Changes in Channel Characteristics

The velocity of a river increases as you go from source to mouth. This is because rivers at the mouth have highest hydraulic radius and greatest cross sectional area. Hydraulic radius is a measure of efficiency – the larger the hydraulic radius, the greater the efficiency. The more efficient a channel is, the higher its velocity and discharge. This is because with a large hydraulic radius (cross sectional area / wetted perimeter) a smaller proportion of the water is in contact with the bed and banks. This means that friction is lower, so less energy is lost. As we move towards the mouth, the channel bed roughness decreases; this means that there is much less friction, so less energy loss and a greater velocity. In the upper course, lots of boulders increase the wetted

perimeter and so the hydraulic radius is smaller, meaning more energy is lost through friction. Discharge increases downstream because more tributaries and surface run off joins the main channel.

Landforms of Fluvial ErosionWaterfalls eg Thornton Force, River TwissA waterfall is a place where flowing water rapidly drops in elevation as it flows over a steep region or a cliff. Waterfalls are found in a river’s upper course, or in a rejuvenated lower course, where the river has large amounts of potential energy. Waterfalls are often ‘knick points’ marking a change in the bed gradient due to differing rates of erosion. A waterfall can be formed where:a) A river has been rejuvenated and therefore has new erosion powerb) The river flows over the edge of a hard cap rock with a softer rock underneathc) The river flows over alternating bands of hard and soft rockFormation of a waterfall: Water flows over hard limestone cap rock. Below this, there is a softer slate rock, known as greywacke. The planes of parting are in the same orientation as the flow of the water, and so this is eroded more easily. A plunge pool is created by hydraulic action, and undercutting of the soft rock eventually causes the overhang to collapse. The fallen rock can be used to erode the plunge pool further by abrasion, and also creating potholes. The waterfall retreats upstream, headward erosion, leaving a gorge.

PotholesPotholes are found in the upper and middle courses of a river, where the channel is high above base level and therefore has lots of potential energy. The bed load is angular and the flow turbulent due to high velocities. Pebbles become trapped in hollows and due to strong vertical eddies, the load drills holes in the rock through the process of abrasion. The edges are smoothed by the action of the swirling, eddying water.

RapidsRapids are areas along the rivers course where water becomes more turbulent often creating white water. It is caused by a localised increase in gradient along the rivers gradient or where the river flows over alternating bands of harder and softer rocks. These are often linked in with pool and riffle sequences (the rapids form the riffles). The pools are areas of deeper water whereas the riffles are areas of shallower water. The pool is an area of greater erosion as the water is deeper and therefore flows faster, whereas the riffles encourage deposition because they are shallower. Landforms of Fluvial Deposition Braiding

Braiding is when load is deposited in the middle of a channel, forcing the water to diverge into a number of distributaries which seek to find the easiest route past the obstructing sediment. Braiding occurs in areas where there are frequent significant changes in discharge;Glacial streams have a high discharge in spring due to melt water. This increases the river’s competence, and so more

load can be transported. However, as temperatures decrease and there is no longer melt water adding to the river’s discharge, the river is no longer competent for the transportation of the load and so deposits it as braids in the middle of the channel. Braided channels occur where the gradient is steep, there is a large sediment load and the river discharge fluctuates daily or annually. Braids are mobile landforms; they change their positions rapidly.

LeveesA levee is a natural raised embankment formed as a river overflows its banks. Larger material is deposited by the river closest to the channel; this is the heaviest load and so is dropped first. This heavier load settles first while the finer material travels further over the floodplain. Over time, these deposits build up, creating the natural embankments.

Flood plains

A flood plain is an area at the side of a river channel, built up from alluvial deposition. Flood plains develop when rivers at bank full spill over onto the surrounding land. This results in a sudden increase in the wetted perimeter (the area of land with which the river is in contact), and so a decrease in hydraulic radius (a measure of the efficiency of the channel). These factors increase the amount of friction that the water must

flow against, which decreases the water’s energy. It is no longer competent to carry its load, and so any material that is being carried in suspension is deposited. Over time, the layers of deposition build up to create the floodplain. A flood plain can be widened by meander migration. As the river becomes more sinuous, as more erosion and deposition occurs, the flood plain is extended as eventually areas of high land (such as interlocking spurs) are removed.

