River Channel processes and landforms

Question 1

Long profile

Answer 1

a graph of a river that marks the change in altitude from the
upper course to its lower course. river processes are related to the long profile because every river is
trying to achieve a smooth, concave long profile
* when this is not the case, the river will be working to smooth its long
profile out by erosion, transport or deposition to establish balance.

Question 2

Dynamic equilibrium

Answer 2

the river channel shape is constantly adjusting to changes in inputs of
water and material, so that it can transfer the water and material
downstream

Question 3

Abrasion

Answer 3

the wearing away of the bed and bank by the load carried by the river. Its effectiveness is dependent on the concentration,
hardness and energyof the impacting particles, as well as the resistance of
the bedrock. Abraision increases as velocity increases.

Question 4

Attrition

Answer 4

when load particles collide against each other and wear one another off. Creating smaller and rounder particles.

Question 5

Hydraulic Action

Answer 5

Force of air and water on the sides of the river and in cracks. Includes the direct force of flowing water and cavitation. As fluids accelerate pressure drops causing air bubbles to form.

Question 6

Cavitation

Answer 6

air bubbles trapped in the water get compressed into small
cracks in the river’s banks; these bubbles eventually implode, creating a
small shockwave that weakens the rocks
—> the shockwaves are very weak, but over time, the rock will be
weakened to the point at which it falls apart

Question 7

Solution

Answer 7

The removal of chemical ions esp calcium. Key factors controlling solution are velocity, solute concentration, discharge and bedrock. Maxiumum rates occur when fast flowing streams pass over soluble rocks (limestone).The water’s acidity will only affect certain types of rocks and
dissolve them
—> highly effective if the rock in the channel contains calcium
carbonate.

Question 8

Types of Fluvial Erosion

Answer 8

Vertical erosion
Lateral Erosion
Headward Erosion

Question 9

Lateral Erosion

Answer 9

River erodes the river banks and widens the river channel

Question 10

Vertical erosion

Answer 10

River erodes downwards, deepening the river channel

Question 11

Headward Erosion

Answer 11

River erodes in an upstream direction thus lenghtening the river valley.

Question 12

Soil strucure affecting rates of erosion

Answer 12

differences in the coastal crust make some rocks more solid than others,
hence rendering them more difficult to erode. Soft unconsolidated rocks such as sand and gravel are easily eroded.

Question 13

pH affecting rates of erosion

Answer 13

Increasingly acidic pH increases rates of solution

Question 14

Human Activities affecting rates of erosion

Answer 14

removing trees, fertilising soils and overgrazing by livestock disrupts the
roots that stabilise sediment and soil, making easier to erode

Question 15

Load affecting rates of erosion

Answer 15

the heavier and sharper the load the greater the erosion

Question 16

Gradient affecting rates of erosion

Answer 16

as the slope becomes steeper, water gains velocity, heightening the
frictional force and enabling it to erode the banks and bed more rapidly.

Question 17

Suspended load

Answer 17

smallest particles (silts and clays) carried

Question 18

Saltated load

Answer 18

Larger particles (sands, gravels, very small stones) are transported in a series
of ‘hops’

Question 19

Traction load

Answer 19

Pebbles are shunted or rolled along the bed as the bed

Question 20

Solution

Answer 20

areas on calcareous rock material is carried as dissolved load.

Question 21

Capacity of a stream

Answer 21

largest amount of debris that a stream can carry

Question 22

Competence of a stream

Answer 22

diameter of the largest particle that can be carried.

Question 23

Causes of deposition

Answer 23

a reduction in gradient, which decreases velocity and energy
hence resulting in the heavier load being
dropped
a decrease of volume of water in the channel the force of the river is reduced, hence
it becomes unable to carry the heavier load, which is eventually
deposited
an increase in the friction between water and channel the heavier sediment rubs against the river bed and
banks, causing it to eventually become stuck to and deposited on the
river bed.

Question 24

Deposition

Answer 24

Mostly takes place in lower course. Sometimes takes place in middle course on inside bends of river meanders whilst outside bends experience erosion

Question 25

Alluvium

Answer 25

the finest (lightest) material may continue to be carried as the river approaches the sea.

