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Principles of Physical Geography

2007 Alan Rice Osborn Geography Department, SDSU Page 1 of 18

The Processes that Shape the Land

Landforms Made by Running Water

Fluvial Processes in Arid Climate Regions

Landforms Made by Waves & Wind

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Fluvial Processes

Most of the worlds surface has been shaped by running water fluvial processes.

Running water is the most important of the processes ofdenudation anderosion.

In some places other processes ocean waves, wind, or moving glacialice may be more important than running water, but overall, running

water is the most important factor in shaping the land.

Erosional & Depositional LandformsAll the various agents of denudation erode, transport & deposit,

transforming initial landforms into sequential landforms.

Landforms created by erosion are callederosional landforms.Landforms created by deposition are calleddepositional landforms.

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Slope Erosion

Overland flow runoff that is not in a channel usually erodes slopesvery gradually.

Slope erosion can be accelerated if:

There is little vegetation cover (either naturally or because peoplehave removed it).

The slope is poorly consolidated.Slope erosion can produce small channels

(rills) that can merge to form large channels

(gullies).Source: http://www.ecy.wa.gov/programs/sea/pubs/95-107/under01.html

Slope Erosion in an Arid Environment

Badlands National Park, South Dakota

Note the rills andgullies that have formed in the soft, easily eroded material.

Source: http://www.nps.gov/badl/exp/home.htm

Soil Erosion: Global Scale

Source: http://www.nrcs.usda.gov/technical/worldsoils/mapindx/erosh2o.html

Colluvium & Alluvium

Soil that is transported by overland flow (and mass wasting) is eventuallydeposited at the base of slope in a layer ofcolluvium.

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If sediment is transported by a stream (rather than just by overland flow)it may eventually be deposited as a layer ofalluvium.

The Work of Streams

Streams shape the earths surface by doing three things EROSION removing material from the sides and bottom of the

stream channel

TRANSPORTATION moving material, in solution, suspension,or along the bottom of the channel

DEPOSITION depositing transported material wherever a streamempties.

Stream Erosion

Streams erode their sides and bottom in three ways HYDRAULIC ACTION

o Hydraulic action is the force of running water. By itself, hydraulicaction is capable of eroding large amounts of material from the

sides and bed of a stream.

ABRASIONo Streams that transport lots of solid material can wear away

(sandpapering) the sides and bottom of their channel (well get

to transportation in a moment).

CORROSION (or DISSOLVING)o Streams can also corrode dissolve away the sides andbottoms of their channels.

Stream Transportation

This is a little complicated!All the material a stream transports is the stream load

All the solid material is the solid loado The solid load is made up of Suspended load (all the fine material that floats), and Bed load (all the material that rolls or bounces along the stream

bed).

In addition to solid material, theres the dissolved load all thatmaterial the stream dissolved.

So, to sum it up: Stream Load = Solid Load + Dissolved LoadHow much material a stream can transport (usually measured in metric

tons) is the stream capacity and capacity depends mostly on velocity.

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Stream Capacity

A streams capacity is a measure of its ability to transport solid material.Capacity is usually given in tons/day.Capacity varies with velocity faster streams have greater capacity.Velocity has a tremendous influence on capacity double the speed of

a river, and capacity increases by the third or fourth power (x3or x

4)

in other words, a small stream can suddenly be able to move a lot.

Stream Gradation

Streams modify their environment they wear down steep hills, widentheir banks, transport tons of material every day.

Eventually, the gradient the steepness of a stream becomesadjusted gradual, instead of steep orgraded.

Stream PiracyIn stream piracy (or stream

capture), part of one drainage

system is diverted into another.

This can happen through severaldifferent processes, and results in

distinctive landforms, including capture elbows and wind gaps.Source: http://www2.nature.nps.gov/geology/usgsnps/noca/sb16river.html

Responding to Changes

Graded streams are in balancewith their environment.

If the environment changes, thestream landscape changes as the

stream tries to get back into balance.Source: http://www2.nature.nps.gov/geology/usgsnps/noca/sb16river.html

Aggradation & Alluvial Terraces

If the solid load increases, then a streams capacity wont be enough totransport all the incoming material.

The stream bed will rise aggradation and the stream becomesbraided.

