to the whole stream system's base level,
slopes are gentler (though discharge is higher, because
small mountain streams join together to form higher order
streams). The stream is less able just to cut down through
its valley, as it encounters particularly resistant rocks.
a. When it find a resistant rock mass in its way, the
stream switches some of its erosive energy to lateral
cutting.
b. This sets up stream meanders, which will persist even
after the original obstacle eventually gives way.
c. This is because the fastest, most erosive current in the
stream hits outside bends in the meanders, and thus
concentrates its erosive force on the outside banks.
These are undermined, so the meander gets more extreme
in amplitude.
d. At the same time, there is really slow flow on the
inside bends of the meanders, which reduces capacity
there, and the stream begins to deposit materials on the
inside bends. It builds up point bar deposits there,
which further reïnforces the amplitude of the
stream meanders.
e. Over time, the mix of lateral erosion and lateral
deposition activities begins to form a floodplain, or a
relatively flat valley bottom.
f. The width of such valleys is usually just about the
meander wave height or displacement (that would be a
horizontal line twice the wave amplitude, in this case,
that is, running from a line passing through the crests
to a line passing through the troughs).
![[ wave characteristics ]](http://astro.uchicago.edu/cara/outreach/se/ysi/1999/diagram.jpg)
g. You can see this in this false color Landsat image of
the Missouri River (false color means that wavelengths
we can't see, e.g., infrared, are assigned colors, which
means that familiar colors are assigned other colors
that seem odd to us, to create "room" in the visible
light spectrum for the wavelengths we can't see. So,
here, regular green vegetation (such as crops) are
assigned a color of red, while an infrared wavelength is
shown as green.). Don't focus on the funny colors:
Focus on the relative width of the flat floodplain and
the wave height of the stream meanders. See how they
pretty well coïncide?
![[ false color image of Missouri River, NASA
Goddard Space Flight Center Scientific Visualization Studio, via M. Pidwirny,
Okanagan University College ]](http://web.archive.org/web/20030104081100/http://geog.ouc.bc.ca/physgeog/contents/images/floodplain.jpg)
iii. Closer to base level (the level of the water body the
stream eventually pours into), stream gradient is nearly
flat. This reduces the stream's capacity to move its bed
load, especially, though not as much as you'd think from
the deceleration (remember, these streams have much larger
discharges, as more and more low order and medium order
streams have come together and contributed their discharges
to the main trunk river). So, the stream's geomorphic work
is dominated by deposition. Such streams are called
alluvial rivers. You have all sorts of distinctive
landforms associated with them.
a. These rivers really meader like crazy, looping and
snaking all over their floodplains. The low gradient
and approach to base level means there isn't much energy
for downcutting, so nearly all erosion takes the form of
lateral cutting. This is what makes the meanders really
extreme looking. Snakes on steroids! In this
photograph, you can actually make out some undermined
banks on the outsides of some of the meanders in the
foreground. You can also clearly see point bar deposits
on the insides of the meanders in the foreground.
![[ meandering stream and oxbow, Beaver River,
Labrador, Norm Catto, geomorph.org ]](http://www.geomorph.org/gal/mslattery/IAG5.jpg)
b. Often outside bends of adjacent meanders cut into one
another and the river will just take a shortcut and
abandon the former meander.
1. It becomes an oxbow lake. You can see one in the
middle of the photograph above, a little above
center.
2. These stagnant water bodies (bayous) eventually silt
in (no capacity to hold anything) and fill up with
dead organic matter and become solid land. You can
often see where they were, though, because the
vegetation is a little different, made up of species
that favor heavy clay soils and lots of organic
content and probably a high water table. You can see
some old oxbows in that photograph on the right side,
as areas with greener vegetation.
3. The lesson here is that these alluvial, meandering
streams really do meander: They are notorious
for changing course during a flood.
c. Related to this are ribbon lakes or abandoned channels.
1. As time goes on, the stream builds up higher and
higher levees, but that means the sediment it's
carrying is confined to the channel more of the time.
Whenever its capacity drops, it drops that sediment
in its own channel (rather than on the floodplain,
where the silt and clay would actually raise soil
fertility). The result is a rising channel bed.