Question 26

Sedimentation

Answer 26

the sediment deposited settles on the river bed, building up
in layers and reducing the river’s depth

Question 27

Sediment yield

Answer 27

the amount of sediment reaching or passing a point of
interest in a given period of time.

Question 28

Denudation

Answer 28

the geological process in which moving water erodes the river
bed, leading to a reduction in elevation and relief or landforms and
landscapes

Question 29

Critical erosion velocity

Answer 29

the lowest velocity at which grains of a given size
can be moved

Question 30

critical erosion curve

Answer 30

the minimum velocity required to lift a particle of a
certain size

Question 31

critical deposition curve

Answer 31

the maximum velocity at which a river can be
flowing before a particle of a certain size is dropped

Question 32

3 important features on Hjulstorm curve

Answer 32

smallest and largest particles require large velocities
Higher velocities are required for entrainment than for transport
When velocity falls below a certain level those particles with a particular
settling or fall velocity are deposited.

Question 33

Factors controlling Velocity and discharge

Answer 33

gradient of the channel bed: steeper gradient = higher velocity
volume if water in the channel: controlled by precipitation levels
—> low levels of water = slower velocity
—> higher levels (banfull) = more rapid flow
channel roughness: rough channels can create friction, which slows down
flow
channel shape: bends/curves in the river can slow the slow, reducing the
velocity.

Question 34

wetted perimeter

Answer 34

the entire length of the river bed, banks and sides in
contact with water. the greater the wetted perimeter, the greater the friction between
the water, bed and banks, and the slower flow of the river

Question 35

River velocity upstream

Answer 35

large angular boulders
create a rough channel shape and therefore a large
amount of bed friction and reducing the velocity

Question 36

Mannings Equation

Answer 36

a measure of how efficient the channel is at
transporting water and sediment
—> it is the ratio between the length of wetted perimeter and cross–
section of a river channel

Question 37

laminar flow

Answer 37

when water travels smoothly in a regular path
* a smooth, straight channel with low velocity is required
* this occurs when water travels over the sediment in the riverbed without
disturbing i

Question 38

Turbulent Flow

Answer 38

characterised by disorder in the movement of water and
irregular flowing patterns
* occurs when there are higher velocities and complex channel
morphology such as meanders, or when the riverbed is uneven. turbulence is associated with cavitation and hydraulic action
* turbulent water swirls called eddies will form an obstacle like a boulder
in a river
* the water within the eddy may reverse the direction of flow and make
it flow upstream
* vertical (downward) turbulence creates hollows in the channel bed
* pebbles and gravel can get trapped in these hollows, and get swirled
through eddying and eroding the riverbed through abrasion creates potholes in time

Question 39

Eddies

Answer 39

turbulent water swirls that form an obstacle like a boulder
in a river
* the water within the eddy may reverse the direction of flow and make
it flow upstream

Question 40

helicoidal flow

Answer 40

corkscrew movememt of water transporting eroded
material downstream from one meander bed to the next

Question 41

Pools or riffles

Answer 41

will develop around water
twists and turns around obstructions
—> results in areas of slower and faster water movement.

Question 42

Riffle-pool sequence

Answer 42

as a stream’s flow
structure alternates between areas of shallow to deeper water

Question 43

Thalweg

Answer 43

line of maximum velocity and it travels from one outside meander bank to the next.

Question 44

Formation of meanders

Answer 44

:1. Due to banks of sediment at the bottom of the river (which are deposited at times of low flow, that is, a low velocity and low discharge, meaning deposition increases), the river weaves around these alternating shallow and deeper sections (riffles and pools) on what was an initially straight 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 5× the length of the channel.

