River Features Notes PDF

RIVER FEATURES Typically, a river valley can be divided into three sections; the upper course, the middle course and the lower course. There are particular river features which we can expect to find in each of these sections.. The diagram below shows the three sections of a river. Notice the diffe...

RIVER FEATURES Typically, a river valley can be divided into three sections; the upper course, the middle course and the lower course. There are particular river features which we can expect to find in each of these sections.. The diagram below shows the three sections of a river. Notice the differences in gradient and the shape of the river valley from one section to the next Features formed at the Upper Course of the River V Shaped Valley-This is a narrow V shaped river valley with steep sides. They are formed near or at the source of the river channel is narrow and where the gradient or height of the land is steep and the river flows very quickly or rapidly such that vertical erosion by way of hydraulic action (also solution) the force of the water cut downwards or vertically to erode the bed of the river by removing material. In addition the rock particles/load within the river bed knock against it (corrosion/abrasion) resulting in the bed of the river also eroding or cutting downwards thereby removing material in the bed. The processes continue to occur along with the weathering or break up of rock along the sides of the river valley such that a narrow V shaped steep sided valley is formed. N.B. A V shaped valley may occur at the lower course when downward erosion is renewed where the sea level falls or the land level rises or by capture of another river. This is referred to as rejuvenation, Waterfalls- This is the fall, drop or cascading of water over steep gradient(hard/resistant rock). Waterfalls are formed where river beds of waterfalls have hard/ resistant rock, and softer/less resistant rock. The hard/resistant rock therefore lies horizontally, vertically or dips upstream above the softer/less resistant rock. The river in flowing passes over the hard rock and erode the softer rock through hydraulic action, abrasion and solution, in that the force of the water along with its load continually beat or knock against the softer rock creating areas of weakness/cracks. The softer rock is therefore undercut or eroded such that the hard rock overhangs of forms a ledge. Without the support of the soft rock below and constant hydraulic action, solution and attrition along the base at the soft rock the overhang collapses or falls to the base/plunge pool. This results in a steep gradient or slope occurring over which the water falls or drops or cascades to form a waterfall e.g. Dunn’s River and Reach Falls in Jamaica A diagram of a Waterfall Rapids- Rapids are the rapid, fast or turbulent (white water) flow of water over steep outcropping hard/resistant rocks along the river bed. They are formed where the river bed is made up of alternate layers of hard and soft rocks and as the river flows the softer rocks are removed or eroded much faster by solution, hydraulic action and abrasion, that is, the force of the water and its load scraping and hitting against the soft rocks causing its removal. This results in the bed of the river being uneven with outcropping or emergent hard rocks which increases the speed/flow of the water hence creating rough turbulent water. Plunge Pools- They are deep pools, holes or hollows found at the base of the waterfall. They are formed when the water from the river falls steeply and its motion erodes the soft rock at the bottom or base of the waterfall to form deep pools or holes. Also, the material produced through the erosion of the softer rock and a collapse of the overhang knock against the base and swirls to deepen the plunge pool. Gorge- A gorge is a deep, narrow and steep river valley. It is formed when a waterfall retreats or move upstream. This occurs when the soft rock/less resistant rock below the hard rock/resistant rock is continually eroded or undercut by hydraulic action and solution in that the force of the water and its dissolving action removes the soft rock. Eventually an overhang or ledge of hard rock emerges above the soft rock. This overtime is unsupported as there is no soft rock below to hold it in place and erosion continues to weaken its base. This results in the overhang collapsing to allow for the waterfalls retreat. The process continues to occur such that more overhang emerge from the undercutting of the soft rock. They eventually collapse to form a gorge or a steep, deep and narrow river valley which is cutting back into the hillside or upstream e.g. Bog Walk Gorge. Interlocking Spurs- Interlocking spurs are ridges or highland areas (promontories)of hard or resistant rocks which jut or project into a river channel/valley alternately around which the river swings or bends to form a zig-zag pattern. They are formed where hills or highland areas of hard rocks on both sides of a river valley project, jut or emerge in a river channel. When this occurs they become and obstacle or barrier within the river channel hence the river is forced to swing from side to side or wind or bend to avoid the obstacles of hard rock while eroding the outside of the bend. At the same time the bed is being eroded downwards or vertically to form V-shaped valleys along the spurs. Potholes- Potholes are circular or rounded depressions, hollows or holes found in/along a river bed. They form along a river bed which is uneven. As the river flows along the bed carrying small rocks and pebbles it falls in the uneven section of the river bed and the turbulence or force of the water cause the rocks to swirl or spin in a circular motion. Overtime, as the rocks swirl or spin abrasion or a drilling or grinding action occurs in the river bed such that circular depressions or holes called potholes are formed. Features formed at the Middle Course of the River Meanders- A meander is the bending or winding of a river in its channel around its load to form S like patterns. The formation of meanders is due to both deposition and erosion. As a river flows over land with a gentler gradient it flows more slowly than in the youthful stage hence its reduced energy makes it unable to transport eroded material that is heavy or large such as boulders hence it deposits it in the bed. Not being able to remove the load which is now an obstacle in its flow the river takes a different path or flows from its direct path by flowing around these obstacles or load. This causes the river to follow a winding path creating a feature called a meander. Overtime because of erosion on the outer bend or bank of the river due to the rapid flow of the river and deposition on the inner bend due to slower movement the meander becomes more pronounced or developed. How do meanders form? Meanders 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 River Cliff - This is a