- Created by: charlia
- Created on: 14-04-15 17:24
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. When a river meets a stationary body of water, its velocity falls causing any material being transported by the river to be deposited.
The bottomset bed - mainly made up of clay because clay & salt particles clump together (flocculate) due to an electrostatic charge developing between the particles. This 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.
The foreset bed - composed of coarser sediments that are deposited due to a fall in the river’s velocity and aren’t transported very far into the water that the river flows into. It is dipped towards deep water in the direction that the river is flowing in.
The topset bed - composed of coarse sediment but, unlike the foreset bed, the topset bed doesn’t dip, it’s horizontally bedded.
Arcuate deltas (The Nile Delta, Egypt) are shaped like a triangle and form when a river meets a sea with alternating current directions that shape the delta so that it looks like a triangle.
Cuspate deltas (Ebro Delta, Spain) 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.
Bird’s foot deltas (Mississippi Delta) They extend quite far into a body of water and form when the river’s current is stronger than the sea’s waves. Uncommon because there are very few areas where a sea’s waves are weaker than a river’s current.
Meanders are bends in a river that form as a river’s sinuosity increases. The sinuosity of a river is a measurement of how much a river varies from a straight line.
Meanders develop when alternating riffles & pools form along a river channel. A riffle is a a shallow section of a channel while a pool is a deep section.
In a pool, the channel is more efficient while at a riffle, the channel is less efficient. This causes the flow of the river to become irregular and the maximum flow is concentrated on one side of the river. This increases erosion on one side of the river and increases deposition on the other causing the river’s channel to appear to bend. Erosion is greatest on the outside bend and deposition is greatest on the inside bend. Example of a meander: Amazon River.
Potholes are cylindrical holes drilled into the bed of a river that vary in depth & diameter from a few centimetres to several metres. They’re found in the upper course of a river where it has enough potential energy to erode vertically and its flow is turbulent. In the upper course of a river, its load is large and mainly transported by traction along the river bed. When flowing water encounters bedload, it is forced over it and downcuts behind the bedload in swirling eddie currents. These currents erode the river’s bed and create small depressions in it.
As these depressions deepen, pebbles can become trapped in them. As a result of the eddie currents, the pebbles drill into the depressions making them more circular, wider & deeper. Pebbles will only be able to erode a river’s bed though if the rock the pebble’s made of is stronger than the rock the river bed is made of.
Example: River Clyde, Scotland
Found in the upper course, formed when water flows over a band of hard rock followed by a band of soft rock.
Waterfalls develop when a change of lithology takes place along the river’s course resulting in differential erosion. When the rock type of the river’s channel changes from a resistant rock to a less resistant one (e.g. granite to limestone), the river erodes the less resistant rock faster producing a sudden drop in the gradient of the river with the resistant rock being higher up than the less resistant rock. As the river flows over the resistant rock, it falls onto the less resistant rock, eroding it and creating a greater height difference between the two rock types, producing the waterfall.
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 and it eventually collapses into the plunge pool causing the waterfall to retreat upstream. Over thousands of years this will produce a gorge or recession.
Example: High Force in upper teesdale.
Rapids are stretches of fast-flowing water tumbling over a rocky-shallow riverbed. They are caused by different resistance among various rocks, that leads to sudden drops and rises in the river bed. That in turn causes instabilities in the flow of a rivers currents. They are formed when the water goes from one hard rock that resists the water's erosion to a softer rock that is easier eroded. The debris formed by the erosion breaks up the flow of the river, but are not big enough to form a waterfall. Over time, rapids are formed.
Example: River Inver
Oxbow lakes are an evolution of meanders that undergo extensive deposition and erosion.
As can be seen in the formation of meanders 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 follow 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.
Example: amazon river
A braided stream is a type of stream made up of many small interconnecting channels separated by small islands of deposited material
Braiding is caused by fluctiations in discharge levels and low river velocity which leads to the river losing both capacity and competence
As the capacity and competence fall, the amount of deposition increases which forms the eyots and causes the stream to divide into a series of smaller channels
Conditions associated with braided channels include a high stream gradient, a large supply of sediment and rapid/frequent variations in river discharge
Often found in streams or rivers with very variable discharges for example in semi-arid environments or Glacier fed stream e.g. the Copper River in Alaska
Levees are natural, raised embankments formed as a river overflows its banks.
During a flood, material is deposited across the whole flood plain as the river loses velocity and energy due to increased friction
The heaviest material (e.g. sand/gravel) is dropped first, closest to the river channel. Over time, this material builds up on the river bank, creating a levee.
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. Levees increase the height of the river’s channel though, so the bankfull discharge is increased and it becomes more difficult for the river to flood.
Created as a result of both erosion and deposition - predominantly depositional features.
Floodplains are 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 (river deposited silts and clays). 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.