2. After a while, the stream will abandon that course
for a new one at a lower level elsewhere on the
floodplain, an act called avulsion.
3. The old course is stranded as a ribbon lake, which
eventually fills in.
4. Alluvial rivers, thus, pose a serious flood hazard
all over their flood plains, because they clog their
own channels enough to raise them well above the
elevation of the surrounding floodplain.
![[ Avulsion, D. Mlekuz, Univ. Ljubliana,
Slovenia ]](http://www.kiss.uni-lj.si/~k4ff0495/images/fig8.gif)
d. Levees are features found on meandering streams on
floodplains. They are areas along the bank, which are
built up as a result of regular overbank flooding.
1. Whenever discharge exceeds the bankful capacity of
the stream, water will begin pouring over the banks
onto the floodplain beyond.
2. As this water leaves the stream, it slows down. This
is because the layer of water that leaves the stream
is much thinner than the water still racing down the
channel, which means that frictional resistance
affects it more and, so, the sheet of water
decelerates.
3. Deceleration reduces its capacity to transport earth
materials, so it begins depositing its now surplus
load of sediment right on the banks.
4. This builds up a higher and higher levee, which then
raises the stream's bankful capacity for the next
flood by raising the banks.
5. It is also ultimately an unstable situation, because
the stream now must deposit all its sediment in its
own channel, raising the water level above the
floodplain. Sooner or later, some flood will happen
along that will breach the levee somewhere and you
get a serious flood, and perhaps the stream will
entirely change course.
6. This photograph is especially interesting, because it
shows a flooded, well, floodplain, and you can make
out the path of the stream itself by the levees.
![[ levees during a flood, Jürgen
USGS Biology ]](http://biology.usgs.gov/s+t/SNT/images/gc138f32.jpg)
e. The backswamp is the part of the floodplain just back of
the levees, and, yes, it often is waterlogged and
swampy.
f. Pools, bars, and riffles are changes in the channel
shape (in cross-section) produced as the fastest current
switches from side to side, going from one outside bend
to the next.
1. Pools are found in the stream bed on the outside
bends of the meanders, where the cutting force of the
fastest current is concentrated. Point bars, as
mentioned before, are found on the inside bends of
the meanders. So, the channel in cross section forms
a flattened slope on the inside of the bend, becoming
progressively deeper as you approach the outside
bend, and then it forms a steep slope as you approach
the outside bank, which the stream is actively
undermining.
2. Riffles are found in the relatively straight
stretches between meanders, as the fastest current is
reflected off the outside meander, often breaking
into several discrete flows, separated by areas of
slow or chaotic flow. This reflects the stream
adjusting to all the alluvium it acquires by
undermining the outside banks: All of that gets
dumped into the stream, where it is deposited between
the separate fast currents in the straighter stretch
between bends. You often see these bar deposits
emerging above the base flow, especially during low
discharge times of year. The result is a braiding of
the stream during low flow.
g. Migrating meanders: Meanders tend to migrate slowly
downstream, because erosion is concentrated more on the
downstream side of outside meanders and on the
downstream-facing bank of the straight stretches between
them. You can see evidence of this very clearly in the
photograph below. Those curvilinear features on the
backswamp between the two meanders mark the former
location of the meander that is now on the lower left
(the backswamp between the two meanders marks the former
location of the meander (the stream is flowing southwest
here).
![[ migrating meanders, M. Pidwirny, Okanagan
University College ]](http://web.archive.org/web/20021212233408/http://www.geog.ouc.bc.ca/physgeog/contents/images/pointbar.JPG)
h. Yazoo streams are streams that "wannabe" tributaries to
a major alluvial river but are prevented from flowing
into it by the latter's levees. These are sometimes
called streams of belated confluence or of deferred
junction (but I like "yazoo" better, myself). They are
named after the Yazoo River in Mississippi, which tags
alongside the Mississippi River for 300 km!
i. Alluvial fans are texture-sorted fans of alluvial
deposits, formed whenever a stream suddenly finds itself
flowing over much flatter terrain than it had been
before.
