Salt Lake Community College
Geography 1000 – Physical Geography
AJ Allred, Spring
2015
Quiz 13
Chapters 15-16
Notes to Erosion and Fluvial Processes
1. A weathering type associated with curved and concentric sets of joints that break away in successive
layers is called ________.
a. exfoliation
b. hydrolysis
c. landslide
d. creep
e. solifluction
Foliation refers to layering, like layers of an onion. A non-technical term is “peeling”. Rock that is exposed to
the surface will react differently than rock underneath. That differential activity caused splits, cracks, fractures,
separations that often follow the grain or layering that exists in most rocks. As these weaknesses develop,
exfoliation or peeling will be visible. You can often slide a knife blade inside foliated layers.
Hydrolysis is similar to soaking a cookie in milk. Water in the milk is absorbed, softening dry structures and
causing them to weaken and eventually dissolve.
A landslide is a mass waste event, not weathering, and often occurs along bedding planes or shear planes where
a weak layer or boundary allows overlying material to “shear off”.
Creep is very slow mass movement of surface layers downward along a slope. The simple effect of gravity will
do that, and the presence of water (hydrolysis enhances the effect by adding weight, lubrication, dissolution
(hydrolysis) and freeze—thaw cycling. The weight of buildings and the activity of animals and plants often
assist the gradual collapse and separation of surface layers.
It is important to note that even though large-scale processes such as solifluction and landslides do represent
movement and loss of outer surfaces, exfoliation is really a micro-scale process involving individual rocks. A
landslide may occur along a weak layer but that is not really foliation. Exfoliation is more like peeling an
onion.
2. Streams which flow for only a part of the year and carry water that originated someplace else are
called ________.
a. superimposed and exotic
b. consequent, perennial and intermittent
c. intermittent and exotic
d. antecedent and exotic
e. perennial, intermittent and ephemeral
The word intermittent means “on again – off again”, holding water for only a few weeks or months each year.
An exotic stream is water that comes from somewhere else and is “just passing through”. Perennial means
‘year-round’ – like perennial flowers that come back every year on their own.
An ephemeral stream flows even less often than an intermittent stream. After a sudden, sharp rainstorm on
hard, dry, or steep land surfaces, an empty stream bed that was dry for months or years may suddenly fill with
flood water that may cause brief but severe erosion.
Antecedent streams existed before a landform rose beneath it, so it often just carves its way back down to a flow
line, even as a mountain tries unsuccessfully to rise through the stream bed.
A consequent stream developed along with emerging landforms and tends to persist along its original slope or
direction. Likewise, a superimposed stream cuts its way down into new layers of rock, after overlying rock has
eroded away.
3. The most important type of mechanical weathering process is ________.
a. salt wedging
b. oxidation
c. frost shattering
d. hydrolysis
e. solifluction and creep
The textbook says that freeze—thaw action does more to weather rocks than any other process. Water is
virtually everywhere, even in the air and underground. Water is one of the few substances in the known
universe that expands when it freezes. Virtually everything else contracts when cooled to freezing because
kinetic motion and pressure subside with loss of temperature and heat energy.
4. Surface indicators of turbulence in stream flow include ________________.
a. eddies and braids
b. whirlpools and eddy currents
c. fluves and interfluves
d. evaporation and whirlpools
e. All of the above
A fluve is a small-scale cut or rivulet in soil or rock that accumulates into a stream, and then to a river.
Interfluves are the overland surface that water crosses on its way to a fluve, or flow channel.
A braided stream is like braided hair: a group of flow lines that cross each over time. Eddy and whirlpool
motions are caused by water curving and flowing around or over solid obstacles in a stream. Likewise, a wind
current flowing around a building can lift and spin a leaf or piece of paper.
