Geography 417 California for Educators
Californias Geomorphology
Landforms
What is denudation?
What is the opposite of denudation?
Weathering
Weathering is the term
applied to all processes that break down rock into smaller rocks (sand, silt,
etc.)
Slopes and Regolith
Slope is the tilt
of land away from the horizontal.
Almost all land
has some degree of slope.
Recall that regolith is broken pieces of the parent material rock
called bedrock. Regolith,
when further weathered, become a constituent component of sediment and soil
Outcrops and Alluvium
Outcrops are spots
on the land where the parent bedrock is visible at the surface.
Alluvium is the
sediment, or weathered rock material that has been transported and deposited by
a stream (also called in this text transported regolith).
Physical Weathering
Involves
the breakdown of rock into smaller pieces by the mechanical action of wind,
rain, water and ice.
Frost Action
Frost action is an
important weathering process in cold climate regions.
Frost can break
apart rocks when it gets into the cracks and tiny pores of a rock and then
freezes.
Because water
expands when it freezes, it can wedge apart most rock types.
Block Disintegration
Many times the
water gets into joints, or geometric cracks in rock.
The rocks break
apart along the joint fractures and the resultant pieces of rock may be in turn
geometric (as blocks or planes).
Porous rocks break
up in a less geometric pattern, creating granular rocks in a process called
granular disintegration.
Salt-Crystal Growth
Much like the
growth of ice crystals is the growth of salt-crystals that are likewise capable
of breaking apart rock.
Most
common in dry climates.
Occurs
when water is sponged upward through rock (mostly sandstone) and then
evaporates. The residual salts accumulate, form crystals
and break apart rock.
Most
common at the bases of cliffs where water is more plentiful.
Damage
Both salt and
frost crystallization is very damaging to buildings, roads and other man-made
structures that are subjected to these types of weathering.
Unloading or Exfoliation
Unloading is a
physical process that results from the release of pressure upon compressed
rocks deep beneath the surface as overlying rock layers are removed.
Rocks may break
off in sheets.
Sometimes the
formation in bowed/arched upward and called an exfoliation dome.
Other Physical Weathering
It is likely that
the daily cycle of heating and cooling of rock surfaces generates weathering.
Plant roots, both
small and large can get into cracks in rock and break it apart.
Chemical Weathering
In addition to
mechanical weathering, the application of water, oxygen and other substances
can break down rock.
Some rock types
are much more susceptible to chemical weathering than others.
Hydrolysis and Oxidation
Water and Oxygen
induce hydrolysis and oxidation respectively.
This is most
common in hot, humid climates, like Louisiana.
Chemically
weathered rock can be a problem for construction engineering because the bedrock
is so greatly weakened.
Oxidized Rock
Acid Action
Major chemical
weathering process that takes place because there is dissolved CO2 in water
which creates a weak acid (carbonic) that dissolves certain types of rock,
especially limestone and marble.
Can
produce very interesting rock formations and caves in underground rock.
Atmospheric acids,
associated with air pollution can damage old buildings via acid rain.
May also have salt
crystal mechanical weathering as well.
Mass Wasting
Mass wasting is
the spontaneous downhill movement of rock, soil and regolith.
It comes in many
forms, but is generally classified by the rock/soil type and the speed of
downward motion.
The common
characteristic is that it is all generated by the force of gravity.
Mass Wasting
Soil Creep
The slow movement
of soil and regolith down a slope is called soil
creep.
It is caused by
the multiple weathering processes, including animals, combined with the
constant pull of gravity.
Many bits of
evidence are evident on most hillsides of the slow type of mass wasting.
Earthflow
Earthflow is caused when soil and regolith
(and occasionally bedrock) is saturated with water and it is pulled downward by
gravity.
The water reduces the
ability of the soil to retain its solid state and it flows like water, but much
slower. Slumps are created.
Solifluction is a related process that effects
cold climate with a layer of permafrost.
Mudflow
Is like an earthflow, but much more rapid because it is much more
watery and fluid.
Can
be very damaging and life-threatening.
They are like
floods, but with much more force because they are thick and may carry large
objects.
Common
in desert, deforested areas.
Lahars are
volcanic mudflows.
Landslide
Occur in
mountainous regions and involve the movement mostly of bedrock that has broken
loose from a steep hillside.
Frequently
caused by seismic activity.
Can
be catastrophic for any human activity in its path.
Costly
natural disaster in mountainous regions.
Induced Mass Wasting
Humans are often
responsible for creating conditions that favor mass wasting.
Building houses on
steep slopes, clear-cutting and forest fires can all create ideal conditions
for mass wasting.
Horrible coal mine
slag pile disaster in Appalachia was one such event.
The PCH was cut
off at Malibu in the early 1990s.
Induced Earthflows
Often occurs on
land that overlies a shale and clay bedrock formation.
Water, either from
heavy rains or from human sources can make the bedrock
slide downward on the clay once the clay is made more plastic and slippery by
excess water.
