1.3 River processes and landforms

Question 1

What are conditions and landforms upstream?

Answer 1

Steep gradients, rough beds, narrow and shallow channel, high friction and turbulence, low speed, vertical erosion.

V shaped valleys, interlocking spurs, waterfalls, gorges, rapids, potholes.

Question 2

What are the conditions and landforms downstream?

Answer 2

Gentle, smooth bed, low gradient, wide and deep, low friction, low turbulence, high speed, lateral erosion.

Wide floodplains, levees, oxbow lakes

Question 3

How do processes change as you move downstream? What occurs in between up and down stream?

Answer 3

Load particle size, roughness and gradient fall as you go downstream.

Discharge, width, depth, velocity and load quantity get bigger as you get downstream

Rivers meander in between up/downstream

Question 4

What is erosion and what are the 4 types?

Answer 4

Erosion is the wearing away of the bed and bank by the load, discharge or processes of the river. The material is broken down and removed due to the energy of the river.

Abrasion is mechanical action of load wearing away the bed and bank, caused by debris from eroded banks. It depends on the concentration, hardness and energy of the load and so increases with velocity.

Attrition is when sediment in the load bump into each other, making the sediment smaller and rounder.

Hydraulic Action is the direct force involved from flowing water and air on the sides of the river. This leads to cavitation as pressure forces bubbles to explode and exert a force due to its own pressure. They also implode, evicting jets of water which damage rock over time.

Corrosion/solution is the removal of soluble rocks such as chalk as ions are removed. It is determined by bedrock, solute concentration, discharge and velocity.

Question 5

Factors influencing the rate of erosion:

Answer 5

Load size - heavier, more energy and sharper leads to more erosion
Velocity
Gradient
Geology - soft rocks such as sand easily eroded
pH
Human impact - deforestation, dams, bridges all interfere with natural flow.

Question 6

What are eddies?

Answer 6

Obstruction in the water can compress and decompress water in the river, causing explosive outbursts of bubbles. This can cause water to flow back upstream in an area of lowered pressure as it spirals.

Question 7

What are the 4 methods of transportation?

Answer 7

Solution: dissolved load from soluble rock such as limestone and chalk. The removal of chemical ions causes material to be dissolved into the water by corrosion.

Suspension: holding small particles such as silts and clays due to the turbulent waters. Lightest particles are the highest

Saltation: small rocks, gravels and sands are transported in a ‘hopping’ motion along the river bed. Faster eddies may pick them up and then they move until they drop again when energy falls.

Traction: rocks and pebbles are rolled along the bed by the force of the water.

Question 8

What is capacity and competence?

Answer 8

Capacity is the amount of debris in a river

Competence is the diameter of the debris in the river

Question 9

What is deposition and sedimentation? What is flocculation?

Answer 9

When a river slows down deposition occurs as it loses energy. This occurs if velocity falls, discharge falls, gradient shallows, friction rises, volume of water rises. It may also occur when there are dry spells, low discharges, high loads (during deforestation), inside of meanders, river enters sea or lake, river banks burst

Sedimentation occurs when the sediment is deposited from still water

Flocculation is when negatively charged clay ions react with the salt water which make some of them positive, causing them to floc together. This may then coagulate on obstacles such as grass or rock where it is then settled out of suspension.

Question 10

What is the Hjulstrum Curve?

Answer 10

Shows the relationship between velocity and diameter of particles. It shows the velocity required for rock to be eroded/picked up as well as the velocity required to pick up material or slow to the point of deposition.

The critical erosion is the lowest velocity at which a grain of a given size is moved.
The settling velocity shows how slow a river must be to deposit sediment.

Question 11

What are some important features of the Hjulstrom curve?

Answer 11

The order of particle size goes: clay, silt, sand, pebbles, boulders.

Smaller and largest require the highest velocities to lift. This is because clay is cohesive and gravel is too light .

The velocity need to keep moving is always greater than needed to start off.

Sand is the easiest to erode and be picked up at the lowest velocities.

Once fine clays are picked up they are always in suspension.

When a river slows the coarsest material is dropped first and the finest is dropped last.

Question 12

What is the velocity and energy of a river controlled by?

Answer 12

Gradient and friction - some highlands may have high gradient but low friction so smooth

Friction leads to turbulence which slows down the river, measured in roughness and radius.

Precipitation and characteristics of drainage basin

Volume of water in the channel

Shape of the channel

Question 13

How is bed roughness calculated?

Answer 13

This is called Manning’s law, stating that

Roughness = radius x gradient/velocity

Or N = R^0.67 x S^0.5/V

Question 14

What is the hydraulic radius?

Answer 14

Cross sectional area/wetted perimeter

Question 15

What is turbulent flow?