If the solid load then decreases, a new stream channel will be created,cutting down through the layers of alluvium degradation leaving

alluvial terraces wherever obstructions prevent the stream from carrying

material away.

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Alluvial Rivers

Alluvial rivers (sometimes called mature rivers) are large rivers withvery low gradients, flowing across a large floodplain of alluvium.

Typical landforms of an alluvial river include

Bluffs Natural levees Meanders, cutoffs, ox-bow lakes

Federal Levees in the Mississippi Valley

The Federal Government builds levees ("artificiallevees") along the main stem of the Mississippi

River, and along its major tributaries.

The levee system in this area is 2,203 miles long.Of that, 1,607 miles lie along the Mississippi River

itself and 596 miles lie along the south banks of the

Arkansas and Red rivers.Source:http://www.mvn.usace.army.mil/pao/bro/mrc_map.gif

Rising Artificial Levees

Levees keep floodwaters from flowing outside the riverbed.

When the flood is over, some of the sediment the riverwas carrying will be left behind in the river bed.

Over time, it becomes necessary to keep raising theheight of the levees to keep up with the rising river bed.

Meanders

Meanders are characteristic of mature (graded) streams.The exact mechanics of meanders is still not completely understood, but

the process is fairly simple:

As soon as a bend or curve starts to form in astream, the water moves faster on the outside

of the bend, and slower on the inside of the

bend.

The outside of the bend erodes quickly, andthe eroded material is deposited on the inside

bend.

Over time the bends in the stream get moreand more extreme. When a flood comes, the river cuts off the

meander. Sediment blocks off the bypassed meander, and a curved,

freshwater oxbow lake is created.

Source: http://www.nps.gov/miss/features/misshist/intro.html

Mississippi River meanders & oxbows

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Entrenched Meanders

Entrenched meander at "the gooseneck" on the Colorado River.Source: http://www.ut.blm.gov/wilderness/wrpt/wrptsegooseneck.html

But rivers cant cut meanders in solid rock! How can this be?Answer: Uplift.

The Geographic Cycle

Landscapes change over time. Just how they evolve is something thatpeople have been arguing over for at least two hundred years.

William Morris Davis (1850-1934), in 1889 proposed a theory oflandscape change he called the cycle of erosion.

The Cycle of Erosion

Daviss idea is fairly simple: Landforms move through a predictable series of changes. These stages in the process are labeled youth, maturity, and old

age.

Landscapes can also be rejuvenated by uplift.Other Concepts of Landscape Change?

Daviss ideas have not exactly been discarded, but there are problems: It assumes an incredibly stable landscape; It does not take into account landscape changes produced by plate

tectonics;

It assumes erosion takes place at the same rate in all situations.Alternatives, such as equilibrium theory have helped to explain

landforms in new ways.

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Fluvial ProcessesYes, arid lands are, by definition, lacking in water.Yes, it does sound odd to talk about how water shapes a place that

doesnt have much water.

But the fact is, water is the most important shaper of arid lands andlandforms.

Slope Erosion

As weve already seen, slope erosion (erosion by overland flow byrunning water that is not in a

channel) can be very

significant in areas with Little vegetation Loosely consolidated

surfaces

This is the case in manydesert areas, and many desert

areas have extensive badlands. Source: https://reader009.{domain}/reader009/html5/0403/5ac32cd1be574/5ac

Streams in Arid Areas

Streams erode, transport and depositIn arid lands:

Streams tend to be ephemeral (they have very short active periods). Streams feed groundwater when they are flowing (in more humid

environments, groundwater supplies stream baseflow).

Aggradation andbraided streams are common. Streams are short, and may end in alluvial deposits ordry lakes. Streams are fed mostly by runoff, and are prone to flash flooding.

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Ephemeral Streams

Ephemeral desert streams (calledwashes, arroyos, wadis, etc.) are

frequently braided, and carry

enormous quantities of debris from

mountainous areas into desert basins.

Source: http://www.mojavedata.gov/mdep/geomorphic/classifications/wash.html

Flash Flooding

Many desert surfaces are impermeable, which means little infiltration,andlots of runoff.

Lack of vegetation also means rapid runoff.The result flash flooding is common in desert regions.

Making Alluvial Fans

Streams transportenormous amounts of

eroded material from

mountainous areas

into desert lowlands.

