1 Fluvial landforms: AQA past questions Landforms of erosion: Rapids and waterfalls Label Figure 1 (shows a waterfall) to describe the characteristics of the landforms shown. (4 marks) The obvious landforms are the waterfall, plunge pool and the associated gorge in the foreground. Allow 1 mark for each of these – maximum 2 for identification. The command word is ‘describe’ so there needs to be more than just recognition of the landforms for further marks. Labels may include reference to the two stages of the descent / the deeper part of the bed just after the descent / the presence of a plunge pool marked by the white water (if not awarded as an identification mark for recognising landform) / the channel occupying the width of the bed in the gorge or narrow channel / the stepped profile of the gorge or narrow channel / the eddying of the water in the gorge / or turbulent flow. Features must be correctly arrowed to be worthy of a mark – arrow must connectwith landform / characteristic. 4x1 1 (a) (ii) Explain the formation of the landforms shown in Figure 1. (7 marks) The landforms to be explained are the waterfall and the gorge. There is likely to be reference to bands of hard and soft rock that are horizontally arranged, with the hard rock forming a cap rock, underlain by the soft rock. Differential erosion results and the softer rock is eroded faster than the hard rock, creating a steep gradient. There may be reference to specific processes of erosion, such as abrasion and hydraulic action. This causes the undercutting of the soft rock and an overhang develops above the plunge pool and its swirling waters. This ultimately collapses, causing the waterfall to retreat and to leave a narrow, steep-sided valley – the gorge – in its wake. This is the most likely explanation, but there may also be reference to rivers plunging over the edges of plateaus in southern Africa and South America and also to knick point waterfalls. Level 1 (Basic) 1-4 marks Begins to explain the formation of the waterfall/one landform. There is a partial sequence and general reference to process, such as erosion. Some use of appropriate terminology present at the higher end. Level 2 (Clear) 5-7 marks Seeks to explain the sequence of the formation of the landforms – waterfall and gorge. There is some reference to both landforms, although one may be dealt with in greater detail. There will be greater reference to gorge through the level. Sequence is clear and there is reference to specific processes, such as abrasion. Appropriate geographical terminology is used. Potholes (c) Figure 2 shows potholes; landforms resulting from fluvial erosion. Describe these potholes and explain their formation. (7 marks) Potholes are circular shaped; cylindrical; vary in depth; some merge with each other; some are exposed above the river level. The water can be seen to be swirling/eddying. This creates a shallow hollow that is deepened by the same process. The water contains some of the load being carried by the river and this hits the base and 2 sides of the hole to both widen and especially deepen it, smoothing the edges (1). This is a particular application of the abrasion process, known as pothole drilling. It is most effective at times of high velocity and discharge. As vertical erosion is the dominant process, potholes tend to form well above base level where potential energy is relatively high. Level 1 (1-4 marks) Describes the landform generally. Begins to explain. Answer may be imbalanced - and clearly emphasise one element. Some use of appropriate terminology present at the higher end. Level 2 (5-7 marks) Description refers partly to the photograph. Response is more balanced. Explanation is clear. Appropriate geographical terminology is used. Explain the formation of channel landforms typically found in the upper course of a river. (15) This question requires both description and explanation of relevant features. Do not penalise those who include the ‘V’ shaped valley, but credit in particular waterfalls, rapids and potholes. Credit diagrams, up to Level 3, if the annotation is detailed. 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 each may be the result of a resistant band of rock occurring across the course of the river or the rejuvenation of the area, giving the river renewed energy for vertical erosion due to a lower sea level.In the case of the waterfall the river falls over an edge into a deep plunge pool. At the foot of the fall the layers of weak rock are excavated more quickly, by corrasion and hydraulic power, than the overlaying resistant rock. The force of the water underneath the waterfall swirls around rocks and boulders and enlarges and deepens the plunge pool by the same processes. This undercuts the resistant (cap) rock above. Eventually the overhanging cap rock collapses andthe waterfall retreats upstream, leaving a gorge ahead of it. This is an example of headward erosion. Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high velocity water loaded with pebbles. Pebbles can become trapped in slight hollows and vertical eddies in the water will be strong enough to allow the sediment to grind a hole into the rock by corrasion / abrasion. Potholes can vary in size from a few centimetres to several metres in width. They are generally found in the upper course of the river, where the valley lies well above base level, giving more potential for down cutting, and where the riverbed is more likely to be rocky in nature. Level 1 (Basic) The answer is predominantly description of a typical upper course river, perhaps with some simple explanation of how waterfalls form. It is likely that non-channel features, such as interlocking spurs, gorges and V shaped valleys will be described. Level 2 (Clear) The answer will concentrate on the channel and there will be a clear explanation of at least one channel feature. Although there will be a sound understanding of the way in which channel features form, the specific terminology, such as vertical and headward erosion, are likely to be absent. Level 3 (Detailed) At this level the response will use more sophisticated geographical terms such as vertical erosion, headward, hydraulic power and corrasion accurately and in context. The formation of at least two channel features must be covered. There may be use of supporting case studies, such as High Force Waterfall on the River Tees. 3 Landforms of erosion and deposition: Meanders 1 (b) Study Figure 2 which shows part of the Cuckmere River in East Sussex. 