Answer 15

When high velocities and complex morphologies cause flow to be subject to friction with the bed and the banks. The water closest to the bank eddies and air is trapped in pores, cracks and crevices under high pressure. As the eddy swirls away, pressure is released similar to hydraulic action.

This friction slows water closest to the bed and banks and so the water in the middle overtakes the outer water. The water on the edge then starts to eddy towards the centre as a result. Eddies occur vertically and horizontally creating chaotic turbulent flow.

Vertical turbulence creates hollows in the bed and lead to potholes. Cavitation and abrasion can deepen a channel into a valley, or can lead to a gorge.

Question 16

What is laminar flow?

Answer 16

A smooth, straight surface at low velocity allows water to flow in sheets with no eddies or meanders e.g. on pavements or road surfaces. They are very rare as most surfaces have enough friction.

Question 17

What is helicoidal flow?

Answer 17

Like a plughole, water spirals at it flows downwards in a corkscrew motion. The line of fastest flow (thalweg) follows a spiralling path down a river, travelling from outside bank to outside bank of a meander, as well as up and down. The vertical movement creates pools and riffles and the horizontal creates point bars and cliffs, scraping material off one and and depositing it on the other.

In a cross section, the thalweg slows down as it reaches the bank, scraping material off which is then swooshed to the other side and deposited as a slip off slope.

Question 18

What are the 3 channel types? How is sinuosity calculated?

Answer 18

Straight - rare as helicoidal flow is in most rivers - even straight thalwegs move side to side. Sinuosity less than 1.5, have a central ridge of deposition

Meandering - sinuosity greater than 1.5, common due to constrained helicoidal flow.

Braided channels

Sinuosity is calculated by the stream length/valley length

Question 19

How are braided channels formed?

Answer 19

Braided channels contain multiple islands called bars or eyots which are made up of sediment. They are vegetated and long lived islands, or short unvegetated bars.

The conditions required for them to form are:
Steep gradients with lots of coarse material
Easily erodible bank material
Highly variable discharge

They begin with a mid channel bar downstream due to increased friction, increased load or reduced streamflow. After a flood, the discharge falls and deposits the finer sediment, forming bars.

The upstream becomes stabilised with vegetation and creates an island which localises and narrows the channel, increasing velocity to a point where it can transport the load. This creates river splits following around the deposits and meeting behind them.

ADDITIONAL WHITTLE SHYT

Braided channels – wide and shallow, unconsolidated material, variable discharges, changing gradient can cause it.
Low discharge comes down, but high sediment load of coarse material mixed with fine material. As discharge falls it drops as loose sediment bars, creating small narrow channels which speeds up channels. This spreads and slows down the flow causing more deposition behind it creating eyots, which become accreted and vegetated – more wide and shit.

Question 20

How do pools and riffles form?

Answer 20

Eddies cause deposition in the centre of a meander, where there is less velocity as there is more friction in the central section of the river, creating small ridges of material midstream called riffles. The outer edge of the flow loops downwards, vertically eroding at the sea bed and undercutting it, forming alternating pools of deeper water.

Helicoidal flow in the river means that the thalweg abrades the sides of the river and scrapes off the sediment, leaving behind deeper sections of erosion called pools from the ‘loops’ of erosion’. This erosion leads to the formation of river cliffs.

This also causes deposition of the outer banks creating slip off slopes as the material scraped off from the eroded banks returns to the centre of the river where it is slower and deposited.

Question 21

What conditions help cause meanders?

Answer 21

• Friction causes turbulence leading to helicoidal flow and bars. Water is raised on the outer surfaces and return at depth in the inner banks, causing erosion on the outside and deposition in the centre.
• Sinuosity is best on moderate gradients able to create pools and riffles.
• Helicoidal flow amplifies erosion rates.
• Variations in cross section, erosion and deposition lead to rising sinuosity
• Deep water and a river cliff is found on the outside of the bend due to erosional processes
• Deposition occurs on the inner bank, creating slip off slopes/point bars.

Question 22

How do meanders develop into ox-bow lakes?

Answer 22

1. Lateral erosion predominates as the soft alluvium becomes destroyed in the neck of the meander. Erosion continues on the outer bend and deposition on the inner bend. Helicoidal flow erodes the banks, carrying sediment to the middle forming a larger point bar.
2. Sinuosity increases, erosion undercuts the river cliff which continues to retreat. Deposition inside also continues to occur and so the neck becomes closer together due to lateral erosion.
3. During a flood the rapid movement erodes away the opposite cliffs, removing the neck between them and leaving behind the cut off oxbow lake and a direct line of flow as a result of the increased gradient of the river bed. After this, deposition cuts off the old meander due to the still water.