When a stream exitsfrom a narrow canyon,

the velocity of thestream drops; with the

drop in speed, the

ability of the stream to transport material also drops, so material is

deposited at the mouth of the canyon.

Source: http://www.mojavedata.gov/mdep/geomorphic/classifications/wash.html

Playas

Dry lakes playas are common inmany desert areas.

Many are remnants of pluvial lakes lakes that were large and permanent

when the climate was wetter.

Playas often fill with runoff, making ephemeral lakes.Source: http://www.mojavedata.gov/mdep/geomorphic/classifications/playa.html

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Landscapes of Mountainous Deserts

Areas, which have been recently uplifted, such as the basin and rangeregion of the United States, include an array of distinctive and

characteristic landforms.

Evolution of Desert Landscapes

Few desert drainage systems actually reach the sea. Most are interiordrainage basins.

In theory, over time, mountains will be eroded, and what will be left willbe a peneplain (almost a plain), with just a few mountain remnants.

Pediments

At the base of mountains in desert areas an eroded area of exposedsloping bedrock and debris a pediment may be found.

The formation of pediments is not completely understood.In many areas pediments are covered by alluvial fans.

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Coastal Places & Processes

SHORELINE: the shifting line of contact between water and land.COASTLINE: zone in which coastal processes operate or have a strong

influence.

LITTORAL: pertaining to the coast or shore.Coastal Definitions

BEACH: a thick wedge-shaped deposit of sand (well, usually).BAY: a body of water sheltered by the coast from strong wave action.ESTUARY: a place where a river empties into a bay.COASTAL WATER BODIES: arm, bight, cove, fiord, firth, gulf, inlet,

lagoon, narrows, sound, strait.

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Waves

Wherever a large body of water and the landcome in contact, the land is shaped by the

energy of wave action.

Although waves can be made by lots ofthings (earthquakes, landslides, volcanoes),

most waves are made by the wind moving

across the surface of the water.

Source: http://pao.cnmoc.navy.mil/educate/neptune/quest/wavetide/BIRTH.HTM

Wave Energy

A wave IS NOT A FLOW OF WATER! A wave is a flow ofenergy.For example, water in waves created in the Gulf of Alaska does not flow

to San Diego the energy does.

Classifying Waves

Waves can be classified based on: WAVE HEIGHT (trough to crest) WAVE LENGTH (crest to crest) WAVE PERIOD (how many waves

per unit of time) Source: http://www.ndbc.noaa.gov/educate/waves.shtml

Wave Heights

Wave height (and wave period, too) depend on WIND VELOCITY (how fast the wind blows) WIND DURATION (how long the wind blows) FETCH (the distance over which the wind blows)

RULE OF THUMB wave height (ft) ~

wind speed (mph)

Fetch & Wave Height

Note that average waveheight is indicated on

this map by colors red

indicates an averagewave height of 5 meters

(16 feet), white 6 meters

(about 20 feet) or more.

Source: http://topex-www.jpl.nasa.gov/discover/image-gallery/gifs/gallery-4/P41497.jpg

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Breakers

When a wave trough touches bottom, the wave: Slows down; Friction impedes the base;

The back of the wave (the crest) overtakes the front of the wave; Water is forced into a peak; The peak curves forward; The peak breaks, dissolving into a mass of falling water and foam.

To experiment with waves, try the National Geographic Wave Simulator:

http://www.nationalgeographic.com/volvooceanrace/interactives/waves/index.html

Waves & Coastlines

What happens to land when waves strike the shore depends on what theshore is made of.

If the material is soft, then there will be a lot of erosion, and a steepmarine scarp will be created.Coastlines of Resistant Rock

Where the coastline is made of hard, resistant rock, a number ofcharacteristic landforms will be created:

Arches Stacks ("sea stacks") Sea Caves Wave cut notch (this is where most

erosion takes place)Abrasion Platforms

As the rocky shoreline is eroded away, aflat planed off area is created just

offshore the abrasion platform

Abrasion platforms can become exposedif sea level drops or the land rises

Beaches

The word beach can mean: shore pebbles; the shore of a body of water covered by

sand, gravel, or larger rock fragments;

a thick wedge-shaped deposit of sand.Beaches are in constant motion, day to day, season to season, year to

year, but are usually stable in the long term.Source: http://www.photolib.noaa.gov/coastline/index.html

Anacapa Island, California

Raised Abrasion Platform, Sunset

Cliffs, San Diego, California

Cobble beach, Block Island,

Rhode Island

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Drifting Along

When a wave reaches the shore, water moves up the beach, carryingmaterial with it SWASH.