In a pool, the channel is more efficient, while at a riffle, the channel is less efficient creating irregular flow. .2. This movement targets one bank of the section after the riffle, subsequently leading to erosion (carried out by hydraulic action and abrasion) – this becomes the outside bend of the meander.3. This leads to the formation of a river cliff on the outside bend. The material eroded further upstream is deposited on the opposite bank (inside bend) on the slip off slope. This is due to a corkscrew movement in the river (helicoidal flow) which results in water levels on the outside of a meander bend to be elevated, giving a faster velocity (the Thalweg line is located here). The deposition on the inside bend gives an asymmetrical cross section of the channel. .4. Erosion and deposition continues which leads to the meanders becoming more curved.

Question 45

River cliff

Answer 45

The Helicodal flow (cock screw like flow of water that goes through rivers) causes surface water to flow towards the outside bank which causes lateral erosion through hydraulic action. Erosion of the outside bank also takes place through abrasion; as the velocity is greater here the river is able to carry larger sediment which increases the force of abrasion. The combined effect of this erosion causes the outside bank to be undercut and eventually causes it to collapse thereby forming a river cliff.

Question 46

River Sinuosity

Answer 46

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.
Increases as depth increases

Question 47

slip off slope

Answer 47

The build-up of deposited sediment is known as a slip-off slope. Gently sloping bank found on the inside of a river bend.

Question 48

Cross section of a Meander

Answer 48

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.

Question 49

Straight channels

Answer 49

rare and typically have a central ridge of deposited material

Question 50

Braided River channels EG- Tagliamento’s channel in Italy.

Answer 50

channel that is divided into smaller sub-channels by small, temporary islands called eyots. develop in rivers with a lot of sedimentary load, a steep gradient where the discharge of the river changes regularly and where there is easily erodable bank material.

Question 51

Braided River channel formation

Answer 51

When the volume of load exceeds the river’s capacity or the discharge of the river drops, the river is forced to deposit its load in the channel thus creating a mid channel eyot. As the discharge decreases after a flood the bedload is deposited forming eyots that grow downstream.

Question 52

Eyots

Answer 52

Bedload that is deposited after discharge of a river decreases. These grow downstream.

Question 53

Islands

Answer 53

Eyots upstream that become stabilizd with vegetation. Helps to localise and narrow the channel in order to increase velocity so river can transport load.

Question 54

Wavelengths of Meanders Influenced by

Answer 54

Channel Width
discharge and nature of bed and banks

Question 55

Meander Wavelenghts

Answer 55

6-10 times channel width
5 times radius of curvature
wavelength increases in streams that carry excess load

Question 56

Meander belts

Answer 56

peak to peak amplitude of meanders usually 14-20 times channel width

Question 57

Factors causing meanders

Answer 57

Friction-causes turbulence making flow unstable producing bars.
Sand bars
sinuosity-
helicoidal flow

Question 58

How meanders change over time

Answer 58

Migrate downstream and erode river cliffs
Migrate laterally and erode floodplain
Become exagerrated and become cut offs
Become intrenched or ingrown

Question 59

Incised meanders

Answer 59

well developed on horizontally bedded rocks as river cuts through sediment and into underlying bedrock. Come in 2 types intrenched and ingrown

Question 60

Ingrown Meanders- in the lower part of the Seine

Answer 60

are a result of lateral meander migration and are aysmmetrical in the cross section

Question 61

Intrenched Meanders- EX GOOSENECKS OF SAN JUAN USA

Answer 61

Symmetrical and occur when vertical erosion is faster than the lateral migration of meanders, typically when sea levels drop.

Question 62

Pools and Riffles

Answer 62

Formed by turbulences. Eddies cause the deposition of coarse materials (riffles) at high velocity points and fine sediment (pools) at low velocity points.

Question 63

Waterfall Formation

Answer 63

Develop when there is a change in rock type from more resistant to less resistant (granite to limestone) river erodes less resistant rock faster resulting in a sudden change in gradient. As the river flows over the resistant rock, it falls onto the less resistant rock, eroding it and causing an undercut. 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 which will collapse causing the waterfall to migrate upstream

Question 64

Gorge Recession

Answer 64

Occurs when waterfalls migrate upstream, common when local rocks are not resistant to powerful river erosion. Similarly in arid areas (water for weathering is scarce) gorges are formed by long periods of river erosion

Question 65

V shaped valleys

Answer 65

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

Question 66

Levees

Answer 66

During a flood the largest & heaviest load is deposited first and closest to the river bank, often on the very edge, forming raised mounds. The finer material is deposited further away from the banks causing the mounds to appear to taper off. Repeated floods cause the mounds to build up and form levees.