1. All of a sudden, its sharply reduced capacity is far
below the load it had been happily moving along
before, and so it is forced to start dropping some of
its load.
2. The first things to be dropped are the larger clasts
and, then, if that doesn't bring its load into line
with its capacity, it drops progressively smaller and
smaller clasts, sometimes all the way down to silt
and clay. That's what's meant by texture sorting.
3. The act of deposition, however, builds up an obstacle
in the stream's course, so it frequently changes
course and typically has a very braided path. This
is why these features have a fan-like shape as the
stream switches back and forth (often visibly criss-
crossed by the braided channels of the stream).
4. Alluvial fans are especially common in the abrupt
togography of the American West, where faulting often
creates a sharp demarcation between steep mountains
and flat valleys.
5. Alluvial fans can fuse together, too, in extensive
features called bajadas (pronounced "bah-HAH-dahs").
6. Here are an alluvial fan in Death Valley seen from
overhead and a bajada coming off the Panamint
Mountains into Death Valley:
![[ Death Valley alluvial fan, NASA ]](http://vathena.arc.nasa.gov/curric/land/landform/alluvfan.gif)
![[ Death Valley bajada, Badwater, C.M.
Rodrigue ]](https://home.csulb.edu/~rodrigue/jps/art/bajada.jpg)
j. A delta is a rather similar feature, developed when a
stream debouches into a sea or lake.
1. The stream velocity drops sharply as it encounters
the relatively still water, which forces it to drop
its load.
2. This builds up a mildly texture-sorted fan of solid
land extending into the sea.
3. Overpowering its tendency toward texture-sorting is a
tendency for extension outward, building out and
burying older deltaic deposits, so the spatial
pattern in texture is often very complex.
4. Just as alluvial fans induce braiding, so, too, does
the main trunk of the stream branch out into
distributaries, which build out the delta in several
directions.
![[ Balize Delta of the Mississippi, NASA
Goddard Space Flight Center ]](http://biology.usgs.gov/s+t/SNT/images/gc138f04.jpg)
g. The longitudinal profile is the X-Y graph you could construct by
surveying the elevation of a stream at a lot of points from its
source down to its mouth, noting the distance from the source as
you did so.
i. If you plotted your elevation readings on the Y (vertical
axis) and distance on the X (horizontal) axis, you would
have created an image of the stream system seen in
longitudinal profile.
![[ longitudinal profile, Jeremy Dunning, Indiana
University, Geology 103 ]](http://www.indiana.edu/~g103/G103/week6/rivprof.jpg)
ii. You'll note that the whole system (as seen above) has a
concave-up shape, with channel slope decreasing as it
approaches its base level.
iii. The system can adjust its slope angle through erosional and
depositional activities to maintain an equilibrium between
discharge and sediment load.
a. If there's a sudden increase in sediment load (as, for
example, from an area in the watershed experiencing
accelerated erosion due to denudation), the stream
engages in deposition, which slightly raises its
gradient, so it can eventually haul the material
downstream.
b. If there's an increase in average discharge (climate
change) or a decrease in sediment load (e.g., someone
put a dam upstream that blocks sediment from migrating
downstream), that stretch of the stream will become more
erosive to reduce the slope angle and/or raise the
sediment load to restore the equilibrium between
capacity and load.
h. A stream that, on average, balances its load with its capacity
is said to be graded, and grade is the dynamic equilibrium
towards which all streams adjust their activities and slope
angles.
i. The resulting longitudinal profile is a relatively smooth
concave-up curve, as seen above.
ii. Once grade has been attained, the overall graded profile is
ever more slowly lowered throughout its length, ever closer
to (but never completely flattened out to) base level.
i. Evolution of a graded stream system.
i. Imagine a landscape newly raised through tectonic activity,
full of irregularities and discontinuities in slope angles.
ii. Runoff will collect in any shallow depression in this
irregular landscape, and the resulting lakes will collect
alluvial deposits, which raise the local topography a bit.
