Running water is the most active agent in humid areas which receive maximum rainfall. It has two components. First, it flows overland as a sheet and directed by the land surface configuration. Second, it flows as streams and rivers in valleys. Many erosional and depositional landforms are associated with the running water some of which are as follows:
Erosional Landforms
- Valleys: The friction of running water overland initiates removal of surface material forming small and
narrow rills. These rills gradually develop into long and wide gullies due to the constant flow of water in the same direction. With time, it further deepens, widens and lengthens and gives rise to valleys. - Monadnocks: In the middle stages of valley formation due to running water, lateral erosion of valley sides becomes severe reducing the valley sides to lower and lower slopes until they are almost completely flattened. It leaves a lowland of faint relief with some low resistant remnants called monadnocks standing out here and there. The type of plain forming as a result of this stream erosion is called a peneplain.
- Potholes: These are circular depressions formed in hilly areas as running water while finding the channel erodes and wear down rocks. Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water and consequently the depressions grow in dimensions.
- Plunge pools: At the foot of waterfalls, large potholes, quite deep and wide, form because of the sheer impact of water and rotation of boulders. Such large and deep holes at the base of waterfalls are called plunge pools.
- Incised or entrenched meanders: When a stream flows rapidly over steep gradients, it mainly erodes the bottom of the channel rather than the sides of the valley. However, when it flows in gentle slopes, it causes active erosion of the sides and develops meandering courses. When this meandering takes place on hard rocks, it is called incised or entrenched meanders.
- River terraces: When a stream vertically erodes into its own depositional floodplains, it limits the reach of water towards the banks and the left untouched formation is known as river terraces.
- Badland Topography: In arid regions occasional rainstorms produce numerous rills and channels which extensively erode weak sedimentary formations ultimately leading to the formation of Badland topography, for example – Chambal ravines
Depositional landforms
- Alluvial fans: River flowing in mountain slopes carries a heavy load of coarse material and when it breaks into foot slope plains of low gradient, it becomes too heavy to carry load further. Therefore, it dumps load which gets spread as a broad low to high cone-shaped deposits called alluvial fan.
- Meanders: When the river enters in plains, it flows slowly due to extremely low gradient slope and starts lateral erosion instead of down erosion. But the irregularities along the banks does not allow uniform erosion because of which small curvature get formed in the banks. These curvatures further deepen with deposition on the inside of the curve and erosion along the bank on the outside which consequently develops a tortuous channel pattern known as meander.
- Floodplains: As the river enters into a gentle slope channel, first it deposits large-sized materials. It carries only fine-sized materials in gentler channels in the plains and deposits over the bed and above the river bed (when the waters spill over the banks during flooding). These formations are called as Floodplains
- Braided channels: Apart from mountains slopes, the river also carries coarse material in lower areas. This material gets deposited in the course of the river and forms a central bar which diverts the flow towards the banks. The water directed towards banks causes erosion of sides which widens the valley and the water level gets reduced. This constant deposition in river course and lateral erosion of sides leads to the formation of various channels which develops as braided channels.
- Deltas: Deltas are similar to alluvial fans but develop at a different location. It is the load carried by the river which is not dumped into the sea and get distributed along the coast in a very well stratified manner. Consequent, low cone accumulation is termed as Delta.
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