Fluvial Landforms produced from deposition *AS and some IG courses*

Fluvial Landforms  and resultant effects produced from deposition

Thalweg: (this is not a landform but has to do with erosion and deposition)This is the line of fastest flow in a stream and is usually exaggerated variation of the stream channel shape that crosses to the outside of each meander at the point of inflection. Because erosion is greatest where the stream flow is fastest, the thalweg is also the deepest channel in the stream. It is found in the top middle of a straight channel because this is where the water is the deepest and is where there is the least friction.

Riffle and pool sequence: River channels have irregularities in the bed, which cause the thalweg to shift from the middle. These are known as ‘pools’ and ‘riffles’. In a flowing stream, a riffle-pool sequence (also known as a pool-riffle sequence) develops as a stream's hydrological flow structure alternates from areas of relatively shallow to deeper water. This sequence is present only in streams carrying gravel or coarser sediments. Riffles are formed in shallow areas (the shallow points of inflection) by coarser materials such as gravel deposits on river with a turbulent flow with a lower velocity. Pools are deeper and calmer areas of laminar flows with higher velocities, whose bed load (in general) is made up of finer material such as silt. Streams with only sand or silt-laden beds do not develop the feature. The sequence within a streambed commonly occurs at intervals of from 5 to 7 stream widths. Meandering streams with relatively coarse bed load tend to develop a riffle-pool sequence with pools in the outsides of the bends and riffles in the crossovers between one meander to the next on the opposite side of the stream. The pools are areas of greater erosion where the available energy in the river builds up due to a reduction in friction. The material eroded tends to be deposited in the riffle area between pools as energy is dissipated across the riffle area. Pools and riffles are responsible for the initiation of a meander. The pools are areas of high velocity and the thalweg is fast in a pool. Its energy is reduced and diffused (spread out) as it crosses the riffles. This is because the water is shallower; the bed is covered with bed load, is rough and creates turbulent flow. Therefore in order to overcome, these obstacles the river uses up more energy become slower.

Point bars: On a meander, material deposited on the convex inside of the bend may take the form of a curving point bar. Material is deposited here where velocity is at its lowest round a bend.

 Ox Bow lakes: Continual erosion on the outside bends, results in the neck of the meander getting narrower until the river undercuts through the neck and shortens the coarse. The current will take the path of least resistance, giving it renewed energy. The faster current will now be flowing in the centre of the channel and deposition is more likely next to the banks. The original meander will now be blocked off to leave a crescent shaped ox bow lake with a meander core in the centre. The lake will slowly dry up except during heavy rain. This lake can also be become filled with alluvium over time (marshland).

Flood Plains:

It’s a flat wide expanse of alluvium covering the valley floor formed due to deposition when the river is in overbankful. As the river floods, the river slows down, loses energy and consequently deposits its large (capacity) load of small material (competence) usually silt (alluvium). Rivers have the most energy at their bank full stage. Should the river continue to rise, and then the water will cover any adjacent flat land. The land susceptible to flooding in this way is known as the floodplain. As the river spreads over its floodplain, there will be a sudden increase in both the wetted perimeter and the hydraulic radius. These results in an increase in friction, a corresponding decrease in velocity and the deposition of material (alluvium) previously held in suspension. The thin veneer of silt, deposited each flood, increases the richness of the soil, while each successive flood causes the floodplain to increase in height. The floodplain may also be made up of material deposited as point bars on the inside of meanders and can be widened by the lateral erosion of the meanders. Prominent slopes known as bluff lines often mark the edge of the flood plain. These bluff lines can change as the flood plains become wider and more sinuous as they migrate downstream – which in turn widens the valley. 

Levees: When a river overflows its banks, the increase in friction produced by the contact with the floodplain causes material to be deposited. The coarsest material is dropped first to form a small, natural embankment (levee) alongside the channel. During subsequent periods of low discharge, further deposition will occur within the main channel causing the bed of the river to rise and the risk of flooding to increase. To try to contain the river, the embankments are sometimes artificially strengthened and heightened. Some rivers flow above their floodplains so if levees increase the river can cause serious damage to properties.

River Terraces: They are the remnants of former floodplains which, following vertical erosion caused by rejuvenation, have been high and dry above the maximum level of present day flood plains. If a river cuts rapidly into its floodplain, a pair of terraces of equal height may be seen flanking the river and creating a valley-in-valley feature. However, more often than not, the river cuts down relatively slowly, enabling it to meander at the same time. The result is that the terrace to one side of the river may be removed as the meanders migrate downstream. If the uplift of land continues, the river may cut downwards to form incised meanders. There are two types of incised meanders. Entrenched meanders have a symmetrical cross-section and a result from either a very rapid incision by the river, or valley sides being resistant to erosion. Ingrown meanders occur when the uplift of the land, or incision by the river, is less rapid, allowing the river time to shift laterally and to produce an asymmetrical cross-valley shape. As with meanders in the lower course, incised meanders can also change their channels to leave an abandoned meander with a central meander core.

Deltas: Deltas are usually composed of fine sediment, which is deposited when a river loses energy and competence as it flows into an area of slow-moving water such as a lake or the sea. When the river meets the sea the meeting produces an electric charge, which causes clay particles to coagulate and to settle on the seabed, a process called flocculation (larger coagulated particles carried out into the shallow water offshore and deposited, and the river loses energy on meeting the sea water). The water flows into a delta via distributaries. They are usually highly populated, not very navigable and have a great risk of flooding. Crops are usually grown on these deltas and are usually staple crops e.g. Rice. Deltas are named after the fourth letter in the Greek alphabet (∆). Yet Deltas range in geomorphology into three main types:

·      Arcuate: (Wave dominant) Having rounded, convex outer margins. They also have smooth coastlines and have well developed beaches/ sand dunes. Lagoons form near coastal areas e.g. the Nile Delta.

·      Cuspate: (Tide dominant) Where material brought down by a river is spread out evenly on either side of the channel. It is tide dominant and is covered by the high tide and left dry at low tide e.g. the Bangladesh Delta.

·      Bird’s foot: (River Dominant) Where the river has many distributaries bounded by sediment and which extend out to sea like the claws of a bird’s foot. The river has a large load from a huge drainage basin, a low energy river into the Gulf of Mexico, and a small tidal range e.g. Mississippi Delta.

Alluvial Fans: In order to form, alluvial fans require a flat or a gently sloping plain near the foot of a hill or plateau, where a stream carrying sediment emerges abruptly from a mountain front and spreads out. As the stream reaches the flat plan, known as the piedmont, its velocity slows and it loses competence to carry sediment load. The coarse sediment is therefore deposited at the junction of the hill and the piedmont, and a fan-shaped deposit builds up. Arid environments are well suited to alluvial fan development because they are prone to flash flooding. Furthermore, they have hill slopes that erode easily and therefore provide alluvial material suitable for deposition. Alluvial channels are considered disconnected from the channel.