iii. Water flows out of these lakes towards base level and pours
over any sharp decrease in elevation (like waterfalls over
a cliff or rapids) at greater velocity, which gives it the
force to erode that part of the landscape down at a faster
rate.
iv. Between deposition in the local depressions and rapid
erosion in the local steep slopes, the longitudinal profile
is gradually smoothed out and brought closer to a graded
condition. A nice graph is provided by V. Divener of the
C.W. Post campus of Long Island University at
http://myweb.cwpost.liu.edu/vdivener/notes/gradation.gif.
v. At this point, we start to see branching outward from the
main stream, as the stream system begins to extend into the
rest of the tectonic landscape, dissecting it into a
fluvial landscape. The stream systems at lower elevations
begin to capture other similar stream systems by providing
them a lower elevation to flow into. These other stream
systems become tributaries to the more aggressive, now main
system. Branching and stream capture create a drainage
basin and a defined watershed in the area.
j. The branching of stream systems takes on a variety of distinct
forms, depending on regional geological conditions. Some common
types are:
i. Dendritic or tree-like, where the main river serves as the
"trunk," its distributaries in its delta at base level
serve as "roots," and the tributary streams serve as
"branches." The confluences of the tributaries makes a "V"
with the tip pointing downstream. These commonly develop in
regions of fairly uniform rock materials, such as
horizontally-bedded sedimentary rock. Here's a nice image
of a dendritic system:
![[ dendritic network, Yemen, NASA, through
Solarviews.com ]](http://www.solarviews.com/browse/earth/yemen.jpg)
ii. Trellised drainage systems are a lot like dendritic, but
the tributaries go off from the main trunk at closer to a
right angle, so the whole effect is rather like a grape
vine trained to grow along a trellis. This is common in
landscapes featuring long parallel ridges and valleys, such
as that created by alternating anticlines and synclines.
The tributaries flow along the valleys, connecting up with
the main stream wherever a gap is found in the ridges.
Much of Pennsylvania looks like this.
iii. Rectangular drainage also creates confluences between
tributaries and main streams that are at right angles, and
the streams themselves often take roughly right angle turns
in their paths. This sort of drainage is often associated
with rock that is deeply fractured and the streams just
helped themselves to the pre-notched paths in the
landscape.
iv. Parallel drainage is a lot like trellised, except there are
loads of low order tributaries that flow directly into a
main stream without having first joined together to create
medium order streams, which then fuse into the highest
order main trunks. Again, this reflects faulting or
fracturing in the underlying rock with a strong linear
character.
v. Radial drainage is often seen, well, radiating outward from
a single peak, each tributary joining different drainage
basins. This is especially common around volcanoes.
vi. Centripetal drainage is the opposite of radial: This time
all the streams converge on a single depression. This
occurs in areas of internal drainage, with no outlet to the
sea. Internal drainage is most common in desert and
semiarid environments, with a playa (or ephemeral or
intermittant lake, sometimes called a dry lake) at the
center. The southern San Joaquin Valley or Tulare Basin
has this pattern, with streams (most of them intermittant
as well) flowing into Lake Tulare.
vii. Deranged drainage occurs when there's no rhyme or reason to
how water descends through a nearly flat landscape and
heads for base level. Water may take the "scenic route"
all over the place before it finally finds a sea or lake.
There may be all sorts of little interconnected
depressions, bogs, and lakes. This is most common in
landscapes that have recently (as in the last 25,000 years)
experienced ice cap glaciers, such as Canada and the
American Upper Midwest ("Minnesota: Land of 10,000 Lakes").
![[ drainage network drawings, D. McConnell, The
Good Earth ]](http://www.mhhe.com/earthsci/geology/mcconnell/streams/images/drain_basins.gif)
k. Stream order.
i. I've mentioned low order, medium order, and high order
streams here and there in this lecture.
ii. This concept has to do with how many confluences a given
stretch of stream is from the spring(s) that are their
sources.
iii. If the stream you're looking at is coming directly from its
source and you are in a stretch above its first confluence
with another stream or river, you would call it a first
order (or a low order) stream. This tiny stream would have
the tiniest drainage basin or subwatershed.
