River Channel Adjustment Answer ALL questions. The marks for each question are given (in brackets). 1. For letters A-H on this idealised river long profile diagram, state whether the fact is True (T) or False (F). (8). 2. What is a knick point and describe how one might form. (2) 3. Use diagrams to explain how river terraces form (4) 4. What factors may result in the base level of a river changing ? (2) 5. How can the sinuosity of a river be calculated ? (2) 6. Draw diagrams to illustrate how meanders develop through time (3) and discuss their formation with reference to these terms: channel thalweg; point bar; riffle; pool, helicoidal flow (5) 7. Discuss the factors which are likely to give rise to braiding in a river. (4) 8. With reference to the term dynamic equilibrium, discuss how river's adjust to the environmental conditions which prevail (8) Total = 38 Answers 1. A: T B: F C: T D: F E: T F: F G: F H: F 2. A sudden break or irregularity in the gradient along the long profile of a river. 3. A river terrace is a remnant of a former flood plain, which has been left at a higher level after rejuvenation of the river. Where a river renews its downcutting, it sinks its new channel into the former floodplain, leaving the old floodplain above the level of the present river. The terraces are cut back as the new valley is widened by lateral erosion. If renewed rejuvenation takes place, the process is repeated and a new pair of terraces is formed beneath the original ones. 4. A rivers base level may change if (think about this as a system): a. There is a long dry spell of no to little rain followed by high temperatures which increase the potential evapotranspiration (therefore reducing the base level) b. A period of heavy rainfall which will increase the base level. c. Frozen impermeable ground preventing infiltration and therefore percolation (reducing the base level and increasing surface run off). d. Deforestation reducing water uptake through roots therefore decreasing base level. e. Urbanisation and the creation of impermeable surfaces therefore decreasing base level. 5. Sinuosity (the curving nature of a meander) is measured: actual channel length divided by its straight line distance 6. The development of meander: 8. 9. The term ‘dynamic equilibrium’ describes a system where both inputs and outputs are balanced. In the context of rivers, this helps us to understand the graded profile which typically exhibits a smooth concave shape from source to mouth. W.M. Davies argued that irregularities in the long profile would reflect changes in underlying geology and are eventually worn away by river erosion. As a result, dymanic equilibrium is achieved by the interplay between the processes of erosion and deposition (and therefore energy). Subsequently, there are many environmental factors which affect these processes: 1. The gradient of the slope for example – as rivers approach their base level there is a decrease in gradient and therefore kinetic and potential energy. This reduces velocity (and overall energy) which encourages deposition and perhaps aggradation to occur (resulting in features like flood plains, levees and deltas being formed). Oppositely, the further up a river’s course where the gradient is much steeper, the more dominant erosion becomes (producing features like waterfalls which are examples of positive feedback mechanisms away from dynamic equilibrium and therefore the long graded profile). Some geographers define the graded river as that which has been attained when the river uses up all its energy in the movement of water and sediment so that no free energy is left to undertake further erosion. 2. The amount of discharge (resulting from precipitation) – during periods of heavy rainfall, the amount of water flowing in a river increases. This can increase the erosive power of rivers as their ‘competence’ and ‘capacity’ (the maximum size of calibre and total load) is also increased. This may produce a ‘knick point’ in the graded profile. Similarly, the extra material carried in transport may be deposited further downstream. Some other factors… 3. Temperature - which can affect evapotranspiration and therefore the amount of water flowing in a river (frost can make the soil impermeable increasing the likelihood of surface run off and therefore the amount of water flowing in a river. 4. Vegetation 5. Angle of slope There are many other factors but the best way to answer this and many other questions about rivers, erosion, deposition and energy is to think of them as a system where a change in one part, may evoke changes in others (inputs;processes;outputs). Aggradation is the increase in the height of the land due to deposition.