5. Bluffs that run along a river or stream may _________.
a. be evidence that anywhere below the bluff is subject to periodic flooding
b. have been carved by a stream that is eroding everything below the bluff and carrying it toward a base
level
c. be a border line, below which soils are unstable and poor support for buildings
d. be an indicator of geologic rifting, with land sinking or subsiding between edges that are pulling
apart
e. All of the above are good possibilities.
Over time, a stream or river will “snake” back and forth, cutting away the edges of its furthest meander. A
minor bluff or cliff can form at the furthest reach of a stream elbow or cut bank. These bluffs are important
signals or warning signs that a stream may return to that location again in the future. Likewise, the relatively
flat surface below the bluff is often saturated with water and soils are unstable. During ground shaking,
cohesive strength or “friction” between soil particles can break down, causing buildings to sink or crack.
A sunken area below a pair of bluffs can also be caused by rifting, where a land form is being pulled apart from
the sides, allowing the center to “thin-out” and sink. Streams will tend to follow lower ground, which means
that erosion is added as a factor that further lowers the floor.
A block fault can also cause a bluff to form, and stream water will tend to flow on the down-thrown surface.
6. Which of the following statements about streams in Utah is not true?
a. Prevailing ‘B’ climate zones ensure that most streams are intermittent.
b. A relatively higher fraction of all solids carried by streams is by bed load or traction.
c. Exotic streams are relatively more common than in eastern United States.
d. Flash flooding is less common in Utah than in climates where annual rainfall is much greater.
e. Relatively less water is absorbed by land surfaces than in more humid climates.
Flash flooding is more a threat in dry climates than in humid climates. Rainfall tends to be ‘spiky’ or brief but
intense. Land surfaces are often hard and impermeable, so water can’t soak in easily. Land surfaces are often
steep and “slick” so water accelerates during run-off.
7. Which of the following features is most closely associated with high-speed, highly-erosive stream
flow?
a. A strongly developed network of vigorous meander scars.
b. Simple straight-line flow with little meander.
c. Braided streams.
d. Water that is carrying dissolved solids instead of bed load.
e. Evidence of heavy leaching and sharply curved oxbows.
Meander scars are a strong indicator that a stream is flowing across relatively flat or shallow-slope land,
because it does not have a definite path to a base level. Likewise, braided streams cross over each other for lack
of a steep channel to force them into one line.
Bed load that includes boulders, rocks and other heavy solids is associated with intermittent flooding or highspeed flow. Likewise, during more vigorous flow, suspended or “stirred-up” material becomes a relatively
more important component of stream erosion. During calmer periods, dissolved material is still carried.
For illustration, if falling rain is steep and straight, then steep terrain is more likely to produce straight-line flow,
as compared to flat country where there is little definite slope to guide water flow.
8.
Which of the following statements is not true of the Mississippi River?
a. Man-made protective walls or levees often fail or cause flooding elsewhere.
b. Levees have effects that are similar to the causes of “yazoo” rivers.
c. River velocity and river elevation are directly related to location of levees.
d. Flash floods are not a major issue along the main river channel.
e. Since hurricane Katrina in 2005, the U.S. Army Corps of Engineer has begun to “channelize” the
river to prevent future flood events.
For about 100 years, the Corps of Engineers believed that “reaming-out” rivers and streams and even paving
them with straight channels would help prevent flooding. More recently, the trend is to allow rivers to form
natural flow lines and allow natural habitat to re-develop.
Instead of trying to control rivers to make them safe, we should make ourselves safe, by keeping a safe distance
from streams and rivers.
It is true that we are still trying to control rivers in places where we already built cities. If we could go back two
hundred years, we could avoid some mistakes, and allow rivers to “do their own thing.” Much of our current
river control effort still has adverse consequences and we keep making mistakes as we learn.
River levees or side walls do prevent some flooding, but often make conditions worse as well, by constraining
river flow, which increases speed, water elevation and erosive effect. Building a levee at one farm is likely to
just make flooding worse at someone else’s farm. Also, levees often collapse or leak during a severe flood,
causing more damage than if we had just not trusted levees in the first place.
