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Geography 12
(Secret) Agents of Erosion

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The Agents of Erosion: Processes and Landscapes
Station Studies


STATION #1 - Fluvial Processes and Landforms

Refer to the following website to learn about rivers' processes: Virtual River - River Discharge
Virtual River - Flooding

Complete the exercises and then answer the following questions:

1. Where do rivers get their water?
2. Define discharge.
3. What part of a river flows fastest?
4. When does flooding occur?
5. What do the terraces along a river tell us?

Go to Journey Along a River and complete the assignment: Journey Along a River

After completing the online assignment, see how much of the following notes is old hat:

Fluvial Drainage Patterns


Dendritic drainage pattern takes the form of "dendrites" and looks like the branching roots of a tree. Dendritic drainage forms where on flat surfaces of homogeneous subsurface material.
Example of trellis drainage pattern where juvenile streams (top to bottom) join secondary stream flowing from right to left. This is in the Babine Range. Trellis drainage is characteristic of areas with alternating parallel valleys and ridges. Streams occupy the valleys, flowing parallel to the ridges but occasionally cutting across the geological grain at water gaps.
The major landforms of Maui are featured in this photograph - the well-entrenched radial drainage pattern of Puukukui Volcano (1764 meters) at the west-northwest end of the island to Red Hill and Haleakala Crater (3055 meters) at the south end. Radial drainage patterns are typically found on the slopes of volcanoes, where streams flow downslope, parallel to the slope gradient.

River Velocity


Notes on Carrying Processes:
River processes and landforms


See page 260 in your text for the diagram!

Discussion:


Read "Entrainment, Transportation and Deposition of Sediments"



Erosion
A river may erode in any of the following ways
  1. Attrition
    - occurs when boulders and rocks are knocked against each other and gradually reduce this stage.
  2. Corrasion / Abrasion
    - occurs when smaller material gets thrown against the riverbanks and erode them.
  3. Corrosion
    - occurs when acids in the river dissolve rocks, such as limestone.
  4. Hydraulic action
    - when the sheer force of the river dislodges rocks from the riverbanks and bed

These are mentioned in your text on page 259.

So, what do all these actions create?
Here are a few features to know:

Mature rivers meander across wide river valleys and create large loop meanders that develop into oxbow lakes adjacent to the stream.
It cuts and erodes into the outside of the curve and deposits sediment on the inside of the cure because the stream moves more rapidly on the outside of the curve and more slowly on the inside of the curve. (Think about the swing of a baseball bat - what part moves faster? The end of the bat is like the outside of the curve.) As the erosion and deposition continues, the curve becomes larger and more circular. Eventually, the river begins to cut the loop off by eroding the neck of the loop. The river breaks through at a cutoff and forms a new streambed. Sediment is then deposited on the loop side of the stream, cutting off the loop from the stream entirely and a crescent-shaped lake results. They are called oxbow lakes because they look like the bow part of the yoke used with teams of oxen.
As glaciers melt, their runoff includes hundreds of tonnes of fine and coarse material. As the gradient of a river decreases, they begin to deposit gravelly debris. Crossing such flats, the river creates dozens of small, meandering channels. These become interwoven with each other into a braided channel. Since the amount of discharge varies almost hourly, the number of channels in a braided area will also vary.
Alluvial fans form where tributaries enter into broad valleys from the steep hillsides or mountain ranges. Coarser material is usually deposited at the top of the fan, and as a result, any stream flow quickly infiltrates and slows.
A waterfall is formed when hard rock is overlying softer rock
  • the soft rock gets warn away from under the hard rock and so there is an overhang of harder rock which is formed
  • when the overhang gets to big and canít support itís weight it collapses and so a waterfall is formed
  • at the bottom of the fall the water is falling which gains speed
  • when it gains speed it digs into the rock creating a feature called a plunge pool
  • where the waterfall retreats there is a gorge that is formed
Stream Piracy or stream capture is the natural diversion of the headwater of one stream into the channel of another stream having greater erosional activity. A decapitated stream or beheaded stream refers to the lower part of a stream whose headwaters have been captured by another stream. In some cases, decapitation may be accomplished by other geologic mechanisms, such as faulting.