iv. When two first order streams come together, you have a
second order stream, and the catchment drained by the
stretch below the confluence is that much larger.
v. Different people disagree on what you call the stretch
below the confluence of two second order streams,
particularly if a third first order stream joined one of
the second order streams before their confluence.
vi. The gist of it is that the higher the order, the larger the
average stream discharge has to be (because it represents
the union of two stream discharges): There is a direct
relationship between stream order (however you classify it)
and stream discharge. Similarly there is a direct
relationship between stream order and catchment area, for
the same reason.
vii. There is an inverse relationship, however, between stream
order and average gradient. That is, low order streams are
found at higher elevations with steeper slopes than higher
order streams. High order streams, being closer to base
level, are obviously at lower elevations, and that means
the slope gradient is flatter.
l. Now just because a stream network strives for the dynamic
equilibrium of the graded condition doesn't mean it just grinds
away until it attains grade and then just peacefully whittles
the whole longitudinal profile ever closer and ever more slowly
towards base level. The system is subject to shocks of various
sorts, which rejuvenate or advance the whole process and set the
stream's ambitions back a bit.
i. Some factors that can rejuvenate the whole system (that is,
make it emphasize erosion over deposition) include:
a. Renewed tectonic uplift.
1. Really extreme events can essentially wipe out a
given stream system or redefine its relationship with
a watershed out of any meaning.
A. Explosive volcanic eruptions reconfigure a whole
mountain ... and its stream systems.
B. Massive flood basalt flows can entirely bury a
landscape, watersheds and all. An example is the
vast Deccan Traps eruption of some 65 million
years ago in western India, which still covers
over 500,000 square kilometers and reaches close
to 2.5 kilometers in depth! This eruption
consisted of repeated flows of very deep mantle
material through great fissures in the earth.
Over the course of 500,000 to 750,000 years, this
flood eruption (or trap) buried all landscape
features then extant in western India. With this
kind of outflow, whatever old stream systems
existed there then were completely buried and
brand-new stream systems had to evolve to drain
the new plateau landscape. Interestingly, this
seems to coïncide with the Chicxulub asteroid
impact in Yucatán in southern Mexico.
2. Faulting, however, can permit blocks to rise above
the surrounding landscape without destroying a stream
system, restoring slope irregularities and
rejuvenating the whole system, making the overall
system more erosive, incising into its own deposits
in places.
A. One sign of this can be the notching of a stream's
floodplain, carving a V-sided mini-canyon in the
flat floodplain alluvium (this can also result
from non-tectonic sources, too, such as the
construction of a dam upstream, as we saw earlier
in this lecture).
B. A particularly interesting expression of system
rejuvenation is the development of very
steep incised meanders in large streams in high
elevation country, where you would expect to see
many low order, fairly straight streams. A good
example of this is the Colorado Plateau. The
Colorado River is an "antecedent" stream, meaning
it existed and developed a floodplain long before
the uplift of northern Arizona and southern Utah
(as the buried Farallon Plate slid under the
western North American Plate). The rate of uplift
was apparently slow enough to allow it and its
tributaries to downcut the Plateau fast enough to
preserve its old meandering stream course! You see
entrenched meanders in many places here, such as
here at Goosenecks in southern Utah:
![[ Goosenecks, Wikipedia ]](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/GooseNeckStateParkPanorama.jpg/800px-GooseNeckStateParkPanorama.jpg)
b. An ice age will rejuvenate systems all over the world by
causing a drop in base level, as the increasing ice
sheets withhold liquid water from the sea. The locking
up of water as glacial ice causes sea level to drop.