Subsidence
·
What is
subsidence?
·
What human
activities cause subsidence?
·
What are the
consequences of subsidence?
Subsiding Marshlands
Fluvial Geomorphology
This section is
about the ways in which flowing water erodes the land and how that flowing
water also creates landforms.
Along with wind,
ice and waves, running water is a process of denudation, or the wearing away of
landforms.
Fluvial Processes and Landforms
Fluvial (of or
pertaining to running water) processes create fluvial landforms.
Fluvial landforms
can be found in virtually all parts of the globe.
Fluvial processes
may not be the most powerful, but they are the most important and over time,
the most effective.
Erosional and Depositional Landforms
There are two
types of fluvial landforms: those carved out by fluvial processes (erosional) and those created, or built by fluvial processes
(depositional).
Valleys are erosional features and floodplains and sandbars are
depositional features.
Erosional and Depositional landforms (fig.)
Slope Erosion
Erosion begins on
upland areas and is caused by the friction of moving water upon the
surface.
Everything from
colloids to gravel can be transported depending upon the speed of the water.
Dissolved material
may also be transported.
Although soil is
removed, under geologically normally circumstances, enough new regolith is created that the system stays in balance.
Accelerated Erosion
Erosion may be
speeded up considerably by human activities or by certain natural events.
Removal of ground
cover through fire or clear cutting opens the door for increased splash
erosion.
Rainsplash on bare soil may cause the pores in the soil to get
clogged, reducing infiltration rates and increasing downslope
erosion considerably.
Rainsplash (fig)
Sheet Erosion
When soil porosity
is decreased then thin sheets of soil can be progressively removed by a process
called sheet erosion.
In the early
stages water will create rills, small channels of water that may increase in
width and depth, creating gullies.
Erosion
Erosion is the
most degrading force upon soil.
It always goes
some place else.
Sometimes it can
be captured and reused, but if it makes it to the ocean, it is lost to all.
$44 billion
dollars (US) $400 billion lost directly and indirectly per year because of soil
erosion.
Erosion Factors
Vegetative Cover
Slope
Rainfall
(intensity and volume)
Soil type
Land management
How is erosion measured?
Erosion itself is
not measured, but the amount deposited in lakes and streams can be
measured.
The deposition is
measured and it is called the sediment yield rate.
May
only represent a third of the total erosion.
Yearly 3.6 tons
per acre of cropland is lost in the US.
Consequences
Lower yields,
higher fertilizer use, more expensive food.
Loss
of arable lands, fragmentation of agricultural areas, higher food costs and
ecosystem fragmentation.
Sedimentation
of various wetland areas which upsets the habitat of these ecologically
sensitive areas.
Increases the dirt
in water (increases turbidity), which upsets the stream habitats.
Flooding!
Colluvium and Alluvium
At the base of
hills where erosion is occurring a pile of eroded materials may accumulate,
called colluvium.
If the material is
transported away from its source by water, then it is called alluvium.
Alluvium is
generally a good thing for agriculture, but too much can ruin farming.
Too much alluvium
can also increase floods.
Arid Climate Erosion
Where rainfall is
scarce, so is the vegetative cover.
Where normal conditions prevail, erosion can be sustained and it may not
be damaging.
Dry climates are
however far more susceptible to hard rains or changes in land use.
Badlands can develop in areas where erosion exceeds the natural
capability of a region to build or rebuild soil.
Human activity can
create badland conditions, like it has on shortgrass
prairie in the upper plains.
Stream Processes
Consists of three
activities:
Erosion
Transportation
Deposition
Stream Erosion
There are a number
of ways a stream can erode a soil or rock formation.
Corrosion occurs
from chemical weathering (lmst)
Generally water
contains particles which act like sandpaper and/or a jackhammer on contact
surfaces. This process is known as
hydraulic action.
Those rocks
dragged along the bottom of the stream are abraded and broken down.
Rock pieces that
get stuck in a single spot can create a pothole.
All the solid
materials that are carried by a stream is its
load.
Some of it is
dissolved in solution, making it invisible to the naked eye.
A much larger
portion is carried in suspension, mostly clay and silt (muddiness)
Some of it is
dragged along the bottom as bed load.
Most of this is sand and gravel.
The majority of
stream load is in suspension.
Stream Load Capacity
How much a stream
carries can be measured against how much could carry.
The load potential
of a stream is largely a factor of stream velocity and the resultant turbidity.
When stream
velocity is doubled, load capacity can increase exponentially.
The vast majority
of erosion during a year will occur during the infrequent high water moments.
Stream Gradation
The
steeper the grade, the greater the velocity of the stream and the greater the
erosive capacity of the stream.
Streams erode into
the soil and rock they overlie. Once
they have flattened out they are called graded streams.
Stream flowing over
newly uplifted ground have many knickpoints or
waterfalls.
Stream Gradation (Fig.)
Landscape Evolution
Landscapes being
eroded by streams go through a sort of life cycle.
