Waterfalls
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Formation of a waterfall:
1.Waterfalls are found in the upper course of a river. They usually occur where
a band of hard rock lies next to soft rock. They may often start as
rapids.
2. As the river passes over the hard rock, the soft rock below is
eroded (worn away) more quickly than the hard rock leaving the hard rock
elevated above the stream bed below.
3. The ‘step’ in the river bed continues
to develop as the river flows over the hard rock step (Cap Rock) as a vertical
drop.
4. The drop gets steeper as the river erodes the soft rock beneath by
processes such as abrasion and hydraulic action. A plunge pool forms at the base
of the waterfall.
5. This erosion gradually undercuts the hard rock and the
plunge pool gets bigger due to further hydraulic action and abrasion. Eventually
the hard cap rock is unsupported and collapses. The rocks that fall into the
plunge pool will continue to enlarge it by abrasion as they are swirled around.
A steep sided valley known as a gorge is left behind and as the process
continues the waterfall gradually retreats upstream.
Potholes
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Potholes are cylindrical holes drilled into the rocky bed of a river by burbulent high-velocity water loaded with pebbles. The pebbles become trapped in slight hollows and vertical eddies in the water are strong eneough to allow the sediment to grind a hole into the rock be 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 a 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.
Braided channels
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Braiding occurs when the river is forced to split into several channels separated by islands. It is a feature if rivers that are supplied with large loads of sand and gravel. It is most likely to occur when a river has variable discharges. 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:
1. Semi arid areas of low relief that receive rivers from mountainous area
2. Glacial stremas 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.
Meanders
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Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope or point bar. The water in a meander flows in a corkscrew like movement as it moves from the inside of the bend towards the outside of the bend. This is called helicoidal flow.
Remember – a meander is asymmetrical in cross-section (see diagram above). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).
Oxbow lakes
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As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and
narrower.
Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander usually during a flood event when the energy in the river is at its highest. The water now takes its
shortest route rather than flowing around the bend.
Deposition gradually seals off the old meander bend forming a new straighter river channel.
Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.
Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up,
the feature left behind is known as a meander scar.
Levees
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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 waterfalls as it 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 depoisted firest, froming small raised banks (levees) along the sides of the channel. Subsequent floods increase the size of these banks and further deposition on the bed of the river also occurs. This means that the river, with channel sediment build-up, now flows at a higher level than the floodplian. For this reason, the authorities sometimes strengthen levees and increase their height.
Floodplains
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Floodplains are created as a result of both erosion and deposition, although the accumulation of river deposits suggests that they are predominately depositional features. They are 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 meadner bend. Over time, this results in the migration of meanders, leaving their scars clearly visisble 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, point bars and old meanders scars become incorporated into the floodplain, adding to the alluvial deposits. These become stabalised by vegetation as the meanders migrate and abandon their former courses.
Deltas
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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 seawater and clay particles coagulate due to chemical reaction. The clay settles on the river bed.
Deltas form only when the rate of deposition excveeds 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 in the Mississippi and Nile rivers
· The coastal are 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 usally composed of three types of deposit:
· The larger and heavier particles are the first to be deposited as the river loses its energy. These form the topset beds
· Medium graded particles travel a little further before they are deposited as steep-angled wedges of sediment, forming the foreset beds
· The very finest particles travel furthest into the lake before deposition and from the bottomset 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 shoreline and a dendritic pattern of drainage. Many distributatries 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 bird’s food delta. Fingers of deposition build out into the sea along the distrubutaries’ 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 cuurents or longshre