Base level is the lowest level to which erosion by running water can take place
With the aid of diagrams, (i) explain what is meant by ‘base level’ and how this may change. (6 marks) (ii) describe the effects that changes in base level may have on a river valley. (15 marks)(i) Base level is the lowest level to which erosion by running water can take place (1 mark). This is generally accepted as sea level, but local base levels can arise when rivers enter an inland sea or lake, where a tributary enters a bigger river or where a resistant band of rock crosses a valley (1 mark). Base level (1 mark for diagram) Changes in base level can be influenced by (i) climatic conditions and (ii) tectonic activity (1 mark). These cause either a positive or negative change in base level. There will be a positive change in base level when there is a rise in sea level from the melting of glaciers or sinking of the land (1 mark). Also lengthy periods of increased rainfall may cause increased deposition further down the valley and cause a positive change in base level (1 mark). A negative change in base level may result from a fall in sea level relative to the land or a rise in the land in relation to the sea (1 mark).(ii) When there is a positive change in base level, sea level rises in relation to the land or the land sinks in relation to the sea. This will result in a decrease in the gradient of the river valley and a rise in base level. When this happens there will be an increase in deposition (1 mark) and the drowning of the lower sections of the river valley (1 mark). This may also cause the flooding of coastal areas. Increased deposition may cause features such as lakes to become filled (1 mark) which will smoothen the long profile of rivers (1 mark) creating a more concave profile (1 mark).A negative change in sea level occurs when sea-level falls in relation to the land or the landrises in relation to the sea. This movement causes the land to emerge from the sea, steepeningthe gradient of the river (1 mark) and therefore increases the rate of erosion due to the increasein the potential energy of the river (1 mark). This causes the river to rejuvenate. This causes achange in the graded profile of the river as rejuvenation upsets the equilibrium between erosionand deposition (1 mark). A waterfall may develop in the river valley if the rise in the land ismore rapid than the river’s ability to vertically erode to the new base level (1 mark). Riverterraces may also develop as vertical erosion has caused former flood plains to be left highabove the level of the river.
DEVELOPMENT OF KNICKPOINTS, INCISED MEANDERS AND RIVER TERRACESA knickpoint is a term in geomorphology to describe a location in a river or channel where there is a sharpchange in channel slope, such as a waterfall or lake, resulting from differential rates of erosion above and belowthe knickpoint.Knickpoints can also be caused by a change in base level. When a river is rejuvenated (that is, has a negativechange in base level, the sea level falls), the long profile is lengthened as land rises from the sea. This increasesthe gradient of the long profile and causes the river to gain more erosive energy. The first knickpoint marks theposition of the former base level but the knick point moves towards the source of the river as under rock erodesaway to create a graded profile. The river, having gained more potential energy due to gravity, will then proceed to work the knickpoints out of its system by either erosion (in the case of waterfalls) or deposition (in the case of lakes) in order for the river to re-attain its smooth concave graded profile. Knick Point Increased potential energy due to increase in gradient from the upliftment of land or fall in the level of the sea.River terraces are simply where vertical erosion occurs in a floodplain that was previously being formed by thenormal conditions of deposition and lateral erosion. The river cuts downwards and abandons the old flood plainas a river terrace - ideal for building settlements upon! Over time once the profile is regraded, the river mayrecommence eroding laterally and depositing sediment, resulting in a secondary flood plain formation. Wherethis occurs and the river is meandering, the river can maintain its meandering form but its energy is focusedupon vertical erosion. This can result in spectacularincised meanders, as found in the Grand Canyon. River Terraces Incised Meanders
A river uses energy to carry out its work of erosion, transport and deposition of sediment and to move the water in its channel. Thisenergy is produced when water flows down a slope, so the height of a river above sea level determines this. The lowest level of a riveris called the base level, and for most rivers it is sea level. The long profile of a river is a graph drawn along the course of a river fromthe source to the mouth. The study of river long profiles shows that they have a concave shape, with a steeper upper reach and agentler lower reach. River processes are related to long profile because every river is trying to achieve a smooth, concave, long profile.This ideal profile is called the graded long profile and in this ideal situation, the available energy and the river processes will bebalanced. Where this is not the case, the river will be working to smooth out its long profile by erosion, transport or deposition.
Graded streamNear the source of a river there is more vertical erosion as the river flows downhill, using its energy toovercome friction (A). As a result the channels are narrow and shallow and may contain large boulders andangular fragments eroded and weathered from the steep valley sides. The sediment in the river createsturbulence and friction.As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performsmore lateral erosion making the channel wider, and smoother (B) and (C). As a result there is less turbulenceand friction, making the flow of water more efficient.