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Depositional landforms






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    Depositional landforms Depositional landforms Presentation Transcript

    • Depositional Landforms • Floodplains • Levees • Braiding • Delta • Alluvial fan
    • Floodplains• What ? – A wide and flat valley – Alluvium deposits on riverbanks build it up during repeated flooding – Provides fertile alluvium for farming
    • How?• Form when river overflows its banks during flooding and spread large amount of sediment over the banks• When flood subsides or over, there is a decrease in volume so that river deposits its load.• Repeated flooding result in materials deposited on the riverbanks forming a floodplain
    • LeveesWhat?It is a natural embankment on the riverbanksWhere?Form when coarser alluvium is deposit along the riverbanks due to flooding
    • How?• Form during flood• The coarser allluvium is deposited near the banks and finer alluvium deposited further from the banks• The accumulation of the coarser alluvium along the river banks formed a natural raised banks called levees
    • Braiding• A river subdivides into many smaller streams within the channel• Small islands / eyots separate each stream• Eyots are made up of deposited loads such as gravels and coarse sands
    • How?• Depends on load-discharge relationship• Short period of the year, some rivers carry a heavy load which the rivers have difficulty in removing• When the river level falls rapidly, competence and capacity reduced – channel deposit materials within the channel• Channel become broad and shallow with a low hydraulic radius
    • (Braided river in southern Alaska.)
    • EYOTS
    • (Braided river at junction of Gakona and Copper River, Alaska)
    • Main Characteristics1. Small islands or eyots of deposited material within the channel.2. A network of small intertwining channels converge and diverge around the eyots.3. At times of high discharge, braided streams carry a large load of coarse material.4. Braiding is a characteristic of streams with a variable discharge5. Very common in Semi-arid region where torrential rainfall will lead to temporary stream with high discharge.
    • DeltaWhat?• A piece of flat land that is made up of sediments deposited at the river mouth• Streams found on the delta known as distributaries
    • Flocculation• Flocculation occurs as fresh water mixes with salt water - e.g where a river flows into the sea -(then called brackish water), and is the process by which tiny particles in suspension like clay and silt (which under normal fresh water conditions would likely never fall out of suspension) coagulate with the salt in the water by chemical reaction and become heavy enough to ‘sink’ and be deposited
    • Typical Conditions:• Calm and shallow seawater with gentle slope• River deposits its load at the river mouth• Wave in the sea is weak and sheltered coast• More deposition due to active erosion in the upper course
    • How does it form?• Form when a river approaches the calm seawater• Its velocity decreases• Starts to deposits its load at the river mouth• Sediment continued to accumulate into the sea- forming delta, which is a flat land
    • Material deposited: 3 layers1. Bottomset beds – the lower part of the delta that consists of fines material that can be transported furtherest out to sea2. Foreset beds – the middle layer where material deposited is slightly coarser3. Topset beds – the uppermost layer where coarsest particles settle first
    • Types of Delta• Delta’s can be categorised into three main shapes – Arcuate – Birds Foot – Cuspate
    • ArcuateDelta’s•The most commonshape of delta•Characteristics:curving shoreline(smoothed by longshore drift), distinctpattern of drainage(branching ofdistributaries), andtypically more‘gravely’ deposits .•Example: the NileDelta
    • Birds FootDelta•‘Fingers’ ofdeposition buildout into the seaalong thedistributarieschannels givingan appearancelike a birds claw.Typically with afiner sediment•Example: theMississippi delta
    • Cuspate Delta • A cuspatedelta is shaped like a tooth by gentle but regular opposing currents in thewater body the river flows into, or longshore drift• Example: Theriver Ebro delta in Spain
    • Alluvial Fans• Are fan shaped similar to delta but deposited on land• Comes from a stream of a steep mountain course and enters a flatter plain with a low gradient• Sudden change of gradient river lose energy  river starts to deposit its load.• The heavier (coarser) the material will b deposited first then the finer materials