1 (b) (i) Describe the landforms shown in Figure 2. (4 marks) Meanders are the most obvious feature – these are quite sinuous. There is evidence of the build-up of material on the inside bend (to right of photo) forming a gentle slip-off slope. There are levées present following the river’s course. The meanders are surrounded by an extensive flat gently sloping area – the flood plain. The meander belt extends across this. The valley sides (bluffs/river cliffs) rise beyond the extent of the meander belt.Allow 2 marks for identification. There must be reference to at least two landforms. 4x1. Next question is from a Specimen paper (c)Describe and explain the development of meanders. (15 marks) Description should relate to the development of riffles and pools at equal intervals along a stretch of river (pools occur 5-6 times the width of the bed). There should be information relating to what these are – alternating areas of shallow and deep water where energy increases within a pool area due to less friction/greater efficiency and is then lost as the water flows over the shallower riffle where friction is greater. Flow over these becomes uneven and results in the maximum flow being toward one side downstream. The pools become placed on the main curve of the meanders. The inside and outside bends of the meander are very different with shallow water, slow flow, deposition, and the build up of a slip off slope characterising the inner bend whilst deep water, fast flow, lateral erosion (especially abrasion and hydraulic action) and a river cliff characterise the outside bend. Thus, the meanders have a distinct asymmetrical profile. The spacing between the pools and the contrasts between the two bends relate to a corkscrew like movement of the water as it spirals downstream from bank to bank – helicoidal flow. Thus, the material that is eroded from an outside meander bend is subsequently deposited on the inside bend of the next meander downstream. 4 Level 1 Describes the formation of meanders. Will refer to basic processes such as erosion and deposition. Likely to focus on contrasts between inside and outside bend. Partial description and sequence not identified. Level 2 Clear description of the formation of meanders. Will refer to specific processes e.g. abrasion, helicoidal flow. Begins to identify sequence of formation e.g. begins with riffles and pools. Explanation will be present. Level 3 Clear description of the formation of meanders. Will refer to specific processes e.g. abrasion, helicoidal flow with clear evidence of understanding. Identifies sequence of formation e.g. begins with riffles and pools. Explanation will be clear in a more balanced answer. Next question is from Specimen paper 2 (b) When a river is flowing round a bend it is more likely to deposit on the inside of the bend rather than on the outside. Explain why. (4 marks) Water flowing in a river tends to adopt a spiral flow as it moves downstream. This means that it flows slower on the inside than on the outside of a bend (1), and also it tends to spiral upwards on the inside of the bend, using energy (1). As it has less energy available to carry its load of sediment it may deposit some of this material (1). On the outside of a bend it moves faster and spirals downwards, increasing its energy (1) and the increased energy allows it to pick up and carry material from the bed and bank (1) in contrast with the inside bend.. OLD AS (b) (ii) Draw a labelled diagram(s) to show how and why meanders migrate. (5 marks) Level 1 (Basic) The answer shows a simple diagram of a meander with deposition on the inside bend and erosion on the outside, so providing a limited explanation of why meanders change position. There may be reference to how meanders change position, e.g. using an arrow. Level 2 (Clear) A clear understanding is shown of how meanders migrate, i.e. downstream and laterally. The why element of the question is also covered, through use of a well-annotated diagram. Oxbow lakes: no question found 5 Landforms of deposition: Braided channels Levees Floodplain OLD AS (c) How and why do flood plains develop in the lower course of rivers? (7 marks) Floodplains develop due to changing energy conditions either related to one flood episode or over a longer period of time. • Become wider ………. Lateral erosion and meander migration. • Sediments become deeper ……….Flooding leads to deposition/meander accretion/channel bed aggregation• Become incised over time ……….Due to changes in the base level of the river (fall in sea level) Level 1 A basic response, which concentrates on the way a floodplain develops as a result of deposition following a flood, e.g. ‘when a river floods it slows down and deposits material on the valley floor, so it deepens the floodplain. The answer will cover how or why a floodplain changes after a flood. Level 2 A clear answer, which describes how floodplains become deeper and/or wider and starts to explain why they develop over time. Level 3 A more sophisticated and detailed response. Two of the three bullet points will be covered and there will be a balance in that both how and why the floodplain changes/develops will be addressed. Changing energy conditions taking place during a flood episode may be explained. 