1.3 Hydrology and fluvial geomorphology: river channel processes and landforms

Question 1

Describe and explain abrasion/corrasion

Answer 1

Abrasion/corrasion takes place when coarse and angular rock fragments are dragged and bounced along the channel bed and sides, scraping and removing fragments of rock from the channel floor and sides.

Some of the bedload may become trapped in depressions in the bed of the channel and spun round by the current. This often results in the process of eversion which involves the formation of potholes which may enlarge and merge and contribute to the deepening/lowering of the channel bed.

Question 2

Describe and explain solution/corrosion

Answer 2

Solution (corrosion) takes place when the acids in river water put certain minerals in solution. It is particularly effective in calcareous rocks such as limestone and chalk.

Question 3

Describe and explain cavitation

Answer 3

Cavitation occurs during turbulent flow when the water is full of air bubbles – such as in the plunge pools below waterfalls. These air bubbles implode and create small cavities in the rock of the channel bed and sides.

Question 4

Describe and explain hydraulic action

Answer 4

Hydraulic action occurs where the weight and force of the river flow penetrates cracks, joints, and bedding planes in the channel bed and sides, leading to the erosion of bed rock and the undercutting of the sides of the channel.

Question 5

What are the factors affecting the rates of erosion

Answer 5

Factors affecting rates of erosion:
Load: the heavier and sharper the load the greater the potential for erosion
Velocity: the greater the velocity the greater the potential for erosion
Gradient: increased gradient increases the rate of erosion
Geology: soft, unconsolidated rocks such as sand and gravel are easily eroded
pH: rates of solution are increased when the water is more acidic
Human impact: deforestation, dams, and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion

Question 6

Describe traction

Answer 6

Traction is the rolling/sliding/pushing of larger, heavier sediment, such as pebbles, along the river bed by the river flow.

Question 7

Describe saltation

Answer 7

Saltation is the bouncing along the river bed of small particles light enough to be picked up/entrained for only a short distance.

Question 8

Describe suspension

Answer 8

Suspension is the transport of particles light enough to be carried/suspended above the river bed by the flow of the river.

Question 9

Describe solution

Answer 9

Solution is the transport of dissolved sediments in solution that are the products of solution weathering.

Question 10

Define deposition

Answer 10

Deposition is the geological process in which sediments, soil, and rocks are added to a landform of landmass (check this definition)

Question 11

What are the causes of deposition

Answer 11

There are a number of causes of deposition, such as:
- A shallowing of gradient, which decreases velocity and energy
- A decrease in the volume of water in the channel
- An increase in the friction between water and channel

Question 12

Define sedimentation

Answer 12

Sedimentation is generally described as the opposite process of erosion that is the terminal end were the sediment transport (check this definition)

Question 13

Describe and understand where and when erosion, transportation and deposition operate (knowledge of the hjulstrom curve)

Answer 13

The transport of sediment in a river channel depends on:
- The speed/velocity of river flow
- The load particle size
It is sometimes expressed as a measure of a river or stream’s competence.

The Hjulstrom curve represents this in the form of a diagram which displays the river channel flow velocities required for the sediment particles of different sizes to be picked up (entrained) and transported. This will take place whenever the river velocity is great enough to erode or entrain sediment. This could occur in periods of high discharge or where river channels long gradient is steep. It is usually combined with a deposition curve so that it is possible to see the speeds at which material will be deposited. It allows an estimate of the balance between the three river functions of erosion, transportation, and deposition.

The different velocities illustrate the speeds required to erode, transport, and deposit particles of a given size. Larger particles and fine sized particles are, surprisingly, both entrained at high velocities (surprisingly because it would seem logical that the very small clay particles would be the first particles to be picked up).
The reason for this is that the tiny clay particles actually “stick” together as a result of weak electrical bonding. At low velocities sediment settles and is deposited. For coarse particles this is just below the erosion velocity. Clay particles settle at very low velocities.

Question 14

Define river velocity

Answer 14

River velocity is the mean speed of the river flow in metres per second measured at a point in a river by a current meter or over a set distance along the channel (often 10 metres) and time

Question 15

What are the factors that control the velocity of a river channel

Answer 15

The gradient of the channel bed, ie the gradient of the long profile

The volume of water in the channel

The shape of the river channel

The channel roughness, is a measure of how rough or smooth the channel bed and sides are, the amount of friction acting on the water to slow it down and how fast the river channel allows water to flow through its. The Manning flow equation (sometimes called Manning’s “n”) is a method by which channel roughness can be measured using fieldwork data.

Question 16

Define river discharge

Answer 16

River discharge is the volume of water passing a point in a given unit of time, which is calculated by the equation Q = A x V, where:
Q = Discharge, in cubic metres per second (Cumecs)
A = Channel cross sectional area, in square metres
V = Velocity, in metres per second

Question 17

Describe laminar flow

Answer 17

Laminar flow is found in a smooth, shallow, straight channel with a slow velocity. The water flows in layers/sheets sliding over one another parallel to the river bed. It is usually found in the lower section of a river’s course

Question 18

Describe turbulent flow

Answer 18

Turbulent flow is found in rough, shallow channels which have complex and rapid changes in shape, along with an increased velocity. This produces a series of spinning eddies which mix with one another. They often trap air bubbles inside them producing a “white water” effect. It is in this type of channel that cavitation takes place, as all the air bubbles are constantly imploding.