New or young fluvial
landscapes are characterized by lots of waterfalls, rapids and lakes.
Eventually the
waterfalls are eroded into gorges/canyons and the lakes are filled with
sediment.
Very little
sediment is otherwise deposited by the system.
Landscape Evolution
The stream tries
to create a condition where the slope of the stream (gradient) is constant
throughout the entire stream.
The tributaries
extend into highland areas, eroding them and creating ever-extensive
watersheds.
Once a graded
condition has been achieved, then the ability of the stream to carry load will
be matched by the available load.
Deposition will begin to take place.
Floodplain Creation
Slow flowing, well
graded streams cannot carry all the sediment fed to them, so they begin
dropping it off in widening valleys.
Within these
floodplains, rivers will begin to meander, cutting banks on the outside corners
of the stream and depositing sediment in point bars.
The edges of the
gorges will evolve into valley walls that are less steep.
Meanders (fig.)
Floodplain Features
Among those
features common in a mature stream floodplain are:
Meanders
Cutbanks
Point bars
Oxbow Lakes
Natural levees
Backswamps and bluffs
Waterfalls
Some are created
by rifting, like in E.Africa
Others, like Niagara,
were created by the movement of great glaciers in the region and by
differential erosion patterns of layers of bedrock (shale vs. limestone)
Where in California?
Where waterfalls exist,
so does hydroelectric power potential.
Where graded stream conditions are present, dams are necessary
precondition of hydropower production.
Pros
and Cons of dam construction.
Peneplain Creation
If streams could
continually erode an uplifted block of land, it would eventually wear it down
to a broad, flat plain.
The point at which
the river has no erosive capacity is its base level, which is essentially sea
level.
The broad mostly
flat surface of highly weathered regions is called a peneplain.
(Amazon Basin).
If it is uplifted,
the feature is said to be rejuvenated.
Aggradation
Streams, once they
have achieved a measure of gradient balance react quickly to changes in the
balance of erosion/deposition inputs.
If there is an
increase in bed load, from deforestation or glaciation
e.g. aggradation may occur, elevating the stream bed
and in turn increasing velocity and erosional capacity
of the stream.
Stream braiding
may occur in extreme instances.
Degradation
Degradation occurs
when bed load decreases below the capacity of the stream to carry sediment.
The
result in stream scour, or downward cutting by the stream.
Streams will get
narrower and deeper until a new more in-balance gradient is achieved.
Rejuvenation
Rejuvenation
occurs when tectonic activity uplifts the block upon which a river flows.
This will rapidly increase
the erosive force of the river, allowing it to cut downward quickly and
creating an entrenched meander pattern.
Entrenched
meandering can create isolated high hills and even natural bridges.
Alluvial Terraces
Alluvial terraces
are like steps on the side of a valley.
They are a result
of successive changes in bedload to discharge ratio.
They are also a
product of differential bedrock hardness/erodability.
Such terraced
landscapes are generally densely inhabited.
Alluvial Rivers
Alluvial rivers
are those flowing through alluvial floodplains created by the river
itself.
They are prone to
frequently flood events.
Natural levees are
produced by the floods.
The bluffs are
above the floodplain.
Alluvial rivers
often meander enough that they cutoff their own course creating oxbow lakes and
later meander scars.
Rivers in Deserts
Although it may
rain rarely in deserts, because the vegetation is sparse, rain events have a
heightened erosional impact.
Flooding in desert
environments can be extremely rapid and dangerous.
Because seepage is
so increased in dry regions with deep water tables, streams tend to dry up
quickly and remain shallow and frequently braided.
Alluvial Fans
Alluvial fans are
common desert landforms that are built of sediments eroded from highland areas.
They are cone
shaped with the small point emanating from the mouth of a ravine between two
high points.
Alluvial fans
serve as important aquifers in dry/moutainous
climates because the sandy lower layers capture and hold much water.
Cycle of Desert Mountains
Uplift and
faulting create greatly uneven surfaces.
Erosion creates
rugged mountains and begins building large alluvial fans and playas (salt flats
or salt lakes).
Eventually
mountains erode into wide, gently sloping pediments that border wide fans and
large playas.
Bajada is the southwestern term for the pediment-to-playa
landform.
Glacial Geomorphology
A
very nice web site with lots of pictures and maps.
Glaciers in California
Much of the
mountainous region in the Northern part of the state has glacial landscapes.
Most of the glaciers
are now melted, but several smaller ones exist in places.
Where are the
likely locations?
Glacial Landforms
Yosemite is famous
for its stunning glacial landscapes.
Below are graphics
that illustrate the various features, including:
U shaped valleys
Cirque, Hanging Valleys, Waterfalls
Moraines (lateral,
terminal)
Horns, Aretes
Coastal Geomorphology
Not much of a
topic in the book.
Wind Erosion
Not a major
problem throughout California, but can be a serious localized issue.
Where would you
guess that wind erosion is worst?
What problems
might it cause?
Wind Erosion (fig)