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The river carries a load of eroded materials which contain rock fragments and other materials. These materials constantly collide with one another as they move. Over time, the collision causes the materials to break down into smaller pieces. Through this process, larger rocks are slowly broken down and their rough edges are smoothed out. This process of erosion, whereby the load carried by the river is eroded as the rocks collide with each other when they are carried along the river, is known as attrition
Erosive agent: Rock fragments

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As rain falls to the Earth’s surface, it reacts with the carbon dioxide present in the atmosphere, forming a weak acid known as carbonic acid. As this rainwater falls into rivers, the weak acid dissolves mineral present in the rocks on the river bed and banks.

Erosive agent: Carbonic acid in water

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Corrasion/Abrasion occurs when rock fragments are dragged along the river bed or against the river banks. This grinding action causes the river channel to widen and deepen as rocks and soil are scraped off the sides and bottom, and carried away with the water. Corrasion is particularly effective during a flood when the river’s energy is at its maximum.

Erosive agent: Rock fragments

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- a series of very short and fast flowing turbulent falls of water along a part of a river
- relatively shallow water
-when a river flows over an alternating bands of gently inclined resistant and less resistant rocks, the less resistant rock is eroded much faster. the river falls swiftly when it flows over outcrops of the resistant rocks.

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shape of the channel
The river has to overcome friction with the banks and bed of the channel. The shape of the channels or its cross section affects the wetted perimeter.

The wetted perimeter refers to the total length of the river sides and bed.

The greater the wetted perimeter, the greater the friction between the water and the banks and the bed of the channel, and the slower the flow of the river.

If both rivers have the same cross-sectional areas, the one with the larger wetted perimeter flows slower than that of a small wetted perimeter because of more friction.

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-A waterfall is a sudden and rapid vertical fall of water along the course of a river

-A plunge pool is a deep pool excavated to the bottom of a waterfall
-A river flows over alternating  band of resistant rocks. In this diagram, the band of resistant rocks are dipping upstream

-The less resistant rock that lies downstream will be eroded at a faster rate than the more resistant rock

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1. water plunges forcefully over the more resistant cap rock as a waterfall. The impact of the falling water erodes the base of the waterfall, enlarging the plunge pool. The backsplash of the turbulent water at the base undercuts the cliff face of less resistant rock

2. over time, the continuous undercutting of the base causes the cap rock to lose support and collapse

3. the waterfall retreats upstream and  a long, narrow, deep valley with almost vertical sides, called a gorge is formed.

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- Potholes are formed by river abrasion. The abrasive action is especially effective in flood condition

-The pebbles and cobbles that are trapped in sight hollows

-Two or several holes may join together to form larger potholes

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- The force of running water hitting against the river banks and bed and surges int o lines of weakness such a joints and cracks
- a example of hydraulic action is the formation of plunge pools at the foot of a waterfall
- Erosive agent: Fast-flowing water

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Vertical erosion: Occurs when rocks and other materials on the river bed are removed, causing the channel to become deeper

Horizontal/Lateral erosion: Occurs when rocks and other materials along the sides of the channel are worn away. This causes the river channel to become wider.

4 types of processes involved in erosion:

-Hydraulic action

-Corrasion

-Attrition

-Solution

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(a)

meanders are more pronounced along the lower course of the river

lateral erosion and undercutting occurs on the concave bank where velocity is the fastest

deposition occurs on the convex banks because of the slack water

(b)

continuous erosion on the concave banks brings closer the two neighbouring concave banks

the neck between the two concave banks narrows

(c)

the neck continuous to narrow as lateral erosion continues to take place on the concave bank

the narrow neck of land is finally cut through

a new and straighter channel is created and an abandoned meander loop, called a cut-off is formed

(d)

when deposition finally seals off the cut-off from the river channel (new straighter channel), an ox-bow lake is formed

an ox-bow lake may not be a permanent feature as water may dry up and it is then filled with sediments and vegetation

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a horseshoe-shaped or crescent-shaped lake

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a gorge is a deep narrow valley with steep, almost vertical sides. a gorge will develop when a waterfall retreats upstream.

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Rocks which allow water to pass either via a network of pore spaces between particles or along bedded planes, cracks, and fissures are permeable. All rocks are permeable but not to the same degree. Permeability depends on the grain size of the rock and the size of pore spaces between the grains.