Flash floods are mostly in either dry country or in steeper terrain near headwaters and high country.
Downstream flooding is not usually a flashy or sudden surprise, because we know when upstream precipitation
or snowmelt is starting to build and head toward us.
Along the main channel of the Mississippi we know when flood conditions are starting to build, even if we can’t
do anything about it except evacuate.
9. Which of the following concepts or processes was not involved with Mississippi River flooding in 1993
near Nauvoo, Illinois?
a. Where the river is wider, water flows faster than where the channel narrows.
b. The “Great Muddy” Missouri river dumps vast amounts of American top soil into the Mississippi.
c. There are few levees near Nauvoo and the cut bank is on the opposite side of the river.
d. River velocity and elevation are lower on the inside or ‘point bar’ side next to the city.
e. River flow is fastest in the thalweg or middle of the river, and deposition is greatest on the inside or
‘point bar’.
The width of a channel is inversely related to stream speed. Water flows more slowly where it has room to
spread out. Levee walls tend to constrain water, causing it to pick up speed and rise to higher elevation.
The outside of a river bend is where water is most erosive as it collides with the river bank or sidewall and
carries off soil and rock. The inside of an “elbow” or bend in the river actually has lower water level, because
water is flowing away from the inside, toward the outside cut bank. During a flood, water will dome in the
middle thalweg where faster flow occurs and will “pile up” on the outside or cut bank side.
Nauvoo, Illinois, is built on the inside or “point bar” of the Mississippi, so less flooding and erosion occur. In
fact, point bars tend to add soil and rock over time as material cut from a ‘cut bank’ are deposited on the slowerflowing inside of the next river bend. The river is very wide near Nauvoo, so there is a lot more room for flood
water to pass through. Levees in St. Louis constrain the Mississippi so much that flood water reached 48 feet
above flood “stage” – mostly because there was nowhere else for water to go but up.
The Mississippi and Missouri rivers are known for their muddy color because they are both carry millions of
tons of precious top soil that wash away from farms along the way. Americans are trying harder to protect farm
soil, but our goal of extremely high farm productivity still includes farming techniques that lead to vast erosion.
Most of the muddy color is suspended material, because truly dissolved material usually appears clear. Bed
load or traction of large rocks would occur only on occasion during the worst flooding when even buildings and
bridges can be carried away.
10. A yazoo stream or river _______________.
a. is created by natural levees that block entry to the main river flow
b. can flow through backwaters, swamps, marshes, bogs, oxbows and other low-elevation terrain
c. is created by variations in weather, precipitation and climate
d. involves processes that can help prevent damage from hurricanes and other storms
e. is correctly described by all of the above.
Natural levees are shallow piles or material along a river that are pushed up on the shoreline during occasional
flood events from severe weather or climate change. These shallow ridges running along a river bank can
prevent small streams from dumping water into the main channel. So, marshes, swamps and ponds may
develop behind the natural levee, and a “yazoo” river can flow parallel to the main channel because it can’t get
into the main channel.
Backwaters and their specialized flora and fauna can help “buffer” or absorb the energy in a hurricane, helping
prevent inundation. These naturally boggy, wet places are important habitat for plants and animals that we are
learning to value and protect.
The United States has unwittingly drained, obliterated, plowed or paved-over about 90% of all wetlands. We
often regret that now, and we are trying to promote restoration of these wet, natural lands.
The massive Okeechobee in central Florida is really just a super-wide spot in the river. Millions of gallons of
water flow slowly on the surface and through soil and rock underneath. We are learning to protect these vital
natural resources rather than drain them and cover them with parking lots and strip malls.
11. Water is able to dissolve or erode almost any natural substance because of __________.
a. capillary action and dissolved chemicals in the water
b. gravity that causes mechanical action
c. seasonal variations in air temperature
d. its support for biological activity that can turn rocks into soil and “plant food”
e. All of the above are true.