Yazoo stream - tributary that follows parallel to the main channel until a breach in levee occurs

Levees - sediment buildup near riverbank formed from deposition of coarse sediment during flooding

A river rises in high ground ( its source ). It then flows down a slope ( its course ) until it reaches the sea ( its mouth ).

The youthful stage :
Youthful rivers are usually small in size.
They usually flow quickly down steep slopes. Such fast flowing rivers generally erode the land over which they flow. Soft rock is usually more easily eroded than hard rock. Fast - flowing rivers usually erode more powerfully than slow flowing rivers.

The mature stage :
By the time the rivers has reached the mature stage the river will have received the waters of many tributaries.
So the river will have increased in size. As these larger rivers sweep quickly along they transport large loads of materials with them.

A rivers old age :
Old rivers meander slowly and aimlessly over almost flat plains as they near sea level. As they do so the rivers deposit much of the loads which they have carried in their mature stages.
Why old rivers deposit their loads : Rivers slow down when they enter almost-flat plains of old age. Slow moving rivers cannot carry large loads as fast flowing rivers can. So the old rivers deposit their loads. The loads of old rivers consist of light particles such as mud and grains of sand. This material is called alluvium. It is deposited along the river bed.
Deltas are found at the mouth of large rivers - for example, the Mississippi. A delta is formed when the river deposits its material faster than the sea can remove it. There are three main types of delta, named after the shape they create:

Arcuate - the land around the river mouth arches out into the sea, the river splits many times on the way to the sea, creating a fan effect.
Cuspate - the land around the mouth of the river juts out arrow-like into the sea.
Bird's foot - the river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird's foot.

Humid Landscapes Worksheet!!

Choose any 4 of the following case studies to complete your Rivers Summaries and locate their basins on the World Map:
Democratic Republic of the Congo, Republic of the Congo
Amazon, Brazil
Mekong, Thailand
Danube, Europe
Mississippi, U.S.A
Yangtze, China
Brahmaputra, India, Bangladesh, China (Tibet), Bhutan, Nepal
Murray, Australia


STATION #2 - Karst Processes and Landforms


Chemical weathering of carbonate-rich rocks produces a spelunkers paradise of caves, mysterious disappearing streams, and the fascinating stalactites and stalagmites. The name "karst" is Yugoslavian and represents a region with limestone plateaus with many of these solution landforms. Chemical solution of limestone and dolomite can lead to fracturing and wears away the bedrock leaving fissures and in more extreme circumstances may destabilize the surface resulting in sinkholes. Locations of major karst regions are in New South Wales, Jamaica, Yucatan, Puerto Rico, and in several States - Pennsylvania, Maryland, Virginia, Tennessee, Kentucky and Florida.

Conditions for karst to develop:
  1. there must be limestone at or near the surface.
  2. the limestone must be dense, highly jointed, and thinly bedded. When too porous, water will soak into the entire rock structure instead of focussing along particular flow lines.
  3. downward movement of groundwater made possible by upland areas that consist of fractured and soluble rock
  4. at the very least, moderate precipitation,


Human factors to consider:

Impacts by Humans:Groundwater in karst landscapes can be easily contaminated by effluent from private septic systems, agricultural chemicals, animal and livestock wastes, motor oil, industrial waste, and garbage since the fractured bedrock often has little holding capacity. With little in the way of slow-filtering clay-rich glacial till or low- permeability bedrock or thick soil cover, recharge to the water table is rapid (minutes or a few hours) and general unfiltered. Groundwater contamination risk from residential, agricultural, or industrial development is very high.
We can also exacerbate the chemical weathering process by increasing the acidity of water throughout the water cycle.

Impacts on Humans: Besides the obvious repercussions from contaminating our own drinking water, we also have the ability to speed up a process that takes thousands of years sometimes and results in (often) a sinkhole swallowing human structures in a matter of minutes. Poor judgement on the part of developers and buyers alike has been a headache for many members ofcommunities throughout Florida. Check out the Case Study!

A more local example to check out is the Vancouver Island Case Study:
Vancouver Island Case Study



Now, download the Karst Landscapes Assignment!!


STATION #3 - Wind Processes and Landforms



Download the assignment here: Wind Processes and Landforms!!

OPEN a separate Word Document. Here you will be placing images and text.

  1. Create a chart by hand or with the computer(like below) of places in the world where wind would likely be an effective agent of erosion, and explain for each why this may be so.
    Also, provide examples from the world of places where these places are.