steep rock face along the outside bend of a river bank/meander. A river cliff or bluff or concave bank is formed on the outside bend of a meander due to erosion. It occurs when the force of high energy current or the fast flow of water erode or undercut the bank or outside bend and cause material to be removed thereby steepening the bank to form a cliff. Slip off Slope-This is a gently sloping area of sediments found along the inside bend of a meander. A slip off slope or convex bank or point bar is formed where deposition takes place on the inside bend of a meander. Here the river has low energy hence it flows slowly resulting in sediments of sand, pebbles etc which are too heavy for the water to carry to be laid down or deposited on the inside of the bend. The bank therefore becomes gently sloping. Lower Course Features of the River In the lower course, the river has a high volume and a large discharge. The river channel is now deep and wide and the landscape around it is flat. However, as a river reaches the end of its journey, energy levels are low and deposition takes place. The features formed at the lower course are floodplains, deltas, oxbow lakes, levees and braided rivers. Floodplains A floodplain is flat/low lying or lowland area on either side of the banks of a river that contain alluvium deposits and is covered by the river in times of a flood. Floodplains are formed where the river has a greater volume of water and reduced energy. Floodplains therefore develop when the river is in flood, that is, having a greater discharge such that it overflows or floods its bank. As the river floods its bank it carries material in suspension and where the energy is reduced the heavier/coarser material are deposited on the banks causing it to become higher. The finer particles of alluvium are however deposited farther away on either side of the bank which overtime build up a low lying area which is often swampy called a floodplain. Deltas This is a low-lying swampy plain found at the mouth of a river which eventually form land. Conditions necessary for the formation of a delta: 1. Low tides and currents which are not strong enough to remove sediments (deposits). 2. There must be a constant supply of material (silt, sand etc). hence active erosion at the upper course. 3. The speed or velocity of the river must be slow. 4. The coastal water must be shallow so as to allow for the accumulation or build up of deposits. 5. The load much therefore be deposited faster than it can be removed by tide etc. Formation of a delta: A river carrying sediments upon reaching the sea or a lake lose energy and deposits material. The sediment may be sorted as the heaviest material is deposited first. Over time, more and more sediment is added.. If the tides are strong enough the sediment will be washed away and if the river deposits quicker than the sea can erode then the main river channel gets blocked as deposited material obstructs the river, forcing the water to find alternative routes to the sea hence splitting the river into a number of distributaries or several channels to enter the sea or lake. Spits or bars (ridges of sediments) may build up across distributaries/mouth cutting off water flow to create still bodies of water (lagoon). Where constant deposition occurs overtime the area of the lagoon becomes filled in with deposits to form a swamp and then dries up to form land or low lying swampy areas known as a delta. Delta Three main types of delta: Arcuate or fan-shaped - This delta consists of coarse material or sediments. It has an arc-shaped or fan-shaped appearance with a rounded edge. It has many distributaries e.g. Nile in Egypt, Po in Italy and Hwang Ho in China. The land around the river mouth arches out into the sea, the river splits many times on the way to the sea, creating a fan effect. Bird's foot - This delta is also called Bird’s Foot because it has such a shape. It consist of finger like projections in the sea and is made up of very fine materials. There are few distributaries. E.g. Mississippi in the USA. The river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird's foot. Estaurine This is a delta formed along a submerged or sunken river. It is partly submerged. It is formed along a straight coastline where wave action is strong, such that material spread out evenly on either side of the river mouth. Estuary: The point at which a river begins to meet the sea. The river will be tidal, meaning that it will have both salt water and fresh water in it.e.g. Marianne in Trinidad. Braided River: This is a river that has been split into several channels because of deposition in the bed of the river. They form when rivers in dry weather and with low volumes of water deposit there load in the river bed. Braided rivers are therefore formed when the river flow is high or has a greater volume of water hence the river has excess energy and can transport large amounts of load. However, when the flow is low or the channel has less water during dry weather the river is forced to deposit its load. The deposited material forces the river to divide into small channels around the load creating a braided river. The small islands which divides the channel created by deposition are known as eyots. Ox - bow Lake Oxbow lakes This is a shallow crescent or horse-shoe shaped body of water. Where a river is meandering across the river valley, that is, winding and bending around load deposited in the bed, erosion is occurring laterally (from side to side) along the outside bends (bank) and deposition on the inside. This results in the river course changing into loop like patterns which is separated by a narrow neck of land (separates the two ends of the meander). Continuous erosion on the outside bend of the narrow neck of land due to the fast flow of the water makes the neck of land much narrower and deposition on the inside bend during flood cause the loop at the neck of land to be filled in with deposits. Eventually erosion removes the narrow neck of land and deposition occurs resulting in the loop being cut off from the main river. Both ends are cut off completely  sealing off the lake. The river thus assumes its normal channel along a straight path and a crescent body of water is left. Levée Levees Levees are raised river banks. They are formed when a river has a great discharge and a fall in velocity or reduction in energy occurs where the river floods or overflows it banks. This results in the river immediately depositing its largest load on the bank creating embankments that line the river channel. When the river returns to a normal flow in its channel its slow movement results in material being deposited on the river bed causing the bed to become raised. Over time these embankments are raised higher by continuous deposition from the river overflowing or flooding its bank and having reduced energy to form raised banks or levees. These are sometimes strengthened by humans to reduce flooding.