This effectively raises the relative elevation of all
landmasses above sea level, causing them to become more
erosive with the greater force that their elevational
acceleration gives them.
c. Still another rejuvenating factor is isostacy: the
buoyancy of the continents. As streams remove earth
materials and eventually haul them out to the sea floor,
the continents lighten. This causes them to rise up as
they depress the plates on which they ride less and
less. This elastic rebound can restore as much as 80
percent of the elevation worn down by the work of
streams. "A stream's work is never done"! It kind of
reminds you of the ancient Greek story of poor ol'
Sisyphus, the greedy Corinthian king, whom Hades
punished by having him roll a huge rock up to the top of
a mountain, only to have it roll right back down. For
eternity.
ii. Some factors can mess up grade in the opposite direction.
a. Sometimes the landscape subsides below base level, due
to faulting or to undermining of rock layers (nowadays
by humans subtracting oil or water too fast). This
causes the affected system to lose some of its capacity
to transport load and so it engages in aggradation.
b. Sometimes ice ages end, and the fresh water tied up in
great glaciers now flows to the sea and raises sea
level. This produces a global loss of relative
elevation, which decelerates stream velocities, which
reduces their erosive force and capacity. Again,
depositional activities dominate stream work, this time
on a global scale. For example, the Persian Gulf was
reduced as the sediment from the Tigris and Euphrates
systems was dumped at the delta in southern Iraq. Towns
on or near the coast thousands of years ago are now
stranded far inland.
iii. So, grade is a dynamic equilibrium, not a static one.
Streams adjust their erosion, transport, and depositional
activities to approach perfect grade, but it's a moving
target for them. The world is simply a very dynamic, often
dramatically dynamic system, to which streams (and people)
try to adjust.
m. From what you've learned of stream systems, you should come away
with a healthy respect for the main hazard posed by rivers:
Flood. It is normal for the alluvial portions of a stream
system to experience overbank flooding pretty often.
i. For a natural hazard to exist, there has to be an extreme
physical event, such as a flood, and people and their stuff
in the way.
ii. Unfortunately, there are many reasons people live on
floodplains, some of them good.
a. Rivers have traditionally been cheap ways to transport
goods, so there is an economic reason to be "in their
way."
b. Another economic reason is that normal overbank flooding
deposits alluvium on the floodplain, which replenishes
soil fertility: Floodplains are often excellent
agricultural areas, and this has led to high
concentrations of populations on them (including the
cities that formed to process and ship produce).
c. Yet another economic reason is that flat land is
extremely easy and cheap to build on, so developers are
always itching to build on floodplains (but don't you be
the chump that buys one of those homes).
d. Another economic motive is simply convenient water
supply.
e. Another reason is more æsthetic: People are drawn
to the scenic beauty of river views (that same thing
that draws people to mountain views right in the middle
of chaparral fire country!).
iii. Given that rivers are so important to human society, it is
not surprising that we have tried to make life on the
floodplain a little safer. Very commonly, we have
exaggerated natural levees by building them way up, and we
have constructed dams upstream to control discharge (and,
nowadays, to provide electric power). But does that
necessarily make us safer?
iv. This is, like chaparral fire hazard, one of those things
where our adjustments to recurrent, smaller-scale hazards
set us up for the much rarer, high-magnitude event.
v. Building up the levees means the river just drops its
alluvium on its channel bed, instead of all over its
floodplain. This inevitably raises the water in the river
high above the surrounding landscape. Sooner or later,
there will be the rare, epochal flood, which will breach
the levees, flooding the countryside, and the stream will
try to change course entirely (an aside, did you know the
L.A. River emptied into the Pacific, not in Long Beach but
in Venice? It changed course just in the last century!
What we call Ballona Creek used to be the mouth of the L.A.
River!!!
a. The problem is that dams and levees give us false
confidence, and so we approve housing subdivisions in
floodplains and then wonder why we're hip high in water
when our mitigations fail. When this (inevitably)
happens, you'll see politicians implying that the levees
weren't maintained (maybe because of that pesky
Endangered Species Act) and that's why they failed.
Maybe -- but the best-built and highest levee will
eventually fail because it is impossible to outstrip
alluvial deposition on the stream bottom (no matter how
much dredging you do) and there are very rare, really
very high floods that can top anything. Human
vulnerability arises from the false confidence that
these structures will always save us. By relying
exclusively on them, we engage in risky behavior, pure
and simple.
b. Be sure to check out flood hazard when you move (you can
check out Project Impact and learn which hazards are in
your Zip code) and don't move into areas subject to the
100 year flood zone (or, better, the 500 year flood
zone) and/or buy flood insurance and make structural
mitigations to your new home.
c. About "100 year floods." That's a misleading concept.