    LocationWhy wind is effective

    Links: Eolian Landforms
    Eolian Processes
  2. Why is water so effective in transporting desert materials?
  3. Using the link here: Deserts, Arid Lands and Desertification lists several ways man's actions have affected the amount of wind gradation.
  4. Conversely, how has wind affected man's activities?
  5. Water action in desert areas is usually devastating? Why?
  6. If you haven't already defined any of the following words, do so now. Collect an image or diagram from the internet and create a document to paste or insert it into your document. You HAVE to write bibliographic reference of where you got it
    . Save all your work onto your disk!
    • erg
    • reg
    • hamada
    • zeugens
    • yardangs
    • barchan dunes
    • bajada
    • bajada
    • playa lakes
    • bolson
    • butte
    • mesa
    • butte
    • wadis
    • badlands
    • rock pedestals



    The Agents of Erosion: Notes on Deserts

    Factors that help create a desert

    Why is the desert so dry?
    1. Cold current

      The current comes from the poles and is cold so it creates mist and does not carry or evaporate water so the area a few kilometres from the sea is dry.
    2. Rain shadow

      Two things can create the rain shadow: Mountains and continental location.
      • Mountains such as the Rockies force air to go up and so release the moisture and the people the other side gets less rain. The dry chinook wind, a "foehn" wind, is the result.

        This katabetic wind develops all over the world, and has a lot of names:
        Santa ana: California
        Chinook: Rocky Mountains
        Zonda: Argentina
        Puelche: Andes mountains
        Kona: Hawai
        Foeh: Austria
        Tauem: Salzburg
        Vent der Midi: around the city of Lyons
        Karaburan: Asia
        Halny Wiatr: Poland
        Bhoot: India
        Brickfielder: Australia
        Sky sweeper: Majorca
        Aspre: France
        Austru: Romania
        Lips: Greece
        Etesian: Greece
        Pyrn: upper Danube
        Rotetur: Romania
        Canterbury Northwester: New Sealand


        Check out this website where it discusses some important impacts of chinook on people. Mountain Nature

      • Winds blowing over continents are dry by the time they get to the interior of continents. ex.the Gobi
    3. High pressure

      High pressure is when cold air sinks down and so there is less evaporation, less clouds, and less rainfall. Ex.the Sahara and the Great Australian Desert



    Water action in the desert

    • 180,000 years ago the Sahara was much wetter than it is today. At that time rivers ran across the land carving out deep valley. Today these are usually dry and are known as Wadis. Ex. Saharan Algeria


    • When there is a sudden rainstorm floodwaters flow down Wadis. They are short lived but carry sediments in large quantities. This is deposited as alluvial pans at the foot of steep slopes. Several fans join together to form a Bajada. Ex. Here the bajada is completely covered by the green suburbs of Glendale, California.


      Aquifer

      • wells can be sunk in order to get to the water table and then pump water up to the surface
      • hollows in the sand may be due to deflation - their downward limit is marked by the water table (e.g. Alfayom depression on Egypt, which produces a wide range of crops)


      Features produced by wind:
      • deposits of sand often form seas called Erg. Example: Grand Erg occidental in Algeria)


      Two other types of deposition are:

      • Barchans
        Barchans may form around an obstacle. They are crescent shaped and face down wind. They migrate and may advance a few meters a year (e.g. Sahara) Here is a computer simulation of the progressive development of a barchan dune and the reversal of it as seasonal winds change direction.:
        Barchan


      • Seif dunes
        Seif dunes form long ridges of sand which are oriented parallel to the prevailing wind. The smaller dunes are 3 m high and 60 m long; the larger dunes are 100 m high and may extend for 100 km. They are found in the Thar, Libyan and Arabian deserts, and are usually knife-like ridges with many peaks and sags. The diagram shows the shape and wind directions creating the dune.