Question 19

Describe helicoidal flow

Answer 19

Helicoidal flow occurs where the river flow takes on a corkscrew shape and is found in water that travels around the bend of a meander. The flow, while still moving downstream, also moves across the top of the water surface of a meander from the inside bank towards the outside bank where it then drops/descends towards the bed of the channel and returns towards the inside bank as a reverse flow

Question 20

Explain what the thalweg is

Answer 20

The thalweg is the name given to the line of both maximum depth and maximum speed/velocity found along a river channel. It tends to swing across a river channel and is found on the outside of meaner bends, where it causes the outside the bank to be undercut and collapse, forming a river cliff.

Question 21

Need to distinguish between a description of the flow and the causes of it***

Question 22

Draw and describe a straight channel

Answer 22

These are rare in nature and will not last for long – rarely for longer than 10 times their channel width. For example, they may follow the route of a fault line. They are found when rivers have:
- Low energy
- Small discharges
- Gentle gradients

Question 23

Draw and describe a meandering channel

Answer 23

Meandering channels are a much more common form; about 80% of channels are meandering.
Meanders are characterised by a winding shape in their plan view and have an asymmetrical cross profile. Meanders are produced when the line of fastest and deepest flow – the thalweg – starts to follow a winding/sinuous path through alternating sections of deep and shallow water called pools and riffles. Meanders form when there is: moderate gradient, moderate flow and the channel sides and bed are formed in alluvium.
When the thalweg runs past the outside of the river bank, the more powerful river current erodes and undercuts the outer bank, which will eventually collapse and the process will be continually repeated, increasing the size of the meander.
It also produces a secondary movement/flow called helical (or helicoidal) flow which is a spiral shape flow which piles up/raises/elevates the water on the outside of the meander and produces a return/counter current which moves across to the inside of the meander. In a meander cross profile/section the features include point bars/slip off slopes on the inside bend where deposition occurs as the current slackens.
On the outside bends undercutting and collapse of the river channel banks takes place by the faster current (following the line of deepest flow – the thalweg) which will produce river cliffs/bluffs). Lateral/sideways erosion of the river channel is then taking place. This lateral movement is called meander migration and it is responsible for widening the floodplain.

Question 24

Draw and describe a braided channel

Answer 24

Braided channels consist of two or more channels in an interlocking pattern. They are divided by low bars and islands made up of sands and gravels which have been deposited by the river.
Braided river channels are found in rivers that have extremes of river discharge, which are sometimes called “unstable flow regimes”. In periods of high discharge these rivers pick up and transport very large amounts of sediment (ie they have large bed loads and incoherent, easily eroded banks). Sand and gravel bars and islands are formed in periods of low discharge. Two conditions often cause this reduction in river discharge:
1) A reduction in the gradient of the stream
2) The reduction of discharge after precipitation or the spring melting of snow and ice.
Vegetation may quickly colonise these island sin the braided channels which will help trap more sediment and increase the height of the eyot. This vegetation may become quite permanent and well established with trees and only then the eyot will only be covered briefly by water during very high discharge.

Braided channels occur where there are:
- Abundant amounts of bedload
- Easily eroded banks
- High and variable discharges
- Steep long gradients/channel slopes
Such conditions are most often found in tropical climates, deserts, and in arctic tundra climates.

Question 25

How is a river cliff formed

Answer 25

Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This faster flow causes lateral erosion on the outside bank which results in the undercutting of the river bank and the formation of a steep sided river cliff.

Question 26

How are point bars formed

Answer 26

On the inner bend water is slow flowing. As a result, deposition occurs resulting in the inside of the meander becoming a shallower channel. This increases the amount of friction acting on the river flow, which further reduces the speed/velocity and further reduces the river’s energy which encourages further deposition. Over time, a small crescent shaped area of material builds up on the inner bend; this is called a point bar or slip-off slope.

Question 27

How are oxbow lakes formed

Answer 27

Oxbow lakes are the result of both erosion and deposition. Lateral erosion, caused by helicoidal flow, is concentrated on the outer, deeper bank of a meander. During times of flooding, erosion increases. The river breaks through and creates a new steeper channel. In time, the old meander is closed off by deposition to form an ox-bow lake.

Question 28

What is a riffle pool sequence

Answer 28

River channels often have alternating shallow (riffle) and deep (pool) sections.

o A riffle is an accumulation of river channel sediment – called fluvial sediment – which forms a bar. It is caused by the deposition of sand and gravel on the bed of a river channel causing the bed to be shallow and rough over which water flows in a turbulent flow. The space between the riffles is about 6 times the channel width.
o A pool is a deeper area within a river channel in which water flows in a calmer, smoother, laminar flow.
- Riffles and pools cause the river current to swing from side to side in its channel and therefore encourage the development of a meandering river channel. The deeper pools tend to be found where erosion is greatest, such as on the outside of developing meander bends, while the shallower riffles are regularly spaced along the straighter part of the channel and are separated by the deeper pools.

