River Landforms of Fluvial Erosion

V-shaped valleys in interlocking spurs

Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valleyOver time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valleyGradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.This material is then gradually transported away by the river when there is enough energy to do so.An Interlocking spur is a ridge of more resistant rock. It forms as the river flows through the valley it is forced to wind around protrusions, hills or ridges of spurs which appear to interlock. As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of higher land creating interlocking spurs. E.g. V-shaped valley: The upper course of the Inn Valley, AustriaE.g. Interlocking spurs: The upper course of the River Eden, Cumbria, England

Waterfalls and Rapids

As a river flows from 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 consequence 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 occurring 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.E.g. Waterfalls, rapids and gorges: Iguaçu Falls, Brazil and Argentina Potholes

Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity water, loaded with pebbles. 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 smoothes 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 down cutting, and where the river bed is more likely to be rocky in nature.Bourke's Luck Potholes in the Blyde River Canyon of South Africa

River Landforms of Fluvial Deposition

Braided channels

Braiding occurs when the river is forced to split into several channels separated 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 generally unstable and easily eroded. As a consequence, 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 receive 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 deposited as islands of deposition in the channel.

E.g. The Waimakariri River, in the South Island of New Zealand

Meanders

Meanders are sinuous 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 their 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 convex bank. Consequently, although the river does not get any wider, its sinuosity 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 undercut 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 opposite 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 to migrate slowly, both laterally and downstream. Imprints of former channels can be seen on the floodplain.E.g. Amazon River

Oxbow lakes

Oxbow lakes are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake separated 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 sinuosity 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.E.g. Amazon River

Levees

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 overflows the banks. This results in deposition, because the competence of the river is suddenly reduced. It is usual for the coarsest material to be deposited first, forming small raised banks (levees) along the sides of the channel. Subsequent 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 build-up, now flows at a higher level than the floodplain. For this reason, the authorities sometimes strengthen levees and increase their heights. On the Mississippi River levee straightening began in 1699. By the 1990's the length of reengineered levees was 3200km.The River Glen is a river in Lincolnshire, England

Floodplains

Floodplains are created as a result of both erosion and deposition, although they accumulation of river deposits suggests that they are predominantly 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 their 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 their former courses. Studies in the USA suggest that point bar deposits account for around 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.The land around any river, e.g. Amazon River

Deltas

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 top set beds.2- Medium graded particles travel a little further before they are deposited as steep-angled wedges of sediment, forming a forest bed.3- The very finest particles travel furthest into the lake before deposition and form the bottom set beds.

Deltas can be described according to their shape. The most commonly recognised is the characteristic arcuate delta, for example the Nile Delta, which has a curving short line 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 bird’s claw. A cuspate delta is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea current or longshore drift.

River Landforms of Rejuvenation

Knick Points

A knick point is a sudden break or irregularity in the gradient along the long profile of a river. Some knick points are sharply defines, e.g. waterfalls, in particular Niagara Falls, whereas others are barely noticeable. Although a number of factors can cause such features, to occur, they are most commonly, attributed to rejuvenation. When a river is rejuvenated, adjustment to the new base level starts at the sea and gradually works its way up the river’s course. The river gains renewed cutting power, in the form of vertical erosion, which encourages it to adjust its long profile. In this sense the knick point is where the old long profile joins the new. The knick point recedes upstream at a rate which is dependent on the on the resistance of the rocks, and may linger at a relatively hard outcrop. It can be difficult to determine whether a waterfall occurs due to variability in rock type or to rejuvenation. Headward erosion upstream may mean that a waterfall cuts back through the valley towards its source until the long profile eventually adjusts to its new energy equilibrium.

River Terraces

A river terrace is a remnant of a former floodplain, which has been left at a higher level after rejuvenation of the river. Where a river renews it down-cutting, it sinks its new channel into the former floodplain, leaving the old floodplain above the level of the present river. The terraces are cut back as the new valley is widened by lateral erosion. If renewed rejuvenation takes place, the process is repeated and a new pair of terraces is formed beneath the original ones. The river Thames has created terraces in its lower course by several stages of rejuvenation. Terraces provide useful shelter from floods in a lower course river valley, and natural routeways for roads and railways. The built-up areas of Oxford and London are mainly located along the River Thames. E.g. South Fork of the Shoshone River, Wyoming

Incised Meanders

If a rejuvenated river occupies a valley with well developed meanders, renewed energy results in them becoming incised or deepened. Incised streams and rivers have cut deeply into the landscape in many parts of the British Isles. The nature of the landforms created is largely a result of the rate at which vertical erosion has taken place. When incision is slow and lateral erosion is occurring, an ingrown meander may be produced. The valley becomes asymmetrical, with steep cliffs on the outer bends and gentler slip-off slopes on the inner bends.E.g. The River Meuse, French Ardennes.

Entrenched Meanders

If a rejuvenated river occupies a valley with well developed meanders, renewed energy results in them becoming incised or deepened. Incised streams and rivers have cut deeply into the landscape in many parts of the British Isles. The nature of the landforms created is largely a result of the rate at which vertical erosion has taken place. With rapid incision, where down-cutting or vertical erosion dominates, the valley is more symmetrical, with steep sides and a gorge-like appearance. These are described as entrenched meanders. E.g. San Juan River at Goosenecks State Park, Utah