Question 23

How do potholes form?

Answer 23

Abrasion can occur vertically when turbulence is forced over an obstacle, forming an eddy. This then leads to abrasion directly behind it as the eddy continues to cause disturbance - although this can happen naturally in uneven beds. This can create a depression. This deepens over time due to abrasion as turbulence and small rocks get into the depression and deepen the pothole with more force, making it more circular and round

Question 24

How do v shaped valleys form?

Answer 24

Rivers naturally erode vertically in the upper course due to gravity and helicoidal flow, creating a trench. This cuts a deep notch due to hydraulic action which also laterally erodes the sides in cracks and load hitting the sides.

Weathering on the side loosen and erode the sides, giving it its V shape. These rivers flow as normal but often find harder rocks in the way which they cannot erode through, creating natural interlocking spurs as they meander around them.

Question 25

How do waterfalls form?

Answer 25

A layer of hard rock lies above soft rock in a valley, followed by soft rock. The soft rock vertically erodes faster and downwards, creating a hillside/cliff face for the river to flow down.
As this continues, this creates a plunge pool through hydraulic action and abrasion at the bottom where the water falls.

Splashing water and eddy currents from the turbulence of the falling water undercut the base off the cliff creating an unsupported overhang, due to cavitation, hydraulic action and abrasion. Mild weathering processes also occur on the cliff face causing it to become weaker and unable to sustain the weight of the overhang, causing it to collapse into the plunge pool and retreat, forming a gorge.

Question 26

Where do gorges form best?

Answer 26

Gorges form best where hard rock is susceptible to erosion but resistant to weather. They may also form in other cases:

Semi arid areas in wet seasons lead to vertical erosion but no weathering at other times of the year.

Antecedent drainage - vertical erosion keeps up with the growth of a mountain range, forming a gorge.

Retreating waterfalls

Glacial overflow channeling

Surface runoff over limestones causes vertical erosion

Question 27

What are floodplains and the features?

Answer 27

Floodplains are flat areas next to a river liable to flooding. Often they are poorly drained with swamps and marshes. They form when a river bursts its banks over a long period of time.
During floods, there is greater capacity, competence and discharge but lower velocities

Alluvial deposits: fine sediment in the load is spread across the floodplain during flooding

Levees: other sediment creates natural barriers around the river as the coarsest material travels the shortest distance and the finer travels further - sometimes creating back swamps, wet areas of sand and silt creating marsh like landscapes. As a result, small hills or levees are created at a gradient

Bluffs: over time, repeated flooding erodes the landscape as far as it can, creating bluffs either side where the old floodplain is eroded away.

Ox-bow lakes: found during floods as erosion increases, breaking the neck and deposition closes the old meander.

Question 28

What are the 3 forms of deposition at a floodplain?

Answer 28

• Silt and mud in the load is deposited in flood as velocity falls and friction rises. The most occurs closest to the river.
• Point bars occur inside meanders
• Sediment on the river bed during low water causes the level to rise.

Question 29

How do deltas form?

Answer 29

When a river meets the sea or a lake, losing velocity and energy. The river must be heavy laden and the body negligible currents. Charged ions in the sea charge the negatively charged clay particles, causing flocculation and clumping, becoming bigger and settling at the bottom due to coagulation. Other factors may be due to type of sediment, local geology, sea-level changes, plant growth and the human impact

The deposition of sediment blocks off the river, slowing it down further and splitting it into distributaries. This continues outward with deposition creating a flat, marshy extension of land.

Question 30

What are some features of the delta? What determines the character?

Answer 30

There are often lakes and lagoons within the delta - fertile alluvium deposits attractive for development however risk flooding.

There is a subaqueous section, a steep, submerged section of the first silts. It has top set, mineral rich soil

There is also a subaerial section above ground with many distributaries, fore/bottom set, matures the deltaic lobe.

Character: rate of deposition, rate of stabilisation (vegetation), tides, longshore drift, human activity

Question 31

What are the 3 layers of material in a delta?

Answer 31

The material is split into 3 types:
Bottom set - stretch out farthest to sea, laid first as suspended material sinks to the sea e.g. clay and fine material.
Fore set: coarser material rolled by traction tipped seaward. Over the bottom set bet, inclined and sloping layers are deposited, each above the previous one, moving by traction.
Top set: fine material most inland form a rich layer of alluvial deposits. Form when distributaries flood.

Question 32

What are the types of delta?

Answer 32

Arcuate deltas: fan shaped, formed due to currents or LSD which keep edges smooth

Cuspate: pointed/arrowhead due to low tidal range and offshore currents

Bird foot: low tidal range, strong currents - no defined current so each distributary builds land out to sea with it