Permeable rocks like sandstone, chalk and limestone which have coarse grain size and large pore spaces allow water to infiltrate easily.

Impermeable rocks like shale, clay and granite which have tightly packed small grains go not allow water to pass through.

When water flows through an area of permeable rock, more water infiltrates the ground. As a result, there is low surface runoff and this leads to a lower volume of flow.

When water flows through an area of impermeable rocks, there will be little water infiltrated into the ground and thus leads to high surface runoff and volume of flow is higher.

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- soluble minerals in the ricks such as salts or calcium and sodium dissolve in the water and are carried away in solution; dissolved minerals cannot be seen.
- solution can occur even when the speed of the river and its volume of water are low

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-In upper course, the river has a lot of gravitational potential energy so it has a lot of energy to erode vertically. The bed of the river is eroded greatly while the banks aren't eroded as much. The vertical erosion of the channel gives the valley the characteristic of a deep, narrow, steep- sided 'V' shape.

-Vertical erosion is further increased by the rough nature of the channel which increases the water's turbulence and its ability to erode. The turbulence of the water causes the bed load to swirl in whirlpools, abrading the bed and causes potholes to form

-The river mainly transports large pieces of angular rock and does so by traction because it does not have enough kinetic energy to move the load in any other way. This increases erosion of the bed by corrosion as a result of the load being dragged along the bed of the river

-Erosion and transportation only takes place in large quantities in the upper course when the river's discharge is falls the river stops transporting the large boulders its transporting and deposits them

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Roughness of the channel
The rougher the river channel, the slower the river flow. Rock boulders on the riverbed and rock outcrops on the river sides increase friction between the water and thus slow down the flow as friction needs to be overcome. A river would flow fasted in a channel made of smooth silts and clays.There would be no large and bulky rocks on the river side which can increase the friction between the water and thus the river velocity is not as affected in a smooth river channel compared to a rough river channel.

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wide and flat-bottomed

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A river tends to flow faster when it flows over a straight course as it expands less energy since there is less friction to overcome. On the other hand, a river tends to flow flower when the river’s course is winding as it has to expand more energy to overcome greater friction at the bends of the river.

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When the temperature is high, there is more evaporation and transpiration which reduces the amount of water in the river.

When the temperatures experienced in a region are very low and much water is held in the form of ice and snow, the water level in rivers is low. When the temperature rises, the volume of water in rivers increase as water from the melting snow and ice flows into them.

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- Loops in the course of the river
- Result of both erosional and depositional processes
- Occurs as vertical erosion is replaced by lateral erosion + deposition within the flood plain
- Perpetuated through a process called HELICOIDAL FLOW
- As the surface flow of water hits the outer bank it corkscrews, flows along the river bed then deposits eroded material on the inner bank

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there is sufficient load carried by the river

this is possible when there is active erosion along the course of the river and there are enough tributaries to contribute to the load of the river

absence or few lakes present along the river which could trap the load

gentle offshore gradient with shallow water

this is necessary for the built up of deposited materials to rise above sea level

quiet coastal waters and the absence of strong currents and waves so that the materials are not carried away

climate that favours the growth of fast growing vegetation to bind and stabilise the materials

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The valley is deep, narrow and steep sided. The steep slopes come down to the sides of the river channel like the letter 'V'. Hence, it is often described as a 'V' shaped valley

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formed at the mouth of the river when the load deposited at the mouth exceeds the amount transported away

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- fine particles like sand, silt, and clay are lifted off the bed and carried away in the water as suspension load.
- this forms the largest part of the river load and is also responsible for giving the river a murky appearance

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The higher the rainfall, the greater the volume of flo win a river. In a tropical monsoon country such as India, many rivers swell during the rainy season from June to September but have a smaller volume during the hot and dry season from March to May. Some may even dry up.

The torrential rain brought about by tropical cyclones causes river levels in the affected areas to rise significantly in places like bangladesh. The amount of water in rivers in equatorial areas increases whenever heavy rain falls in the afternoon.