All of the above are true. Even the simple process of soaking something in water causes it to soften, expand
and loose strength.
12. The chemical erosion of stream channels is known as ______________.
a. abrasion
b. competence and deposition
c. corrosion
d. turbulence and thalweg
e. traction or bed load
Abrasion is mechanical scraping, pounding, and gouging by collision. Freezing is mechanical and chemical: as
water goes to and from crystal state, mechanical pressure fractures rocks and pushes soil particles apart.
Traction and turbulence are just mechanical action.
Corrosion is the effect of absorbing water and thereby being exposed to the chemical effects of water. For
instance, acidity is clearly associated with break-down of soil particles and solid rock. Rust and other forms of
oxidation also chemically weaken rocks because absorption of oxygen also interferes with existing chemical
structures, making chemical bonds weaker. Salts that result from acid and basis mixing are also weaker than
native, original rock. Salt spalling or foliation “peeling” can occur when salts in water are absorbed by rock and
then dry out, leaving weak layers or pockets that fall apart.
13. In the Salt Lake Avenues neighborhood, the process of ‘creep’ is probably more active than
solifluction because __________________.
a. temperature swings in the Salt Lake area are not as severe as in ‘H’ climate terrain
b. most surface water is carried away by hard-surfacing and drain pipes, not through soil and rock
c. man-made retaining structures help hold rocks and soil in place
d. All of the above.
e. None of the above
Solifluction is associated with severe weather and climates when masses of soil and rock move seasonally as
water in soil freezes and thaws. Gravity is active everywhere all the time, moving earth mass downhill each
time expansion and contraction occur, like a one-way ratchet, always downhill.
In the Salt Lake avenues terrain is steep, but does not freeze and thaw as severely as in high mountain areas.
Heavy buildings do put extra pressure on soil and rocks, but storm drains and retaining walls also help keep
water out of the soil, preventing or reducing expansion and contraction cycles.
On any steep slope, creep continues as a slow process because it is impossible to stop gravity and keep all water
and ice out of soil.
14. Which of the following statements is not true about the fate of granite plutons over time?
a. hard crystal granite will not erode or ‘weather’, even though adjacent sedimentary rocks wash away.
b. as overlying layers of granite erode away, “unloading” or release of confining pressure overhead
allows granite crystal to expand or “breathe” at different rates, causing stress fractures.
c. outer layers of the pluton will be exposed to expansion and contraction caused by freeze—thaw
cycles.
d. Newly exposed granite will be weathered and dissolved by salts, animals, plants and water.
e. Newly exposed granite will form cracks and joints that accelerate weathering and erosion over time.
Everything in Question 14 points toward granite being the subject of weathering, mass wasting and erosion.
Hard crystals in granite often allow plutons to endure longer on the surface than surrounding softer rocks like
sandstone and limestone. Other igneous rocks, like volcanic ash and even flood basalt, may erode more quickly
than granite because of pore spaces and vesicles that allow water to get in. The silicates in volcanic rocks also
bond easily with water, promoting hydrolysis that weakens and softens those rocks.
Granite may outlast many other rock types, but is still weathered, wasted and washed away.
15. A knickpoint develops where streams spill over the edge of _________________.
a. surface rock layers that are softer than rock underneath
b. surface rock that is harder or more erosion-resistant than rock underneath
c. over the top of other streams
d. over the top of mountains and down the other side
e. All of the above are good examples of knickpoints
A water fall typically spills over the edge of durable cap rock that covers softer rock underneath. Over time,
falling water erodes the softer material underneath so that the more durable cap rock collapses from lack of
support. The water fall gradually moves upstream as water undermines relatively durable surface rock.
The surface rock does not erode or dissolve as much as it falls when softer rock underneath is washed away.
Streams don’t cross over each other, they just merge. Rivers and streams don’t go uphill and over the top of
mountains.