Deltas

A delta is a flat, low lying area of land at sea level. Deltas are found at the mouth of a river, where it reaches the sea or a lake. The velocity of the river is greatly reduced, and so its competence and capacity decreases. The wetted perimeter increases so the hydraulic radius decreases, making the river less efficient. The bed load and

material being carried in suspension is deposited. The topset beds are the large and heavy particles which are deposited first. The foreset beds are the medium particles, deposited as angled wedges. The bottom set beds are the finest particles which travel furthest into the sea or lake before being deposited. If sediment is coarse grained arcuate deltas form. If it is fine grained then bird’s foot deltas can be created (e.g. Mississippi).

Conditions necessary for formation: Conditions necessary for formation: Large sediment load of riverWeak sea currents that are incapable of removing sediment after its deposition and which have a small tidal range.

ARCUATE DELTA CUSPATE DELTA BIRD’S FOOT DELTA

Landforms of fluvial erosion and deposition Meanders A meander is a large, sweeping curve in a river’s lower and middle course. They are formed by a combination of erosion and deposition.

Ox Box Lakes

An oxbow lake is formed when the neck of a meander is broken through and deposition dams off to the loop, leaving a lake. As rivers become more sinuous, more erosion occurs at the outer bank, and more deposition at the inner bank, meaning that the meander neck gets thinner and thinner. At times of high discharge, the river has enough energy to erode through the meander neck, giving the channel a straighter, shorter route downstream. Deposition of a point bar seals off the meander loop, creating a cut off loop. This can be colonised by vegetation, and eventually, if all of the water evaporates, it will become land once more, leaving a ‘meander scar’ on the landscape.

Slow moving water loses energy due to friction with he bed and banks. Deposition occurs because the river no longer has enough energy to carry its load.

Faster moving water on the outer bank caused by centrifugal force makes erosion occur. Water has more energy and erodes the bed and banks through processes of lateral hydraulic action and abrasion.

Meanders form where alternating areas of deep water (pools) and shallow water (riffles) are evenly spaced along a stretch of river. Where the water is deeper, there is a larger hydraulic radius, meaning that there is less friction and so more energy and a greater erosive power. At riffles, there is more friction, so a loss of energy occurs. The fastest flow (“Thalweg”) becomes uneven, and goes from one side of a river to the other. Turbulence increases around pools as the water speeds up, causing corkscrew like currents, called helicoidal flow, to spiral from bank to bank between pools. The helicoidal flow causes the material eroded at the outer bank to be deposited at the inner bank, creating the asymmetric cross section. Continual undercutting of the outside bank river cliff and deposition on the inside bank slip off slope results in the exaggeration of the meander, making it more sinuous, causing the river to migrate laterally, widening the valley floor and its floodplain

Rejuvenation; Knickpoints, Incised meanders and River Terraces

Rejuvenation is the renewal of the river’s energy as a result of a relative fall in base level. This can be caused by the land rising relative to the sea as a result of crustal movements (ISOSTATIC) or a fall in sea level (EUSTATIC). The drop in base level gives the river greater potential energy, increasing its vertical erosion potential. A knick point relates to the extent to which the river has created a newly graded profile to adjust to the new base level. It is identified by a break in slope (sharp change in gradient) and is usually marked by a waterfall. This is the junction between the original long profile and the new long profile.

An incised meander is formed when the change in base level continues for a long period of time. A river will erode vertically forming one of 2 types of incised meander: Entrenched meanders (eg. River Wear, Durham) have symmetrical cross sections and result from a very rapid incision of the valley sides or due to the resistance of the valley sides to erosion. Ingrown Meanders occur when the incision by the river is less rapid allowing the river time to shift laterally, producing an asymmetrical cross section (eg. River Wye, Tintern Abbey)

River Terraces are remnants of former flood plains which, following vertical erosion caused by rejuvenation, have been left high above the maximum level of present day flooding. If a river cuts rapidly into its plain, a pair of terraces of equal height may be seen at each side of the river, creating a valley in valley feature. More often, the river cuts down relatively slowly enabling it to meander at the same time. The result is unpaired terraces, present at different points at different levels.