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Potholes

Potholes are cylindrical holes drilled into the bed of a river that vary in depth and diameter from a few centimetres to several metres.They’re found in the upper course of a river where it has enough potential energy to erode vertically and its flow is turbulent. In the upper course of a river, its load is large and mainly transported by traction along the river bed.When flowing water encounters bedload, it is forced over it and downcuts behind the bedload in swirling eddie currents. These currents erode the river’s bed and create small depressions in it.

As these depressions deepen, pebbles can become trapped in them. As a result of the eddie currents, the pebbles drill into the depressions making them more circular, wider and deeper. Pebbles will only be able to erode a river’s bed though if the rock the pebble’s made of is stronger than the rock the river bed is made of.

V-Shaped valleys

V-Shaped valleys are found in the upper course of the river and are a result of both erosion by the river and weathering. V-Shaped valleys are deep river valleys with steep sides that look like a letter V when a cross section of them is taken, hence the name.

They’re found in the upper course because this is where the river has the greatest gravitational potential energy and so the greatest potential to erode vertically. It does so during periods of high discharge. When the river’s discharge is high, it is able to transport its large bedload by traction eroding the river’s bed and valley by corrasion, deepening it.

Not much lateral erosion takes place so the channel and valley remains relatively narrow. As the channel and valley deepens the sides of the valley are exposed and become susceptible to weathering. The valley’s sides also undergo mass movements resulting in large volumes of material falling into the river’s channel, adding to its erosive power and causing the valley sides to take up a V shape.

The steepness of the valley sides and whether the valley actually looks like a V is dependent on the climate, vegetation and rock structure among things. In cold, wet climates, freeze thaw weathering is abundant and rainwater can act as a lubricant, aiding mass movements. Vegetation can impede mass movements because it will help bind the soil. If the valley is composed of hard rock the valley sides will be very steep because they won’t be weathered easily.

Waterfalls (geological)

Waterfalls develop when a change of lithology (rock type) takes place along the river’s course resulting in differential erosion. When the rock type of the river’s channel changes from a resistant rock to a less resistant one (e.g. granite to limestone), the river erodes the less resistant rock faster producing a sudden drop in the gradient of the river with the resistant rock being higher up than the less resistant rock. As the river flows over the resistant rock, it falls onto the less resistant rock, eroding it and creating a greater height difference between the two rock types, producing the waterfall. When water flows over the waterfall it creates a plunge pool at its base and the splashback from the falling water undercuts the resistant rock. The unsupported rock is known as the cap rock and it eventually collapses into the plunge pool causing the waterfall to retreat upstream. Over thousands of years, the repeated collapse of the cap rock and retreat of the waterfall produces a gorge of recession.

Rapids

Rapids are sections of a river where the gradient of the river bed is relatively steep resulting in an increase in the river’s turbulence and velocity. They form where the gradient of the river is steep and the bed is composed mainly of hard rocks.

Meanders

Meanders are bends in a river that form as a river’s sinuosity increases. The sinuosity of a river is a measurement of how much a river varies from a straight line. It’s a ratio between the channel length and displacement (straight line distance) between two points in the river’s course:

A sinuosity of 1 means that the channel is perfectly straight. A sinuosity greater than 1 means that the river meanders. Meanders develop when alternating riffles and pools form along a river channel. A riffle is a a shallow section of a channel while a pool is a deep section. These riffles and pools develop at equal points along the river channel with each pool being about five times the length of the channel.

In a pool, the channel is more efficient, while at a riffle, the channel is less efficient. This causes the flow of the river to become irregular and the maximum flow is concentrated on one side of the river.

This increases erosion on one side of the river and increases deposition on the other causing the river’s channel to appear to bend. Erosion is greatest on the outside bend and deposition is greatest on the inside bend.

Characteristics

A cross section of a meander would show that on the outside bend, the channel is very deep and concave. This is because the outside bend is where the river flows fastest and is most energetic, so lots of erosion by hydraulic action and corrasion takes place. River cliffs form on the outside bend as the river erodes laterally.

The inside bend is shallower with a gentle slip-off slope made of sand or shingle that is brought across from the outside bend by the helicoidal flow of the river. The river flows much slower on the inside bend so some deposition takes place, contribution to the slip-off slope. — https://geographyas.info/rivers/river-landforms