AS Geography, Rivers: Erosion & Deposition landforms

Basic and concise notes on the landforms produced by EROSION, DEPOSTITION OR BOTH. 

#Notes taken from OCR Geography textbook#


Landforms produced by EROSION

1) V-Shaped Valleys: Formed by erosion from a river, weathering and mass movements such as slumps, falls and slides which occur on the valley sides, creating the distincitve V-Shape.

2) Waterfalls: Resistant rock is underlain by weaker rock. The less resistant rock is undercut by HYDRAULIC ACTION (due to the force of falling water). The resistant rock overhangs and eventually collapses and retreats back.The fragments of resistant rock plummet and scour at the bed, creating a plunge pool. This type of erosion is known as ABRASION. Some waterfalls are initiated by FAULTING whereas some are created due to a drop in sea level.

3) Gorges: Gorges are narrow, steep sided valleys formed by the retreat of a waterfall or by the collapse of a cave system, superimposed drainage or the opening up of a fault line ie, Victoria Falls.

4) Potholes: Holes in the base of a stream/river, formed by ABRASION of pebbles held up by TURBULENT FLOW in the eddies of stream ---> creates a "drilling effect".

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Landforms produced by DEPOSITION.

1) Braided Channels: When a river separates into a number of smaller, interlocking channels ---> highly unstable. This feature tends to occur when a channel struggles to transport its load in a single channel, dropping a variety of loads containing coarse material. There a few conditions leading to braiding; a high variable discharge is needed and a channel gradient which is slightly steeper than that of a meandering stream.

2) Floodplains: Heavy precipitation leads to an increased discharge. BANKFULL STAGE occurs, The channel variables (depth, width and velocity) cannot cope with the sudden large amount of water. The river's banks burst and ALLUVIUM (fine sediments) are deposited, creating a floodplain. Floodplains may also be formed by the erosion of bluffs (steep banks or cliffs).

3) Levees: Raided ridges that are parallel to the edge of the channel and formed by repeated flooding of a river. Flooding causes velocity to drop, causing water to spill out of channel. The heaviest/coarse sediment are dropped. Time is required (centuries) to build/create the raised banks known as levees.

4) Deltas: A delta develops when the river meets it's end at the sea or lake. Any material carried is dumped and accumulated, forming a Delta. Deposition is increased if the water is salty. Salt particles group together, become heavier and deposit.

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Landforms produced by EROSION/DEPOSITION

1) Meanders: A meander is a bend in the river along a floodplain. The channel velocity increases but transporting a finer load which does not use up all of the river's energy. In relatively straight channels, meanders are developed to use up this energy. It must be noted that a meander has an ASYMMETRICAL CROSS-SECTION; the outer bend being the deepest part and the inner bend being the most shallow part. Meanders begin with the development of pools and riffles in a straight channel and the THALWEG (line of maximum velocity in a river) flows side to side. HELICOIDAL FLOW occurs on the outer bank whilst the bottom flow is towards the inner bank. These variations cause the asymmetry of the cross-section and creates river cliffs on the OUTER BANK and SLIP-OFF SLOPES on the inner bend.

2) Oxbow Lakes: The result of both erosion and deposition. Lateral erosion occurs on the outer bend of a meander, widening it. During flooding, erosion will increase which impacts the bend even further, causing the river to break through, joining the meanders. Deposition occurs between the meander and new channel, eventually cutting of the "loop", resulting in an oxbow lake alongside a new course of river and a straighter channel.

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