Geography 140
Introduction to Physical Geography
Lecture: Overland fluvial Processes
F. Fluvial processes entail the erosion, transportation, and deposition
of earth materials by running water.
1. Fluvial processes and fluvial landforms dominate land surfaces the
world over, as opposed to the limited effects of glacial, coastal,
and wind processes. This is true even in deserts: Even though
deserts are so dry, when they do get rain, it's often torrential
and can generate flash floods. Most of the geomorphic work of
desert streams is done in a few flood events.
2. There are two major groups of fluvial landforms:
a. Erosional, e.g.,
i. river valleys, canyons, ravines, gullies, and rills
ii. the unconsumed mountains, hills, and ridges in between them
iii. caverns in limestone
b. Depositional, e.g.,
i. Floodplains
ii. Alluvial fans (formed where a stream flows out of a canyon
onto a flatter surface)
iii. Deltas (formed where a stream flows out into the sea or
lake)
iv. Stalactites and stalagmites.
3. There are three major groups of fluvial processes:
a. Overland flow
b. Stream flow
c. Underground flow
4. This lecture concentrates on overland processes.
a. Fluvial action begins on the uplands of drainage basins, when
precipitation intensity exceeds infiltration capacity and
evaporation.
b. Some definitions:
i. Drainage basins are the catchment areas for a given stream.
a. They are separated from one another by drainage divides
(hills, ridges, and mountains): Precipitation that
falls on one side of a drainage divide flows into
Drainage Basin A; if it falls just a meter away over the
divide, it flows into a neighboring drainage basin,
let's call it "B."
b. These are also called catchments or watersheds.
c. An older usage for "watershed" was "drainage divide,"
but that's not common these days.
d. Actually, some precipitation that falls in Drainage
Basin A may still wind up in a stream in Drainage Basin
B, if it infiltrates into the soil and enters an aquifer
(or underground water-transporting rock layer), which
moves the water over to a spring in Drainage Basin B.
Groundwater can complicate catchment "accounting."
ii. Precipitation intensity is the amount of water that falls
in a given period of time, such as in cm/hr or in./hr.
iii. Infiltration capacity is the amount of water a soil surface
can sieve downward into itself in a given period of time.
Infiltration capacity is affected by:
a. Soil texture: Infiltration capacity varies directly
with clast size. In other words, the bigger the clasts
(and the spaces in between them), the greater the
capacity of a material to drain water down through
itself.
1. Clayey soils (made up of very fine clasts, less than
0.002 mm in size) have very small interstitial spaces
or pores and a tremendous amount of surface area
compared to volume.
A. Think of a cube of something, like, oh, Silly
Putty, let's say 1 cubic centimeter in size. So,
its surface area would be, oh, 6 square
centimeters, right (1 sq. cm per side, and 6
sides).
B. Now, cut it in half. You now have the same
volume, 1 cubic centimeter, but you now have 8 sq.
cm of surface area (the two new faces you made
with your cut). Do it again, and you have 10 sq.
cm per cubic centimeter. And so on.
C. With that much surface area, water has that much
more opportunity to bond to clay (adsorb) and hang
out near the ground's surface, rather than
infiltrating downward. This is hygroscopic water.
D. Just beyond the layer of water adsorbed onto the
clay, there is capillary water (water molecules
clinging to other water molecules, including those
adsorbed onto the clay), which basically fills the
tiny pore spaces in a densely packed clay soil:
There is little opportunity for water to migrate
downward with all these sticky attractors keeping
it from responding to gravity. Only the water
beyond the water subject to capillary attraction
is free to move downward in the soil in response
to gravity. This is gravitational water.
![[ chemically and physically bound water
in soil, capillary water, gravitational water, VICAIRE, Virtual Campus in
hydrology and water resources ]](http://hydram.epfl.ch/VICAIRE/mod_3/chapt_2/pictures/p2.4.gif)
E. Even soils containing other, larger materials,
such as sand, can still get clogged if the clays
in them migrate and plug up their larger pore
spaces.
2. Soils dominated by larger clasts (e.g., silts, which
are about 0.002 mm to 0.05 mm in size, and sands,
which are 0.05 mm to 2 mm in size), as long as there
isn't much clay in them, have relatively low surface
area for their volume and large pore spaces. There
is little surface for water to adsorb onto, and the
spaces are so big that there is space beyond the
capillary layer of water stuck to water that's
adsorbed. This space is large enough for
gravitational water to form: water that can respond
to gravity and pour down from pore to pore into the
ground.
b. Soil cover also affects infiltration capacity.
1. Litter and roots restrict overland flow through
frictional resistance and acting like micro-dams,
which give precipitation a little more time to filter
down into the ground.
2. Also, soil cover protects bare ground from splash
erosion.
A. Water droplets smacking into the ground after a
long fall from a cloud have pretty high terminal
velocities.
B. Soil particles smacked by a raindrop are compacted
a bit, which partly seals the surface.
