5. How are the main river features formed? Which processes are involved?






We have looked at the channel network within the river basin.
The source of the river tends to be close to the watershed which occupies the highest ground around the basin. The journey of the river is then downhill to the sea.

Example: River Tees: rises on Cross Fell,seen below, that rises to 890m. The sources of the Tees is 130m below the peak.
1.5E_Tees_Source.png

The path the river follows from its source to mouth is known as the river's course. When studying rivers we often divide it into 3 main sections, the upper course; middle course and lower course. Each part of the river has distinctive features which form and the characteristics of the river and its surrounding valley change.
1.5B_Summary.png

The Upper Course
V-Shaped Valleys
In the upper course of a river, water flows quickly through a narrow channel with a steep gradient; as it does so it cuts downwards. This vertical erosion results in a number of distinctive landforms including the steep sloping v-shaped valley through which the river flows in its upper course.

How does a v-shaped valley form?
1.5A_upper_coaurse_pic.jpg1. Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley
2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley
3. Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.
4. This material is then gradually transported away by the river when there is enough energy to do so.

As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs). As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of higher land creating interlocking spurs.









Waterfall

An other feature found in the upper course of a river, where vertical erosion is dominant, is a waterfall. The highest waterfall in the world is the Angel Falls in Venezuela (see picture right) which have a drop of 979m. Other particularly famous examples include Niagara Falls (North America), the Victoria Falls (on the Zambia / Zimbabwe border) and the Iguazu Falls (South America). Although much smaller in scale, there are many waterfalls in the upper course of UK rivers (e.g. Thornton Falls, Yorkshire - above), but how do they form?

The formation of Waterfalls1.5C_waterfall_diag.PNG

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.

1.5D_High_force.pngHigh Force, the highest waterfall at 21 metres in the UK is on the River Tees. It has very deep plunge pool at its base. Its the cap rock is made of a very resistant igneous rock called whinstone. Below the whinstone there are bands or sandstone and shales as well as some very chin coal seams, which are much softer and easily eroded

Middle Course of the River

1.5G_Middle_Tees.pngIn the middle course, the river channel has become much wider and deeper as the river has been fed by many tributaries upstream. Despite the more gentle gradient the velocity of flow may be as fast as in the uplands, as there is less friction and fewer objects that the river has go around. As well as changes in the river channel, its surrounding valley has also become wider and flatter in cross-section with a more extensive floodplain. One more obvious features of the river in the middle course is its increasingly winding. Unlike the relatively straight channel of the upper course, in the middle course there are many meanders (bends) in the river.

How do meanders form?

1.5H_meander_formation.pngMeanders form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders.

1. Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. The faster moving water tends to travel in a straight line and thus hit the outside of the next bend. It erodes this laterally by attrition and hydraulic action in the main. This also causes greater vertical erosion which deepens the channel, which reduces friction and increases in energy results in further erosion. The lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff.

2. 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 river beach or run-off slope builds up on the inner bend.

Remember - a meander is asymmetrical in cross-section (see diagram). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).

Over time meanders gradually change shape and migrate across the floodplain.

Lower Course of the River -
Floodplains, oxbow lakes and Levées


As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment (hence its muddy appearance).
With the addition width to meander in, the bends becomes pronounced due to further lateral erosion and eventually an ox-bow lake may form.

How does an oxbow lake form?
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. 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.

How do Levées form?
When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain.

When the river water bursts its bank, the shallower depth of water flowing over the surface results in frictional drag and a consequent reduction in velocity (speed) of flow. This results in the loss of energy and therefore deposition occurs. The heaviest materials are deposited first as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as Levées. Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.
>

The mouth of the river:
When the river reaches its mouth, any remaining sediment is deposited as the water slows down. If the river flows into an ocean or a sea like the North Sea with a strong current, the waves, quickly transport the fresh water sediments away. So the river flows out of an estuary, a wide river mouth, for example the River Thames.
If however, the river flows into a lake or a sea such as the Mediterranean which does not have a strong current, then the deposited sediments accumulate and the river water has to flow around the bank created by previous deposits. Then a delta forms, for example the River Nile or the Mississippi. The separate channels which take the river to the sea are called distributaries .

Key Words
V-shaped Valley
- a valley which resembles a 'v' in cross section. These valleys have steep sloping sides and narrow bottoms.
Interlocking Spur - spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course. Interlocking spurs form where the river is forced to swing from side to side around these more resistant ridges.
Load - collective term for the material carried by a river
Cap Rock - layer of hard resistant rock forming the 'step' over which the 'falls' occur in a waterfall.
Waterfall - a cascade of water over a hard rock step in the upper course of a river
Plunge Pool - a deep pool beneath
Gorge - a steep sided valley left behind as a waterfall retreats upstream
Meander - a bend in a river
River Cliff - a small cliff formed on the outside of a meander bend due to erosion in this high energy zone.
Slip off Slope - a small beach found on the inside of a meander bend where deposition has occured in the low energy zone.
Ox-bow lake - a lake formed when the continued narrowing of a meander neck results in the eventual cut through of the neck as two outer bends join. This result in the straightening of the river channel and the old meander bend becomes cut off forming an ox-bow lake.
Meander scar - feature left behind when the water in an ox-bow lake dries up.
Floodplain - the area of land around a river channel which is formed during times of flood when the amount of water in a river exceeds its channel capacity and deposition of rich silt occurs.
Levées - a raised river bank (can be natural features formed by deposition or artificial structures built to increase channel capacity and reduce flood risk)
Estuary – a river mouth that is kept clear of sediments by the currents of the ocean
Delta – a river mouth in which sediments accumulate due to oceans currents being insufficient to remove them.
Distributaries – the channels into which the main river channel break up when a delta is formed

Many thanks to Rob Chambers for many of the pictures, links and ideas for text – for more of his great stuff check out http://geobytesgcse.blogspot.com