1 (b) (ii) Flooding is important in the formation of some river landforms. With the help of Figure 2, describe the role of flooding in the formation of river landforms. (6 marks) There is likely to be reference to flood plains (shown in Figure 2), ox-bow lakes (the subsequent stage from the meander development present); levées are also permissible as are bluffs and braided streams. Reference should be made to the specific role of flooding in formation, e.g. the development of levées when the river bursts its banks, the subsequent loss of energy and the deposition of material, beginning with the largest first. Subsequent flood events lead to the further buildup of the banks. The finer material is carried further to be deposited on the flood plain, leading to its aggradation. This process may be especially marked in deltas and channels may split, following flooding. There could be recognition that the meander is often cut off during times of flooding resulting in a straight course and the formation of an ox-bow lake. Level 1 (1-4 marks) Identifies landforms at lower end. Describes the formation of landform(s) such as flood plains, ox-bow lakes. May be detail on one or a variety of landforms mentioned. Some reference to flooding will be present at the higher end. Some use of appropriate terminology present at the higher end. Level 2 (5-6 marks) There is mention of two landforms. Description is purposeful – related to flooding specifically. (c) Explain how flooding can contribute to the formation of depositional landforms. (15 marks) The river is at risk from flooding during times of high discharge. If the river floods, the velocity of the water falls as it overflows the banks. This results in deposition as the competence of the river is suddenly reduced. It is usual for the coarsest material to be dropped first, forming a small raised bank along the sides of the channel. Subsequent floods will increase the size of the bank leading to the formation of Levees. Floodplains are the most common depositional feature of a river. 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). When flooding occurs the water spreads out over the floodplain and as it does so it slows, thus reducing its capacity for transportation. Ox-bow lakes are features of both erosion and deposition, so their formation is not just as a result of flooding. 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 higher discharge, such as a flood, the river cuts through this neck, and the new cut eventually becomes the main channel. The former channel is sealed off by deposition. Credit may be available to other factors so long as they are linked to contribution of flooding. Allow references to eyots and braiding. Level 1 (Basic) A simple description of one or more landforms of deposition, perhaps ox-bow lakes, formed as a result of flooding, with no useful explanation as to why flooding leads to this landform.Level 2 (Clear) Either a 6 description of two landforms created by flooding but with limited explanation why the flood event actually leads to the development of these or one landform of deposition will be well explained in relation to flooding. Level 3 (Detailed) Two or more landforms of deposition, probably levees and floodplains will be explained and the answer will demonstrate a sound understanding of why flooding leads to their formation. More precise use of geographical terminology will be evident. Deltas 1 (b) (i) Draw a labelled sketch plan to show the characteristics of the Mississippi delta. (4 marks) Sketch plan should show recognisable outline of river channels and build – up of land along them, with infilling between those on the right-hand side of plan. Labels likely to include – distributaries, bird‟s foot shape of delta, land extending along distributaries, levees, evidence of deposition/shallower water on edges of land, large amounts of sediment present in waters off the developed part of the delta. Allow 1 for sketch and 3 for appropriate labels. (4 marks) 1 (b) (ii) Explain the formation of deltas. (7 marks) There are certain conditions that are present which encourage the formation of deltas. The presence of a large amount of sediment is essential – brought down by large rivers. Mississippi transports 450m tonnes per year. The limited removal of this by small range of tides is also significant – thus deltas form in areas such as the Mediterranean, Gulf of Mexico. The presence of relatively calm seas encourages the deposition of material, as does the process of flocculation where the salt present in the water in the sea generates an electrical charge that causes particles to stick together, increasing cohesion and weight and encouraging deposition. There may be reference to the land extending out especially along distributaries and the fact that the largest material is deposited first. This links in with reference to topset, foreset & bottomset beds. (7 marks) Level 1 (Basic) 1-4 marks Begins to explain – ideas separate initially. There is a partial sequence and begins to link ideas at top end. Some use of appropriate terminology present at the higher end. Level 2 (Clear) 5-7 mark Explanation is clear/sequence given. Links between different aspects and interrelated nature is recognised. Appropriate geographical terminology is used. 7 OLD AS Explain the formation of a delta. (5 marks) Deltas generally form when the rate of deposition exceeds the rate of sediment removed as a river reaches the sea / a lake. In order for a delta to form the following is usually true.