Rivers, Floods Management Content book 2 Flashcards
What is the special type of abrasion which forms potholes?
Drilling
Describe the process of drilling
Pebbles and gravel collect in a depression on the riverbed. As water flows past, it causes the pebbles to rotate in the hollow. Overtime they drill down into the bedrock. More pebbles become trapped in the deepening hollow and the process continues.
Between what figures can potholes vary?
A few centimetres to several metres
Where are potholes found on the rivers long profile?
The upper or early middle course
What cause Rapids?
Changes in geology where the river flows over a series of harder bands of rock that dip gently downstream. As the water becomes more turbulent its erosive power increases
Where may potholes form?
Downstream of waterfalls or knickpoints
Which river is marked by Rapids as it flows through the Nubian deserts?
The Nile
Why do waterfalls occur?
Where there are clear changes in the rock type or geology of the river valley, such as where a resistant band of rock cuts across the course of the river. A waterfall may often start as Rapids.
What is an alternative reason why waterfalls may form?
Rejuvenation
What two erosive processes take place in the plunge pools of waterfalls?
Hydraulic action and abrasion
What happens to the overhang of a waterfall if too much undercutting occurs?
It collapses
What is the tough rock in the igneous intrusion which is under High Force waterfall?
Whinstone (granite)
What processes have led to the formation of High Force?
- Hydraulic action
- Abrasion
- The limestone is attacked by the acidic peaty water of the Tees
- The Weaker limestone is eroded at a faster rate
- The Whinstone is left as an overhang after being undercut
- The undercut Whinstone collapses leaving rocks in the plunge pool, which further abraded the plunge pool
- The waterfall retreat leaves a gorge
What two processes form meanders?
Erosion and deposition
In the upper course of the river, the channel bends around interlocking spurs, are these meanders?
No
What happens to the thaweg when a river is at base flow?
It zigzags down the channel between ‘bars’ of sediment on opposing sides of the channel
What alternates in rivers in meanders?
Deeper pools and shallower riffles
What happens when the thalweg swings towards the bank?
It erodes via undercutting
What does slower flow and deposition cause a meander to do?
Bend more acutely
What two erosive processes form river cliffs?
Abrasion and hydraulic action
What process forms a slip off slope?
Deposition
Why do meanders migrate?
Because the zone of greatest erosion of a meander is just downstream of the midpoint of the meander bend, causing the meander to move laterally and downstream
What is the term used for the imprints of former meanders?
Meander scars
Where can a clear example of meander migration be seen?
Rio Negro, Argentina
When the outer part of the meander bend reaches the valley side, what is created?
A bluff
What will meanders do eventually when they migrate downstream?
They will widen the valley floor, to create a wide, flat floodplain
Describe the formation of an oxbow lake
Meander migration may cause the neck of a meander to narrow. During bank-full or flood conditions, the neck can be breached and the river will cut a shorter, straighter course through it. Over time the meander will be cut off through deposition and an oxbow lake will be formed. The oxbow lake will then eventually dry up, forming a meander scar.
What is deposited when floodwaters spill onto a floodplain?
Alluvium
Flood waters are shallow. This means they have an extensive …. meaning they have more ….. and are …..
- Wetted perimeters
- Friction
- Lower velocities
Successive floods deposit more layers of alluvium, what two things does this increase?
Fertility and height
Which river has a ‘classic floodplain’?
The River Till, Northumberland
What is reduced suddenly after water leaves the river channel and what happens as a result of this?
Competence and its heaviest and coarsest load is deposited first, finer material is carried further from the channel
What can levees act as?
Natural flood defences
Where have the authorities, strengthened and heightened levees to act as flood defences?
Mississippi River, USA
What causes rivers to braid and where do they braid?
Braiding is caused by variable discharge, this occurs in the upper course of rivers
Name some examples of braided rivers
Yellow river, China and the Franz Jospeh river, New Zealand
What is the Franz Joseph river fed by?
Melt waters from the Franz Joseph glacier
Why is the discharge of the Franz Joseph river highly variable?
