Introduction & Waterfalls and Rapids
As a river flows froms its source to its mouth a number of changes take place in its morphology. These changes affect the shape and size of the channel and result in distinctive landforms along its course. Some of these landforms are the result of erosion, some are the results of deposition and some are the consquence of both.
Waterfalls and rapids occur when there is a sudden change in the gradient of the river as it flows downstream. Waterfalls are more dramatic features than rapids and may be the result of:
- a resistant band of rock occuring across the course of the river
- the edge of a plateau
- the rejuvenation of the area, giving the river renewed erosional power as sea level falls.
The river falls over a rock edge into a deep plunge pool at the foot of the fall, where the layers of weak rock are excavated more quickly than the overlying resistant rock. The force of the swirling water around the rocks and boulders enlarges and deepens the plunge pool by hydraulic action and abrasion. This undercuts the resistant rock above. Eventually the overhanging cap rock collapses and the waterfall retreats upstream, leaving a gorge ahead of it.
Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity water loaded with pebble. The pebbles become trapped in slight hollows and vertical eddies in the water are strong enough to allow the sediment to grind a hole into the rock by abrasion (corrasion). Attrition rounds and smooths the pebbles caught in the hole and helps to reduce the size of the bedload.
Potholes can vary in width from a few centimetres to several metres. They are generally found in the upper or early-middle course of the river. This is where the valley lies well above base level, giving more potential for downcutting, and where the river bed is more likely to be rocky in nature.
Braiding occurs when the river is forced to spilt into several channels seperated by islands. It is a feature of rivers that are supplied with large loads of sand and gravel. It is most likely to occur when a river has variable discharge. The banks formed from sand and gravel are gernally unstable and easily eroded. As a consquence, the channel becomes very wide in relation to its depth. The river can become choked, with several sandbars and channels that are constantly changing their locations.
Braiding also occurs in environments in which there are rapidly fluctuating discharges:
- semi-arid areas of low relief that recieve rivers from mountainous areas
- glacial streams with variable annual discharge. In spring, meltwater causes river discharge and competence to increase, therefore the river can transport more particles. As the temperature drops and the river level falls, the load is depositied as isalnds od deposition in the channel.
Meanders are sinous bends in a river. Explaining the formation of meanders has caused many problems for geographers. In low flow conditions straight channels are seen to have alternating bars of sediment on thier beds and the moving water is forced to weave around these bars. This creates alternating shallow sections (riffles) and deeper sections (pools). The swing of the flow that has been induced by the riffles directs the maximum velocity towards one of the banks, and results in erosion by undercutting on that side. An outer concave bank is therefore created. Deposition takes place on the inside of the bend, the covex bank. Consquently, although the river does not get any wider, its sinousity increases.
The cross section of a meander is asymmetrical. The outer bank forms a river cliff or bluff with a deep pool close to the bank. This bank is udercut by erosion, particularly abrasion and hydraulic action. The inner bank is a gently sloping deposit of sand and gravel called a point bar.
Once they have been created, meanders are perpetuated by a surface flow of water across to the concave outer bank with a compensatory subsurface return flow back to the convex inner bank. This corkscrew-like movement of water is called helicoidal flow. In this way, eroded material from the outer bank is transported away and deposited on the inner bank. Modern research suggests that the flow is rarely strong enough for the river to transport material across to the point bar on the oppisite bank. Point bars are most likely to be maintained by sediment from erosion at the bluff of the meander upstream on the same side of the channel.
The zone of greatest erosion is downstream of the midpoint in the meander bend, because of the strongest current does not exactly match the shape of the meander. As erosion continues on the outer bank, the whole feature begins ti migrate slowly, both laterally and downstream. Imprints of former channels can be seen on the floodplain.
Oxbow lakes are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake seperated from an adjacent river. The water is stagnant, and in time the lake gradually silts up, becoming a crescent-shaped stretch of marsh called a meander scar. An oxbow lake is formed by the increasing sinousity of a river meander. Erosion is greatest on the outer bank, and with deposition on the inner bank, the neck of the meander becomes progressively narrower. During times of high discharge, such as floods, the river cuts through this neck, and the new cut eventually becomes the main channel. The former channel is sealed off by deposition.
In its middle and lower courses, a river is at risk from flooding during times of high discharge. If it floods, the velocity of the water falls as its overflowsthe banks. This results in deposition, because the competence of the river is suddenly reduced. It is usual for the coarsest material to be depositied first, forming small raised banks (levees) along the sides of the channel. Subsquent floods increase the size of these banks and further deposition of the bed of the river also occurs. This means that the river, with the channel sediment buildup, now flows at a higher level than the floodplain. For this reason, the authorities sometimes strenghten levees and increase thier heights. On the Mississippi River levee straightening began in 1699. By the 1990's the length of energineered levees was 3200km.
Floodplains are created as a result of both erosion and deposition, although they accumulation of river deposits suggests that they are predominatly depositional features. They are the relatively flat areas of land either side of the river, which form the valley floor in the middle and lower courses of the river. They are composed of alluvium - river-deposited silts and clays. Over time, a floodplain becomes wider and the depth of sediment accretions increases. The width of the floodplain is determined by the amount of meander migration and lateral erosion that has taken place. Lateral erosion is most powerful just downstream of the apex of the meander bend. Over time, this results in the migration of meanders, leaving thier scars clearly visible on the floodplain. Interlocking spurs are eventually removed by lateral erosion in the middle course, leaving behind a bluff line and widening the valley. The depth of the alluvial deposits depends partly on the amount of flooding in the past, so floodplain creation is linked to extreme events. Over time, pointing bars and old meander scars become incorporated into the floodplain, adding to the alluvial deposits. These become stabilised by vegetation as the meanders migrate and abandon thier former courses.
Studies in the USA suggest that point bar deposits account for arounf 80% of the volume of sediment contained within a floodplain. In Britain, a large proportion of the accumulated sediment in floodplain deposits was laid down by post-glacial streams following the last ice age, when the volume of water in rivers was higher and frequency of flooding much increased.
A Delta is a feature of deposition, located at the mouth of a river as it enters a sea or lake. Deposition occurs as the velocity and sediment-carrying capacity of the river decrease on entering the lake or sea, and bedload and suspended material are dumped. Flocculation occurs as fresh water mixes with sea water and clay particles coagulate due to chemical reactions. The clay settles on the river bed.
Deltas form only when the rate of deposition exceeds the rate of sediment removal. In order for a delta to form the following conditions are likely to be met:
- the sediment load of the river is very large, as the Mississippi and Nile rivers.
- the coastal area into which the river empties its load has a small tidal range and weak currents. This means that there is limited wave action and therefore, little transportation of sediment after deposition has taken place. This is a feature of the Gulf of Mexico and the Mediterranean Sea.
Deltas are usually composed of 3 types of deposits:
1- the larger and heavier particles are the first to be deposited as the river loses its energy. These form the topset beds.
2- medium graded particles travel a little further before they are deposited as steep-angled wedges of sediment, forming a foreset beds.
3- the very finest particles travel furthest into the lake before deposition and form the bottomset beds.
Deltas can be described according to thier shape. The most commonly recognised is the characteristic arcuate delta, f0r example the Nile Delta, which has a curving shortline and a dendritic pattern of drainage. Many distributaries break away from the main channel as deposition within the channel itself occurs, causing the river to braid. Longshore drift keeps the seaward edge of the delta relatively smooth in shape. The Mississippi has a birds foots delta. Fingers of deposition build out into the sea along the distributaries channels, giving the appearance from the air, of a birds claw. A cuspate delta is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea current or longshore drift.