Levees aren’t permanent structures. Once the river’s discharge exceeds its bankfull discharge, the levees can be burst by the high pressure of the water.

Question 67

Flooplain

Answer 67

large, flat expanses of land that form on either side of a river. The floodplain is the area that a river floods onto when it’s experiencing high discharge. When a river floods, its efficiency decreases rapidly because of an increase in friction, reducing the river’s velocity and forcing it to deposit its load. The load is deposited across the floodplain as alluvium. The alluvium is very fertile so floodplains are often used as farmland.

The width of a floodplain is determined by the sinuosity of the river and how much meander migration takes place. If there’s a lot of meander migration, the area that the river floods on will change and the floodplain will become wider.

Question 68

Backswamps

Answer 68

It is where deposits of fine silts and clays settle after a flood. These deposits create a marsh-like landscape that is often poorly drained and usually lower than the rest of the floodplain.

Question 69

Terraces

Answer 69

remenants of old floodplains which have been eroded.

Question 70

River bluffs

Answer 70

line of relatively steep slopes at the end of a terrace

Question 71

Oxbow lakes

Answer 71

strong erosion takes place on the outside bend of a meander while deposition takes place on the inisde bend. As a result, the neck of a meander narrows. During extremely high discharge (e.g., a flood), it’s more efficient for a river to flow accross the neck of a meander rather than around it. When discharge returns to normal levels, the river continues following this new course. The meander is left connected to the channel as a cutoff. Deposition eventually separates the cutoff from the main channel leaving behind an oxbow lake. With its main source of water disconnected, the lake eventually dries up leaving behind a meander scar.

Question 72

Deltas

Answer 72

Deltas are depositional landforms found at the mouth of a river where the river meets a body of water with a lower velocity than the river (e.g. a lake or the sea). For a delta to develop, the body of water needs to be relatively quiet with a low tidal range so that deposited sediment isn’t washed away and has time to accumulate.

Question 73

Delta formation

Answer 73

When a river meets a stationary body of water, its velocity falls causing any material being transported by the river to be deposited.

Question 74

3 layers of Deltas

Answer 74

bottomset bed- built along the sea floor by turbidity currents. composed of fine material such as clay ( salt water causes particles to flocculate) his makes the clay particles sink due to their increased weight producing the bottomset bed. The bottomset bed stretches a fair distance from the mouth of the river as the fine sediments can be transported a reasonable distance from the river’s mouth.

Question 75

foreset beds

Answer 75

on top of bottomset beds inclined layers of coarse material is deposited.The foreset bed makes up the majority of the delta and is dipped towards deep water in the direction that the river is flowing in.

Question 76

Topset bed

Answer 76

Composed of fine material and are part of rivers floodplain and are horizontally bedded.

Question 77

Types of Delta

Answer 77

Arcuate
Cuspate
Birds Foot
Lacustrine

Question 78

Arcuate Deltas

Answer 78

Fan shaped. Found in areas of regular longshore drift or other currents. These factors keep the seaward edge of the delta smooth. (shaped like a triangle)

Question 79

Cuspate Delta

Answer 79

are vaguely shaped like a V with curved sides. Cuspate deltas form when a river flows into a sea with waves that hit it head on, spreading the deposited sediment out.

Question 80

Birds foot delta

Answer 80

They extend reasonably far into a body of water and form when the river’s current is stronger than the sea’s waves and deposition occurs along the edge of distributaries.

Question 81

lacustrine deltas

Answer 81

When a river enters a lake and it deposits some or all of its load. As the delta builds up and out it may fill the lake basin

Question 82

rapids

Answer 82

When a waterfall retreats the waterfall often
becomes smaller in size until it almost disappears
producing a steeper, more turbulent section of river
profile, often with a rough bed where outcrops of
resistant rock are found