      Notes on wind:

      Wind

      Features trade winds produce:

      1. West coast deserts

        • occur in the trade wind belt on the western side of continents where the winds are offshore
        • on shore local winds do blow across these coasts but they rarely, if ever, bring rain because they have to cross cool currents which parallel the coasts in these latitudes
        • the cool currents cause condensation to take place in the onshore wind which produces mist, fog or light rains. By the time they reach the coasts the winds are dry (e.g. Sahara desert in Africa)

      2. Continental deserts

        • occur in the interior of continents
        • winds have already traveled a great distance across land and have lost much of their moisture
        • this happens when winds blow over a dry land or even high mountains
        • day temperature of these continental areas are very high because they are so far from the moderating influence of the sea
        • the night temperatures are low because the absence of clouds causes these areas to lose their heat rapidly
        • occur mainly in west tropical latitudes but they do extend into the temperate zones
        • sometimes these deserts occur in intermontane zones plateau regions (e.g. deserts of Arizona and Nevada in the Rockies)


      A desert often occurs because of a combination these factors (e.g. the Sahara and the Australian desert are partly of the west coast type and partly of the continent type)

      Desert types

      1. Sandy desert

        Called erg in the Sahara and Koum in Turkestan. This is an undulating plain of sand produced by wind deposition.

      2. Stony desert

        Called reg in Algeria and serir in Libya and Egypt. The surface of this desert was covered with boulders and angular pebbles and gravels which have been produced by diurnal temperature changes.


      3. Rocky desert

        Called hamada in the Sahara. The bare rock surface is formed by deflation which removes all the small loose rock particles. A part of the Sahara desert in Libya has extensive areas of this type of desert.

      4. Badlands

        • develops in semi-desert regions mainly as a result of water erosion produced by violent rain storms
        • the land is broken by extensive gullies and ravines which are separated by steep ridges
        • examples are in the region extending from Alberta to Arizona.


      Features produced by wind Erosion

      Wind abrasion can sculpture rock into fantastic shapes.

      1. Rock pedestals
        Formed of alternate rock layers of hard and soft rock, abrasion is greatest at ground level.


      2. Zeugens
        • a desert area which has a surface layer of hard rock underlain by a layer of soft rock into a ridge and furrow landscape
        • the ridges are called zeugens which may be as high as 30 meters
        • they are eventually undercut and gradually worn away.

      3. Yardangs

        • another type of ridge and furrow landscape
        • bands of hard and soft rocks which lie parallel to the prevailing winds are eroded
        • the belts of hard rock stand up as rocky ribs up to 15 meters high
        • common in the central Asian desert and in the Atacama desert.

      4. Depressions

        • some depressions produced by wind deflation reach down to water bearing rocks. A swamp or an oasis then develops.
        • Qattara depression is 122 meters (400 feet) below sea level


      5. Fault produced depressions

        The formation of a depression may first be caused by faulting. The soft rocks thus exposed are excevated by wind action.


    Desertification
    Worldwide, 70 000 square km of arable land are degraded annually
    Desertification usually involves soil degradation. Soil degradation can be in the form of erosion by wind or water, compaction by trampling, contamination by salt or other chemicals, etc. This occurs as a result of improper use of the soil
    This includes: devegetation and overgrazing which expose soil to erosion, improper tilling practices, etc.

    Soils

    Regolith is the layer of unconsolidated material at the Earth's surface. Soil is the topmost part of the Earth's crust which can sustain plant growth. Soil is formed by weathering of pre-existing bed-rock or unconsolidated deposits.
    Typical soil profile:
    Most soils will show some or all of a sequence of layers that make up an ideal soil profile (click for some photos), from top down:

    • a layer of organic matter (humus; "O" horizon);
    • a layer depleted in certain elements as a result of leaching by percolating water, and rich in organic material ("A" horizon);
    • a layer depleted in certain elements as a result of leaching by percolating water, and poor in organic material ("E" horizon);
    • a layer enriched in certain elements stripped from the A and E horizons by percolating water ("B" horizon);
    • a layer of partially weathered bedrock in which bedrock particles are still recognizable ("C" horizon);
    • basement or bedrock.


    Types of soils
    Pedocals are soils rich in calcium characteristic of dry areas (little movement of material by flowing water, only limited remobilization of more soluble elements, like calcium)

    Pedalfers are soils poor in calcium characteristic of humid areas (extensive remobilization and removal of soluble elements such as calcium, potassium, etc, by flowing water).

    Laterites are soils characteristic over very warm, humid places from which all but the most insoluble elements (aluminum, and to a lesser extent iron) have been removed. These are relatively infertile soils.

    Course Outline Calendar Assignment Descriptions Class Forum Lesson Notes