Because there’s more meltwater during the day time than there is at night due to there being more melting during the day. Also there’s seasonal discharge differences with a much higher discharge during the summer months
When the discharge is high, a lot of what material is carried downstream in the Franz Joseph river?
Glacially eroded material
When the Franz Joseph river’s discharge falls or when it spreads out laterally, what lowers?
The river’s hydraulic radius
The sediment deposited in braided rivers can build up to form…
Eyots that split the river into multiple channels
Sometimes what can happen to eyots?
They can become colonised by plants or stabilised by vegetation, but more often than not they are eroded
The shape of a braided river is…..
Constantly changing
Where do river deltas form?
At the moth of a river or on its approach to a lake
What type of landform are deltas?
Depositional
What conditions are required for deltas to form?
The amount of sediment supply is greater than the rates at which it is removed
What must a river be carrying for a delta to form?
A large sediment load
How much sediment does the Mississippi carry into its delta distributaries each year?
450 million tonnes
What rate must the material be deposited at for a delta to be formed?
At a faster rate than it is being removed by the tide, waves and currents
What conditions do deltas usually form in?
Calm seas which have a gently sloping seabed- most are tideless or almost tideless
When the river meets the sea it spreads out laterally, what four factors fall, when a river meets the sea?
- Velocity
- Capacity
- Competence
- Hydraulic radius
What increases when a river meets the sea?
The wetted perimeter
What process involving clay particles occurs when a river meets the ocean?
Flocculation, where electrical charges cause clay particles to clump together and they therefore sink
The sediment can then cause the channel to split and small islands to form, what is this similar to?
Braids streams
What are the smaller streams, formed by a delta called?
Distributaries
There are usually 3 types of sediment deposit in deltas, what are they?
Topset beds, forset beds, bottomset beds
What are topset beds?
Made up of larger bed load that is deposited first
What are fore-set beds?
They consist of middle-sized sediments and are therefore transported a little further
What are bottom-set beds?
The finest sediments carried by a river and so travel the furthest before low velocity and/or flocculation causes them to be deposited
What is an arcuate delta?
They’re the most common type of delta. They have a gentle sloping shoreline and a distinct pattern of branching distributaries. They tend to be dominated by coarser material
Give an example of an arcuate delta
The Nile delta
What is a braids foot delta?
They are dominated by an extruding finger like branch of deposition. They have fewer distributaries and fine sediments. They are formed due to weaker ocean processes and high discharge
What is an example of a birds foot delta?
The Mississippi delta
What two processes can bring about rejuvenation?
Isostatic rebound and eustatic change
What is isostatic rebound?
This occurs after an ice sheet has been located on a tectonic plate. The plate rises back up once the ice sheet has melted causing a relative fall in sea level. This affects areas locally
What is eustatic change?
The lowering of sea level, during an ice age, caused by ice accumulating on land and so removing water from the oceans. This occurs globally
What does a knickpoint mark?
Where the old long profile joins the new long profile.
What will a river do once it has been rejuvenated?
It will adjust to its new base level, first in the lower course, then moving progressively upstream
What do river terraces create?
Steps in the valley cross profile
What are paired terraces?
They are on the same level on each side of the river and so indicate rapid downcutting, with little lateral erosion
What are unpaired terraces?
These occur when the fall in base level is slower, the terraces will be present on different sides of the channel, at different levels. This is because of lateral erosion through meander migration (so mainly hydraulic action and abrasion).
What are the two types of incised meanders?
Entrenched meanders and ingrown meanders
What are entrenched meanders?
These are caused by rapid vertical erosion or when the valley sides are more resistant to erosion, creating a winding gorge. This can happen when uplift is more rapid and results in a symmetrically shaped cross-profile
Give an example of a river which has entrenched meanders
The River Wear Durham
What are ingrown meanders?
These occur when vertical erosion is slower and draw out over a longer period of time. In this case the meander has time to erode both vertically and laterally to form an asymmetrical river valley with steep cliffs on the outside bend
Give an example of a river with ingrown meanders
The River Wye, Tinton Abbey