Question 29

What are the ways in which a waterfall forms

Answer 29

The smooth long profile of a typical river is often interrupted in several places by vertical breaks which produce waterfalls. They vary enormously in size. There are three common ways for a waterfall to form in a river channel:
1) By a layer of resistant rock passing across the bed of a river channel – causing differential erosion.
2) By a drop in base level (caused by the rejuvenation of the river channel often as a result of a drop in seal level).
3) By an earth movement – often triggered by an earthquake producing a fault line and possibly causing a drop in the river bed.

Question 30

How does a waterfall and gorge form

Answer 30

Typical features found in a waterfall formed by a change in rock type: as the water falls over its edge into a plunge pool, it undercuts the top of the waterfall. The less resistant rock underneath will be eroded backwards, by hydraulic action and cavitation, as the falling water literally explodes into the plunge pool (very turbulent). Eventually, the overhanging rock collapses and the waterfall retreats upstream. This leaves behind a narrow, steep sided valley, known as a gorge. Gorges are narrow, steep sided valleys and can also be formed in other ways:
- By the collapse of an underground limestone cavern
- In arid areas where there is little erosion of the valley sides
- In arctic climates where tundra areas are often frozen for most of the year; during the short hot summers, tundra areas experience rivers with huge discharges and great erosive power, which produce deep gorges.
When a waterfall retreats the waterfall often becomes smaller in size until it almost disappears producing a steeper, more turbulent sections of river profile, often with a rough bed where outcrops of resistant rock are found; these are called rapids.

Question 31

What is a bluff

Answer 31

Floodplains are edged by bluffs where they meet the valley sides. The junction where the valley slopes meet the edge of the floodplain is known as the bluff line.

Question 32

What is a levee

Answer 32

A levee is an elevated bank lying parallel to the river on the edge of the floodplain, where it reaches the river channel. It lies slightly above the level of the rest of the floodplain. Levees are formed during flooding by the deposition of heavier sediments as floodwater comes out of t he river channel and starts to flow across the floodplain. Finer material, such as silts and clays, will be transported further away from the channel and deposited on the floodplain. Repeated flooding will cause layers of coarse alluvium to be built up into the levees.

Question 33

What is a floodplain and how do they form

Answer 33

Floodplain: Floodplains are that part of the catchment that flood when over bankfull conditions occur. They are usually located in the lower parts of the river valley profile and are characterised by having an almost flat, low relief with very gentle gradients. Floodplains are edged by bluffs where they meet the valley sides. The junction where the valley slopes meet the edge of the floodplain is known as the bluff line.
When the river reaches bankfull level, any water that is added to the river will then become overbank flow and floods over the surrounding land. It will quickly lose energy and start to deposit its load.
Regular flooding will result in the building up of layers of nutrient rich alluvium (silt) which forms a flat and fertile floodplain.

Question 34

What are the types of delta

Answer 34

Arcuate
Digital (or bird’s foot)
Cuspate

Question 35

What is a delta

Answer 35

A delta is a fan-shaped feature that will form when vast quantities of sediment are deposited at the mouth of the river when it reaches the sea or a lake.

Question 36

How does a delta form

Answer 36

Deltas are deposited where a slowing of the river speed causes a decrease in its ability to transport its load – its competence – so that its load is deposited. This can occur where velocity i s reduced as a river enters a large stationary body of water such as a lake, lagoon, sea, or ocean. The deposition of the river’s load is increased when it enters salt water because of the process of flocculation This is a process which causes the salts in sea water to join with the fine clays brought down by the river in a chemical reaction. The fine clay particles join together to form larger heavier particles which will then fall to the sea bed.

The material deposited as a delta can be divide into three types:
- Bottomset beds – the lower parts of the delta are built outwards along the sea floor by turbidity currents (currents of water loaded with material). These beds are composed of very fine material).
- Foreset beds – over the bottom set beds, inclined/sloping layers of coarse material are deposited. Each bed it deposited above and in front of the previous one, the material moving by rolling and saltation. Thus the delta is built seaward.
- Topset beds – composed of fine material, they are really part of the continuation of the river’s floodplain. These topset beds are extended and built up by the work of numerous distributaries (where the main river has split into several smaller channels).

Question 37

What is an arcuate delta

Answer 37

Arcuate – these have many distributaries, they are fan shaped and build out radially in a uniform shape. These are found in areas where longshore drift or other currents keep the seaward edge pf the delta trimmed and relatively smooth in shape. Eg Nile and Rhone deltas

Question 38

What is a digital or bird’s foot delta

Answer 38

These are found where the river has vast amounts of load and splits into two or more channels which deposit their load along their sides and extend out into the sea or lake. Where the river brings down enormous amounts of fine silt, deposition can occur in a still sea area, along the edges of the distributaries for a very long distance. Eg Mississippi delta

Question 39

What is a cuspate delta

Answer 39

These form a pointed delta where there is a dominant channel shaped by tidal and longshore currents. They are shaped by regular but opposing gentle water movement. Eg Ebro and Tiber deltas