C. They are also shifted around a little bit and
drift downslope (soil creep) and tend to clog any
natural openings in the soil, which also helps
seal the surface against infiltration.
iv. Evaporation rate is a trivial consideration, generally,
because it drops to near zero in most storms.
c. Overland flow, then, is directly related to Precipitation
intensity and inversely related to Infiltration capacity and
Evaporation:
O = P - I - E
d. The rôle of overland flow:
i. By exerting a dragging force over the slope surface,
overland flow picks up particles of mineral matter and
pulls them downslope.
a. This includes clastic materials (clays to gravel and
larger rocks).
b. It may also include dissolved ions.
ii. This slow movement of soil downslope is a part of the
natural gradation of land surfaces and goes on
continuously.
a. Normally, the rate of overland erosion in humid climates
is slow enough to allow a chemically profiled true soil
to develop and maintain itself and the plant communities
dependent on it.
b. Soil easily develops under stable, humid conditions, and
typically develops a profile of distinct layers:
1. A topsoil or zone of eluviation from which basic
nutrients (e.g., calcium, potassium, phosphorous) are
removed downward by gravitational water.
2. A subsoil or zone of illuviation where these
nutrients are deposited.
3. A layer of weathering rock materials (regolith).
4. And, possibly, below them, a layer of unaltered
bedrock.
c. A profiled soil is a mature soil and it can accommodate
the removal of topsoil by overland flow as long as that
removal isn't faster than the processes that create and
maintain profile development (pedogenic processes).
iii. Accelerated erosion is something else: Overland flow
removes materials faster than pedogenic processes can work
to maintain a well-developed soil.
a. This happens naturally, because of natural changes in
climate regimes through geologic time
b. It can also happen naturally because of tectonic uplift
changing local slope angles, which speeds up water flow.
c. More commonly, over the last few thousand years,
accelerated erosion is the result of human activity:
1. Humans alter fire regimes in vegetation. Fire
naturally leads to temporary removal of the plant
cover and accelerated erosion, but human society will
change the frequency and the magnitude of this
temporary accelerated erosion.
2. Agriculture and animal grazing produce denudation
(clearing) of natural vegetation and soil compaction.
3. Mining activities create slopes vulnerable to
overland flow
4. Urbanization seals off soil, leading to faster
overland flow, which then races across bare slopes.
e. Forms of overland erosion and deposition (especially of
accelerated erosion):
i. Rain splash on bare slopes leads to sealing of the slopes
(a reduction in infiltration capacity), which leads to
sheet erosion.
ii. Steep slopes subjected to heavy precipitation may form
rills, small gouges in the slope, as tiny irregularities in
the surface begin to channel overland flow (this is sort of
the transition to channeled or stream flow, actually). You
can see this here in the Cinder Cones and Lava Beds Natural
Landmark Area in the Mojave Desert (remember that fluvial
erosion dominates even arid landscapes through the
disproportionate effect of the few storm events):
![[ rills on a cinder cone in the north central
Mojave, USGS ]](http://pubs.usgs.gov/of/2004/1007/images/cone.gif)
iii. Rills may grow into larger gullies if not smoothed out by
tilling, which can fuse into one another and become humid
climate badlands.
iv. The material washed off through sheet erosion or from rills
and gullies may be deposited at the base of the slope as
colluvium (overland flow depositional landform): If this
material makes it to a stream, the stream may carry it off
as part of its sediment load and deposit it downstream as
alluvium (stream depositional material).
v. One of the most dramatic examples of accelerated erosion is
seen in the photograph below. Where do you think this
would be? Some desert? Mais non -- it's Copper
Hill, Tennessee. Yes, Tennessee. What happened here is
there used to be a copper smelter nearby, which emitted
sulphur into the air, which became sulphuric acid, which
killed the forest that used to live on these slopes!
Assisting the deforestation were logging operations. With
the trees dead and gone, the denuded soil surface was
subject to sheet erosion and gullying.
![[ accelerated erosion, Copper Hill, TN, TN
History for Kids, Ducktown Basin ]](http://www.sitemason.com/files/llZI5i/copperhill3.JPG/main.jpg)
![[ smelter, 1912, Copper Hill, TN, TN History for
Kids, Ducktown Basin ]](http://www.sitemason.com/files/eUHUyY/full1912acidplant.jpg)
Come away from this lecture knowing the definition of fluvial processes. Be
aware that fluvial landscapes dominate land surfaces. Be able to
differentiate erosional from depositional fluvial landforms and what the three
classes of fluvial processes are.
With respect to overland flow, know when overland flow is likely to take place
and the factors that affect soil infiltration capacity. Know the formula for
predicting the amount of overland flow. Know physically how overland flow
denudes slopes and be able to distinguish normal denudation from accelerated
erosion. Know that mature, stable soils develop layered profiles and know
what the four common layers are. Recognize the major overland flow landforms.
Document and © maintained by Dr.
Rodrigue
First placed on web: 12/03/00
Last revised: 07/08/07