River/Fluvial: Mechanism of Erosion and Landforms Department of Geography (1st Semester) Rhenock Government College

Fluvial processes are associated with rivers and streams and the deposits and landforms created by them. The fluvial processes may be divided into three phases: Erosion. Transportation Deposition   Factors affecting fluvial (river) process: River Volume River speed/Velocity Slope Load (Material carried by a river)

Agents of Erosion: Erosion: The combined action of transportation and breaking down of rocks is known as erosion. Agents: Four main types of agent’s i.e moving water (Fluvial), wind (Aeolian), ice and waves. These agents’ wears and tears the surface of the earth and leads to formation of different landforms. Four agents of erosion: Action of running water Action of moving ice Action of wind Action of waves

Process of Erosion CASH: Corrosion, Attrition, Solution and Hydraulic Action Corrosion/Abrasion: Involves the removal of loosened materials of the rocks of valley walls and valley floors with the help of erosional tools or river loads. Attrition: The mechanical tear and wear of the erosion tools themselves. The boulders, pebbles etc while moving with water collide against each other and thus are fragmented into smaller and finer pieces in the transit. Thus, the rock particles are so broken down that ultimately they are comminuted into coarse to fine sand particles. Solution: Involves the dissolution of soluble materials through the process of disintegration and decomposition of carbonate rocks. The soluble materials are removed from the parent rocks and are mixed with running water. Hydraulic action: Involves the breakdown of the rocks of the valley sides due to the impact of water current of channel. In fact, hydraulic action is the mechanical loosening and removal of materials of rocks by water alone. 

William Morris Davis divided the cycle of erosion into three stages: The variations in latitude , geologic structure, and climatic conditions result in notable departures from the sequence of landforms described below: Youth stage Mature stage Old stage

LANDFORMS IN YOUTH STAGE:

Waterfalls: Sudden descents or abrupt breaks in the longitudinal flow of rivers due to host of factors e,g variation in the relative resistance of rocks, difference in topographical reliefs, earth movements etc.

Pot Holes: The kettles like shaped depressions in the rocky beds of the river valleys are called potholes. Potholes drilling mechanism: the grinding tools (fragments of rocks) when caught in the water eddies or swirling water start dancing in circular manner and grind and drill the rock beds of the valley like drilling machine and thus form small holes which are gradually enlarged by the repetition of the said mechanism. When potholes are very large, they are called plunge pools.

River Valleys: . The valley formed in youthful stage initially is V shaped having steep valley side slope. The valley is very deep and narrow, both the valley sides meet together at the valley floor and thus the water always touches the valley sides. Such V shaped valleys are result of accelerated rate of down cutting (vertical erosion or valley deepening). V-shaped valleys are divided into two types a) Gorges and b) Canyons Gorges: It is extremely deep and narrow valley. Formed due to active down cutting of valleys.   Canyons: V shaped valleys are gradually widened due to lateral erosion (Extended form of gorges).

Gorge Canyon

LANDFORMS IN MATURE STAGE:

Alluvial Fans Alluvial Cones As the river which had a high speed while flowing in the mountains, has a lot of sediment. The stream, due to loss of slope, deposits the sediment. The deposited sediments spread in a semi circular. The section of this sediment spreading in a semi circular form is known as Alluvial fans. Many times these alluvial fans increase in height by the deposition of the sediments and their slope becomes steeper and such formations are called alluvial cones.

River Meanders Ox-bow Lakes When the stream flows in a plain it widens its valley instead of deepening it. The river course is neither plain nor composed of rocks. The stream therefore cannot flow in straight course. Many loop like bends and turns are common in the course of stream. Meander is a serpential flow of river. The lakes are formed due to impounding of water in the abandoned meander loops are called ox-bow horse shoe lakes.

Landforms in old stage: Delta: The depositional feature of almost triangular shape at the mouth of a river is called delta. As the water velocity decreases and loses the power to carry sediment in old stage of river cycle. The sediment is dropped at the mouth of the river.

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