When the energy of the wave is depleted, the water flows back to the sea,carrying material with it BACKWASH.

Because waves always strike the shore at an angle, material carried bythe swash and the backwash moves along the beach BEACH DRIFT.

And More Drift

In addition to BEACH DRIFT, the movement of water along the coastproduces a current along the shore the LONGSHORE CURRENT,

which carries material along the shore

LONGSHORE DRIFT.

LONGSHORE DRIFT + BEACHDRIFT = LITTORAL DRIFT.

Moving waters ability to transport dependson velocity.

When the longshore current reaches a bay, speed drops, and the materialcarried is dumped to form a spit.

Image source: http://www.epa.gov/grtlakes/ecopage/wetlands/glc/plate3.html

Eroding Headlands & Pocket Beaches

When littoral drift deposits more sand along the coast than it carriesaway, the beach grows progradation.

When more material is carried away than deposited, the beach shrinks retrogradation.

Resistant rocky headlands will be worn away and their sand will bedeposited between the headlands in what are called cove beaches or

pocket beaches.

Changing Beaches

If more sand is added than taken away PROGRADATION.If more sand is taken away then added RETROGRADATION.If structures are built that interfere with the beach drift or longshore drift,

people can change the shape of a beach.

If the supply of sand is cut off, beaches can be depleted or destroyed.Tidal Currents

Most coastlines are affected by the tides the periodic rise and fall ofsea level, affected by the movements of the moon and sun.

In some places, the daily cycle of the tides can be massive 16 metersin parts of the estuary of the Bay of Fundy in Canada.

Tidal currents can keep bays open, by preventing the build-up ofsediments.

Sand spit,

Little Tail Point,

Green Bay, WI

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Tidal currents can also fill in bays and estuaries, since they can transportand deposit large amounts of fine material.

Coastlines

There are three categories of coastline:

SUBMERGENCE(partially drowned coasts, caused by a rise insea level or sinking of the crust).o EXAMPLES: ria, fiord, barrier island, fault

EMERGENCE(formerly underwater landforms exposed by a fallin sea level or a rising of the land).

o EXAMPLES: raised shoreline, marine terrace BUILT(created by a process that constructs new land).

o EXAMPLES: volcano, delta, coral reefSubmergent Coastlines

Examples: RIA (drowned river valleys). FIORD (drowned glacial valleys). BARRIER ISLAND (see below). FAULT COAST (down-thrust faulting).

Barrier Islands

Barrier islands are basically offshore sand bars huge submerged dunes up to a few miles

from the actual coast

Barrier islands protect the coast from severestorms and waves, and are important in ocean shipping

Image source: http://easternshore.fws.gov/Fisherman%20Island/FI%20Gen%20Info.htm

Emergent Coastlines

Emergent coastlines are created by the exposure of formerly submarinelandforms, caused by a fall in sea level or a rise of the land

Examples RAISED SHORELINE MARINE TERRACE

Built Coastlines

Built coastlines are created by any process which constructs new landalong a coast

Examples VOLCANO DELTA CORAL REEF

Fisherman Island,

Vir inia

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Deltas

Deltas of the Mississippi, Ganges, and Nile Rivers.

Source: http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_5/geo_images_5/

Coral Reefs

Coral atolls (Midway and Baker Islands)Sources: http://pacificislands.fws.gov/wnwr/nwrindex.html ;http://www.accessnoaa.noaa.gov/images/coralmap1.jpg

The Power of the Wind

Like water, wind can erode, transport, and deposit.However, because air is so much less dense than water, its power is

usually much less evident.

Only in arid or semi-arid areas is the wind a highly significant shaper oflandforms.

Transportation

Because of airs low density, only very fine particles (dust) can betransported in suspension.

Larger particles (sand) can seldom rise more than a few centimetersabove the surface. Heavier material cant be moved at all.

