Interlocking spurs are found in the upper course of rivers in V-shaped valleys where the rivers zig-zags from side to side around ridges on either side of the channel. These ridges, or spurs, ‘interlock’ like the teeth of a zip, restricting the view up the river valley.
2. Describe and explain the formation of V-shaped valleys (4)
A V-shaped valley is found in the upper course of a river close to the source. The large bed load is moved during times of high discharge after heavy rain or snowmelt. This movement results in vertical erosion, mostly by abrasion as the bed load is rolled and dragged over the channel bed. As the channel gets deeper its banks are exposed to weathering processes which weaken them until they collapse, by mass movement, into the channel. Repeated erosion and weathering act together to create the V-shape to the valley.
3. Define what is meant by a floodplain. (2)
A floodplain is the area of flat land found either side of a river in the middle and lower course. It is the product of lateral erosion, caused by the migration of meanders, and the deposition of alluvium during floods.
4. What is meant by the term long profile of a river? (2)
The long-profile describes how a river changes in both height above sea level and channel gradient with distance from the source.
5. What is a valley cross-profile? (2)
The valley cross profile describes the shape of a river valley described across the valley at right angles to the channel. It changes from V-shaped in the upper course to more gentle with a floodplain towards the mouth.
6. Describe and explain the formation of river levees. (4)
As a river bursts its banks in the lower course the sudden loss of velocity, as friction increases, results in the deposition of course sediment along the river’s banks to create raised ridges along the banks. Repeated flooding raises these ridges until the river no longer floods and deposition occurs on the channel bed. This, in turn, raises the level of the river allowing flooding to occur once more. Repeated deposition on the ridges and then the channel bed result in the rising level of these parallel ridges, which are called levees. The rising height of the levees results in the river flowing above the level of its floodplain.
7. Explain why river sediment (the load), tends to become smaller and more rounded downstream. (4)
As a river flows from source to mouth it is able to move sediment by traction (rolling/dragging), saltation (bouncing) and suspension (Held in the water column). As large sediment is dragged along the channel bed in the upper course during times of high discharge, it collides with other sediment breaking pieces off in a process called attrition which reduces the bed load in size. The fine pieces of rock may be moved by saltation or suspension, themselves acting like liquid sandpaper an eroding the bed load by abrasion. Abrasion on the base of the bed load as it is moved by traction along the river bed, combined with abrasion from the suspended load, results in the flattening and smoothing of the bed load
8. Describe the features of a river meander. (4)
River meanders are bends in a river’s channel which form as a river develops its floodplain in the middle course. The inside of a meander bend, which river flow is slowest, is characterised by deposition. Deposited bed load forms a gently shelving slip-off slope. This appears above the mean water level as a river beach. On the outside of the bend, where river flow is fastest, abrasion and hydraulic action erode a deeper channel and a steep river cliff.
9. With the aid of a diagram(s) describe the formation of an oxbow lake. (6)
In the middle course of a river, meanders begin to form as the river laterally erodes its floodplain. Erosion by the fastest flow on the outside of meander bends, coupled with deposition on the inside, act to make the river more sinuous over time. In time, the bending channel becomes a swan-neck meander until the river cuts through the narrow neck of the meander creating a new straighter channel. The fastest flow now flows down the middle of the channel and deposition seals off the old meander loop, creating crescent-shaped oxbow lake.
10. Floodplains, in the middle course of a river, are the product of both erosion and deposition. Discuss the role of both these processes in the creation of floodplains. (6)
Floodplains are flat, or gently shelving land found on either side of a river channel in the middle and lower course. Initially, the floodplain is formed as meanders begin to migrate downstream as a result of erosion on the outside of bends, coupled with deposition on the inside. The meanders become increasingly sinuous and, as the channel becomes wider, the size of each meander bend increases. This results in a wider and wider area of flat land either side of the channel marked at the edge by a ridge or bluff. During a flood, when the river bursts its banks, it deposits fine alluvium across the plain. Repeated flooding slowly builds up the level of the floodplain.
11. Describe and explain the changes in a river’s long profile and valley cross profiles along a river's course. (8)
As a river flows from source to mouth it drains a large and large area of land in its drainage basin. This means it will usually carry more water and, as a result, have more energy with which to transport sediment. In the upper course, the bed load is large and can only be moved during times of high discharge by traction. When this occurs the river bed is deepened helping creates V-shaped valleys and a steep drop in the gradient of the long profile. As erosion of the bed load creates smaller load, erosion begins to occur on the river banks, creating lateral erosion. The rate of downcutting is reduced making the long-profile gentler. Lateral erosion leads to the formation of river cliffs, while deposition creates river beaches in an asymmetrical channel. As meanders migrate downstream they create a wider and wider floodplain. In the lower course sediment has become reduced in size and is easily transported in the wide, deep channel. In the lower course deposition often occurs when the river bursts its banks. This creates levees as well as floodplains covered in layers of alluvium.
12. Rivers tend to flow faster in their lower course despite a reduction in the channel’s gradient. Explain the reasons for this change in velocity. (4)
As a river flows towards the sea the long profile experiences a reduction in gradient as lateral erosion replaces vertical erosion, in part because of a reduction in bed load size. However, at the same the same time the river experiences an increase in discharge as it drains a larger and larger area of land within its drainage basin. The greater volume of water results in a deeper and wider channel which is more efficient and has less friction. This reduction in friction is enough to outweigh the reduction in gradient and, as a result, the river flows faster towards the mouth.