• The river carries a significant load of sediment. • The coastal area into which it empties has a small tidal range and weak currents, so there is limited wave action. The larger and heavier particles are deposited first as the river enters the sea, losing its competence. These form the top-set beds. Medium sized particles, carried a little further out and deposited as steep angled wedges of sediment next form the fore-set beds. The finest material is deposited furthest away from the mouth and becomes the bottom set beds. Deposition within the channel in the final stages of the river’s journey occurs during times of low flow, or immediately following a flood event. Islands of deposition called eyots (chars in Bangladesh) can split the channel and cause it to braid. When salt water meets fresh water the process of flocculation occurs. Fine-grained clays transported in river water are attracted together to form larger particles in saltwater and are deposited on the river banks and bed. Some candidates might refer to the formation of a bird’s foot delta, where fingers of deposition build out into the sea along distributary channels. Level 1 A simple description of a delta with a basic attempt to explain its formation, most likely along the lines ‘deltas form when the river carries lots of sediment and when the sea is not powerful enough to carry this away.’ Level 2 A clear explanation, rather than description, which might account for graded sediments or the process of braiding leading to the formation of distributaries. Landforms of rejuvenation June 2012 1 (b) (i) Outline the cause(s) of rejuvenation. (2 marks) Rejuvenation occurs when the river has renewed/increased energy, as a result of a change in the river height relative to base level, caused by fall in sea level or land uplift. (d) Describe and explain the formation of landforms resulting from rejuvenation. (15 marks) A definition of rejuvenation is likely to form part of the answer – a renewal of the river’s energy as a result of a relative fall in base level. This will lead to a return to vertical erosion. It may be a result of a fall in sea leveleustatic change or an increase in the relative height of the land in relation to the sea - due to isostatic uplift (the ‘rebound’ following the end of glaciation) or due to tectonic activity. Landforms - knick point relates to the extent to which the river has created a newly graded profile to adjust to the new base level. It is identified by a break in slope and is usually marked by a waterfall and reflects the process of headward erosion as well as vertical erosion due to the renewed ability to erode vertically. River terraces - are the remains of the former flood plain – now abandoned as the river has eroded too deeply to access it; these may be paired - i.e. at the same level on either side of the channel. This is indicative of rapid down cutting. If it is slower as a result of more gradual uplift, the terraces will be present on different sides of the channel at different levels - unpaired terraces as the river has time to erode laterally. Incised meanders - there are two types - entrenched when the cross section is symmetrical and ingrown where the cross profile isasymmetrical. Both result from an increase in the rates of vertical erosion - this is more rapid with entrenched meanders or can be the result of the presence of more resistant rock. With ingrown meanders, vertical erosion is less rapid, allowing some lateral erosion also. Reference to specific examples is not a requirement, but would be one way of enhancing the answer. Level 1 (1-6 marks) Describes at least one landform, two at the top end in general terms -may be definition like. Begins to explain - will refer to basic processes - vertical erosion. Answer may be imbalanced - may clearly emphasise one element. Some use of appropriate terminology present at the higher end. Generic waterfalls (hard and soft layers of rock).Level 2 (7-12 marks) Description of at least two landforms is more specific and precise. Response is more balanced. Explanation is more focussed and there is a link between erosion and rejuvenation. Appropriate geographical terminology is used. Level 3 (13-15 marks) Clear, purposeful description that links to the process - a recognition of the different types of terraces, meanders. Explanation is clearly linked to the impact of rejuvenation on base level and vertical erosion. 8 Old AS (b) Why do features of deposition exist throughout the long profile of most rivers? (7 marks) • Features of deposition can occur anywhere because a river deposits its load when it does not have the competence to transport material. • In the upper course, boulders are evident on the riverbed. These will only be transported when the river is in flood, after prolonged or heavy rainfall, or after snow melt. Braiding occurs, particularly when the source of the river is a glacier. In summer, when the ice retreats the river has a high competence and carries a heavy load. As temperatures cool, river levels drop and deposition occurs in the channel, forming eyots. • In the middle course of a river, as it starts to meander, the fastest flow of the river occurs on the outside of the bend. On the inside of bends, where velocity is slack, banks of deposition, called slip-off slopes develop. Also when the river floods it will deposit alluvium on the banks and the floodplain. Levees may be evident. • Most features of deposition are found in the lower course. Deposition occurs after flooding, on the inside bends of meanders and also when affected by tides. If tidal waters invade the lower course, river levels alter on a daily basis. At low tide, the smaller discharge results in a lower carrying capacity and mud flats and islands of deposition might be a feature. Deltas also occur at the mouth and can be classed as features of deposition, as can levees and eyots. Level 1: ( 1-3 marks) A poorly focused answer, which does little more than describe a feature of deposition. Level 2: (4-5 marks) Understands why deposition takes place and can relate this to the middle and/or lower course. Level 3: (6-7 marks) Clearly understands the circumstances under which deposition takes place, through the long profile i.e. upper, middle and lower course.