Topic: Rivers 21. Formation of River Landforms I. Upper Course: a) Valleys b) Gorges Formed by Erosion c) Waterfalls II. Middle Course: a) Meanders Formed by Erosion & Deposition III. Lower Course: a) Meanders b) Floodplains Formed by Deposition c) Deltas 1
LANDFORMS FORMED BY EROSION
2.1 A Valley
1. 2. Lateral erosion River Hard Rock Hard Rock3. V‐shaped Valley Hard Rock
2.1 Formation of Valleys1. River flows over an area of hard rocks.2. Due to the steep gradient and resistant rocks, vertical erosion is faster than lateral erosion.3. Over time, the river channel deepens by hydraulic action, abrasion and sometimes solution.4. Eventually, a steep‐sided valley is formed.
2.2 A Gorge A Gorge is an exceptionally deep and narrow valley.
1. 2. Vertical erosion > Lateral erosion River Lateral erosion Very Hard Rock Very Hard Rock3. Gorge Very Hard Rock
Formation of Gorges (Method 1)1. River flows through an area of very hard rocks.2. Due to the rock hardness and steep gradient, vertical erosion dominates.3. Over time, a deep, narrow and steep‐sided (almost vertical) valley called a gorge is formed.
Side View Front View of Waterfall1. 2. Waterfall retreats backwards Soft rocks Hard rocks Soft rocks3. 4. Waterfall retreats backwards A gorge is formed
Formation of Gorges (Method 2)1. As the soft rocks at the base of the waterfall get undercut by the river water,2. the tip of the waterfall loses support beneath and collapses into the water.3. Overtime, the cycle repeats and the waterfall retreats backwards4. leaving behind a deep, narrow and steep‐ sided valley called a Gorge.
2.3 A Waterfall
1. 2. Soft rock Soft rock Hard Hard rock Soft rock Soft rock rock3. 4. Waterfall SoftSoft rock rock Plunge Hard rock Hard rock Soft Pool Soft rock rock
2.3 Formation of a Waterfall1. As rivers flow through bands of hard and soft rocks.2. Softer rocks gets eroded faster than the hard rocks.3. This causes the gradient to steepen.4. Over time, the river plunges from a great height, hitting the base with great force.5. This sudden, steep vertical flow of fast moving water from a great height is called a waterfall.6. Repeated pounding of the water against the water bed 7. will create a depression at the base of the waterfall called a plunge pool.
LANDFORMS FORMED BY EROSION & DEPOSITION
2.4 A Meander
1. 2. Deposition Outer Inner convex (D) concave bank Erosion bank (E) River D Outer Inner convex Cliff concave bank E bank Outer Slip‐off concave Inner convex D slope bank bank 3. Separated 4. Ox-bow by a narrow lake D neck E DLegend Cut-off Erosion D Deposition
2.4 Formation of Meanders1. As a river flows around a bend, river speed is faster on the outer concave bank.2. Hence, erosion by undercutting occurs.3. Over time, a steep‐sided bank called a RIVER CLIFF is formed on the outer bank.4. As the river speed is slower on the inner convex bank, deposition occurs.5. Over time, a gentle SLIP‐OFF SLOPE is formed on the inner bank.6. With repeated erosion and deposition, the meander becomes more and more pronounced, eventually separated by a narrow neck.7. Eventually, the river breaks through the neck and flows in a straight channel.8. The cut‐off forms an ox‐bow lake.
Deposition at the Erosion on the inner convex bank outer concave forms a gentle slip‐bank forms a off slope.river cliff.
Meander Ox‐bow lake
LANDFORMS FORMED BY DEPOSITION
2.5 A Floodplain
1. 2. Heavy and continuous rain, Finer load river overflows its banks Coarser load3. Floodplain Levee
2.5 Formation of Floodplain and Levees1. After a heavy and long period of rain, the river may overflow its banks causing a flood.2. As the water spreads over a larger area, the friction increases causing the river lose energy and deposit its load.3. The coarser and heavier sediments are deposited on the immediate river banks whereas finer and lighter sediments are carried further away.4. Over a series of floods, these layers of sediments forms a floodplain and the coarser materials that have accumulated on the immediate banks form Levees.
2.6 A Delta
1. 2. Land Sea Land Sea River River Distributaries3. Land Sea River Delta
2.6 Formation of a Delta1. When river enters a larger water body such as a sea, its speed of flow and hence river energy is reduced. Hence, it starts to deposit its load.2. At the river mouth, heavier sediments such as sand is deposit close to the shore whereas lighter sediments such as silt and clay are carried further out before being deposited.3. The layers of deposition at the river mouth block the flow of water into the sea, hence the river branches out into smaller streams called distributaries. 4. Over time, an extensive depositional landform called a delta is formed at the river mouth.