Most material moves by saltation (bouncing) or creep.Landforms of Erosion: Blowout

Wind can remove loose surface material. This process is calleddeflation.

Normally, the process is slow and unnoticed. However, under somecircumstances a large depression up to a kilometer across can be created.

This is called a deflation hollow or blowout.

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Landforms of Erosion: Reg

Desert pavement or reg isproduced by wind removing all fine

material, leaving behind a layer of

gravel and pebbles that armors the

surface.

Dust Storms

Dust fine material, smaller than sand can form dense clouds, and can be

carried enormous distances even

across oceans.

Saharan dust storm, moving out over the AtlanticSource: http://pao.gsfc.nasa.gov/gsfc/gnews/041301/041301.htm#saharan

Dust storm, Al Asad, Iraq (April 26, 2005)

Source: http://www.marines.mil/marinelink/mcn2000.nsf/lookupstoryref/2005426134811

Landforms of Deposition: Sand Dunes

A sand dune is a loose hill of sand, shaped by the wind.Active dunes are constantly changing.Most dunes are made of quartz sand (silicon dioxide).

Types of Dunes

Sand dunes can take an almost infinitevariety of shapes. However, we can

recognize several basic types:

Barchan (crescents) Transverse (waves) Star dunes (mountains) Parabolic (coastal blowout) Longitudinal (lines)

Source: http://www.mojavedata.gov/mdep/geomorphic/mdei_images/dunes.gif

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Moving Sand

Sandcreeps andsaltates up the windward side of the dune, reaches thelip of the slip face and falls.

Dunes migrate as the sand they are made of is eroded from the windwardside and deposited on the steeper leeward side.

Barchan Dunes

Barchan (or crescentic dunes) areisolated hills of sand that move across a

flat surface.

Barchans have a characteristic shape On the upwind side, the slope is

gentle and rounded. Source: http://www.nps.gov/grsa/resources/barchan.htm

On the downwindslip face the slope is steep. The points of the crescent point downwind.

Transverse DunesWhere there is lots of sand a sand sea or

erg dunes form wave-like ridges separated

by troughs.

The dune crests are at right angles transverseto the prevailing wind.

Source: http://www.nps.gov/grsa/resources/transverse.htmParabolic Dunes

Parabolic dunes look a little like barchan dunes but they are curved inthe opposite direction (that is, the points of the curve are into the wind).

Deflation is important in the formation of some parabolic dunes.Parabolic dunes can take a variety of shapes (broad, "hairpin," etc.)

Longitudinal Dunes

Longitudinal (orlinear orseif) dunes form long, narrow ridges ofsand orientedparallel (roughly) to the prevailing winds.

Longitudinal dunes may be dozens of miles long.Coastal Foredunes

Just inland from a beach there is often a belt of sand dunes, stabilized bybeach grasses. These are calledforedunes.

Coastal foredunes can become quite high, and can protect areas inlandfrom high waves unless something damages the vegetation, and ablowout happens.

Coastal Foredune

Deflation Plain

Tree Island

Ocean

Beach

Parabolic Dune

After: http://www.fs.fed.us/r6/siuslaw/recreation/tripplanning/oregondunes/index.shtml

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2007 Alan Rice Osborn Geography Department SDSU Page 18 of 18

Loess

Loess is made up ofsilt (very fine soil). Loess deposits are commonlylocated in or near glacial regions (or areas that were once glaciated).

Glacial debris can be carried away by the wind because there is little

vegetation in glaciated areas to hold sediment. The fine sediment cantravel hundreds of kilometers, with hundreds of tons of sediment being

transportedin a single dust storm.

Loess deposits, though not very strong structurally, tend to form steepcliffs. They are often superb farmland.

Sources: http://www.eos.nasa.gov/globe/soilform/deposits.htm ;

http://earthobservatory.nasa.gov/Study/Paleoclimatology_Speleothems/

Induced Deflation

Human activities farmingmarginal land, draining lakes,

overgrazing, mining, etc.

can induce deflation, making

the surface more prone to be

removed.

Dust (particulates) hasbecome a severe health

problem in many areas.

Dust storm near Owens Lake, California

Source: http://geochange.er.usgs.gov/sw/impacts/geology/owens/fig2d.jpg

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