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1. No rainfall , channel is at normal flow. Materials deposited at the bed of the channel. Built up of material on the river bed --> channel becoming shallower

2. Heavy rain - river overflows its banks and floods surrounding areas. Extensive flat lowlands --> larger wetted perimeter --> Water encounters friction --> increased friction reduces speed of water --> deposition of materials on both sides of river

3. coarser heavier materials are deposited first, hence nearest the banks and finer lighter materials are deposited further away from the banks

4. repeated flooding will cause the alluvial deposits to accumulate
the coarser material at the river banks form raised embankments called levees
the finer materials forms the floodplain
although floodplains and levees are more common at the lower course, they can also develop between the middle and lower course

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- Sudden change of gradient (Leaving a mountain & entering flat lowland)
- Flows into another body of water (Lake / calm sea)
- Increase in wetter perimeter (More friction /River overflowing its banks & entering a flood plain)
- Flowing in shallow section of the river (Encounters friction/ Convex bank of a meander)
- Obstruction (Make the channel rougher & increases friction)(Mounds of deposit / rock outcrops)

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Narrow, bed is rough because of the presence of large angular boulders and stones of different sizes. These bed load are a result of weathering and erosion of the valley sides

Load increase the wetted perimeter and hence decrease the speed of flow. The volume of water is small as the river has few tributaries to bring water to it

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- Less GPE and more KE; erosion shifts from vertical to lateral erosion
- Abrasion is still the main erosive process as large particles are transported by saltation
- Average load size decreases; more load is being tranported in suspension
- River may flood and deposit gravel / sand sized particles onto its flood plain

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the river has next to no gravitational potential energy so erosion is almost exclusively lateral

however, erosion is negligible because the channel is smoother resulting in less turbulent flow

the main place where erosion takes place is where the river meanders

the average particle size is very small, another reason for the reduction in erosion

the river’s load is mainly composed of silts and clays and it is transported in suspension or even solution

the dominant river process along is deposition like in the middle course, when the river floods it deposits its load but deposition now also takes place at

the mouth where the river meets the sea or a stationary body of water

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Gradient of a channel
The speed of flow of the river whose average gradient is steep would be faster than that of a river whose average gradient is gentle. Gradient decreases downstream. The river usually has a steep gradient near its source and a gently gradient towards its mouth. A river flowing down a steep slope or gradient has a higher velocity compared to one which flows down a gentle gradient.For example, the speed of flow in a river that purges down a steep slope in the for of a waterfall is much higher than the speed of a flow in a river that winds its way down a gentle slope.

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When a drainage basin has a thick vegetation cover to intercept the rain and allow more waters to infiltrate into the soil subsurface and percolate through the underlying soil and rock layers, there is less surface runoff. Hence, less water flows off into the river. The volume of water in the river will be smaller than that of an area with little or no vegetation.

When a drainage basin has little or no vegetation to intercept the rain, a greater volume of water in the surface runoff quickly adds to the volume of water in the river.

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- Little to no rainfall
- Flows across an arid area (Hot & dry, high rate of evaporation)
- Flows across permeable rocks (Allow water to sink in, infiltrate into the ground)
- Flood water brought by a storm has drained away (Flood has subsided)

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- is the widest and the deepest
- the river has the largest volume of water and load because of the numerous tributaries bringing their water and load to it as they join it

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Flood plain:

- wide and low-lying plain found on both sides of a river
- covered with alluvium deposited by the river

Levees:
- natural embankments found along the banks of a river
- made up of coarser alluvial deposits than the flood plain

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As the meander swing from side to side, the high velocity on the concave banks cause the current to descend undercutting and eroding the bank.
Eroded material is carried and deposited at the convex bank.
The concave bank will eventually collapse, forming a steep cliff.
At the convex bank, deposition makes it shallow and the built up material result in the formation of a slip off slope

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- Starts to meander
- Wider and deeper channel
- Asymmetrical cross section
- Steep river cliff develops on the concave bank of the meander and a gentler slip off slope develops on the convex bank of the meander

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- when energy increases, coarse sand and other small rock fragments like gravel are lifted briefly then dripped on the river bed so they bounce in hops or jumps

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Begins to deposit its load when volume & speed of the water decreases

Carrying a greater load than it can transport

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- More open V-shape
- Wider floor

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A larger drainage basin will have a greater volume of water. This is because there is a greater number of tributaries bringing water to the river resulting in a higher surface runoff.

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1. traction
- larger rocks like large pebbles and boulders slide and roll along the river bed
- only move is the speed of a river is great enough to move them

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the more river volume the river has, the higher the energy

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the higher the river velocity, the higher the energy