Floods don't recur right on their average recurrence
interval, like clockwork. You can have two "100 year
floods" right back to back! What that means is a flood
so large that it has a 1 percent chance of happening ANY
year (1% is a 0.01 probability of occurring and the
inverse, 1/0.01, of that is 100).
d. There is an inverse relationship between flood magnitude
(say, measured as discharge) and probability of
recurrence. The trouble is, this isn't a nice,
straight, linear relationship: It's more like
logarithmic (like the Richter magnitude scale for
earthquakes). Small events are very common, and bigger
events are much rarer (but the magnitude of the low
probability event is apt to be humongous).
e. Another way of expressing this is the "magnitude-
frequency relationship": The more frequent a given
flood level is, the lower the magnitude of that flood.
The greater the magnitude of a flood, the less
frequently such a flood occurs.
Now, I don't know about you, but I've ABOUT had it with fluvial processes for
today! Make sure you understand the balance among a stream's erosion,
transport, and deposition work in terms of Newton's First and Second Laws of
Motion (what determines a stream's erosive force? why does the fastest current
in a stream tend to move in a straight line in a meandering channel? how is
the acceleration of particles affected by their mass, if the force applied to
them is the same?).
Know the three mechanisms by which moving water can erode its channel.
What are the differences among dissolved load, suspended load, and bed load?
What's the difference between capacity and competence? Know which two factors
govern a stream's capacity and competence. Five factors affect stream
velocity: What are they and how do they interact with one another (sometimes
partially compensating for one another)? What are the two factors that govern
a stream's discharge (hint: They're in an equation)?
Know where in a stream the fastest and slowest currents are and why. What's
the thalweg? Know the difference between laminar and turbulent flow and which
prevails in fast-moving water (it seems a little counterintuitive). How does a
stream react to acceleration and deceleration, and which landforms might
express these reactions?
What is alluvium (and how does it differ from colluvium?)?
Understand the storm hydrograph, especially why there's a lag between peak
precipitation intensity and the crest of flood stage discharge in a stream.
Just for fun, what do you think the storm hydrograph would look like in an
urban stream channel, say the majestic L.A. River, compared to an undisturbed
stream? Think in terms of the lag and figure out why there would be a
difference in the lag (and in which direction).
Understand the predictable changes in the elevation of the stream bed during
the rising limb and the crest of a storm hydrograph.
What is the difference between downcutting and lateral cutting? Which sorts
of streams, high order or low order, tend to be predominantly engaged in each?
Why?
Why are low order stream valleys V-shaped? Why does a floodplain start
developing in medium order streams? Why do the lower reaches of stream
systems develop such insanely meandering paths? What are oxbow and ribbon
lakes and what eventually happens to them? What's a levee and how does it
form? What does formation (or human enhancement) of a levee mean for the
depositional activities of a stream and what are the consequences? What is
avulsion? What are backswamps and yazoo streams? What are pools and riffles,
where do you find them in a stream, and why? What are point bars? Where do
you find them and why? What's an alluvial fan, where do you find them, and
why? What do you call numbers of coalesced alluvial fans? What's a delta, and
how does it resemble and differ from an alluvial fan? Why do meanders migrate
downstream?
What's a longitudinal profile and how would you make one? What are the
features common to longitudinal profiles all over the world? Understand the
concept of a graded stream: How does it evolve? once grade is attained, how
does its longitudinal profile develop through time? what can disrupt the
graded equilibrium? What the heck is isostacy? What are the main shapes of
stream networks and where are you likely to find examples of each? What is
stream order and, roughly, how are streams classified?
Make sure to understand the flood hazard posed by streams. Make sure you
understand the magnitude-frequency relationship, and don't fall into
complacency the year after a "100 year" flood, thinking it can't get you
again! What DOES a "100 year flood" mean? What sorts of measures can you
take to minimize your own vulnerability to flood hazard?
Document and © maintained by Dr.
Rodrigue
First placed on web: 12/03/00
Last revised: 07/08/07