River processes Groundwater Groundwater refers to subsurface water. The upper layer of the permanently saturated zone is known as the water table. The water table varies seasonally — in Britain it is higher in winter following increased levels of precipitation. Most groundwater is found wiehin a few hundred metres of the surface but has been found at depths of up to 4km beneath the surface (Figure 9), an hurd regions quite Major Intermittent Neal charge Unsaturated Artesian discharge aoe echarge ea stnor “~ perennial iseharge bin semiarid regions Aquifer techarge aa Minor perennial dlscharge ates Figure 9 Groundwater Groundwater accounts for 96.5 per cent of all freshwater on the Earth. However, while some soil water may be recycled within a matter of days or weeks, groundwater may not be recycled for as long as 20000 years. Hence, in some places, groundwater is considered a non-renewable resource. Aquifers (rocks that contain significant quantities of water) provide a great reservoir of water. Aquifers are permeable rocks such as sandstones and limestones. This water moves very slowly and acts as a natural regulator in the hydrological cycle by absorbing rainfall which otherwise would reach streams rapidly. In addition, aquifers maintain stream flow during long dry periods. Rocks which do not hold water are impermeable rocks which prevent large-scale storage and transmission of water, such as clay. ‘The groundwater balance is shown by the formula: AS = Qr-Qd where AS is the change in storage (+ or -), Qr is recharge to groundwater and Qa is discharge from ‘groundwater. Groundwater recharge occurs as a result of afiltration of part of the total precipitation at the ground surface seepage through the banks and bed of surface water bodies such as rivers, lakes and oceans groundwater leakage and inflow from adjacent aquicludes and aquifers «© artificial recharge from irrigation, reservoirs ete Losses of groundwater result from: ¢ evapotranspiration particularly in low-lying, areas where the water table is close to the ground surface ¢ natural discharge by means of spring flow and seepage into surface water bodies groundwater leakage and outflow through aquicludes and into adjacent aquifers ¢ artificial abstraction, for example in the London basin in the UK. @ River processes Erosion “The main types of erosion include: © abrasion (or corrasion), the wearing, away of the bed and bank by the load carried by a river © attrition, the wearing away of the load carried by a river which creates smaller, rounder particles @ hydraulic action, which is the force of air and water on the sides of rivers and in cracks solution (or corrosion), the removal of chemical ions, especially calcium, which causes rocks to dissolve. ©2.2 RIVERS ‘There are many factors affecting erosion. These include: ‘© load — the heavier and sharper the load the greater the potential for erosion velocity and discharge ~ the greater the velocity and discharge the greater the potential for erosion © gradient — increased gradient increases the rate of erosion © geology — soft, unconsolidated rocks, such as sand and gravel, are easily eroded ‘© pH — rates of solution are increased when the water is more acidic ‘© human impact ~ deforestation, dams and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion. Transport ‘The main types of transport in a river (Figure 10) include: © suspension — small particles are held up by turbulent flow in the river « saltation — heavier particles are bounced or bumped along the bed of the river «solution ~ the chemical load is dissolved in the water traction — the heaviest material is dragged or rolled along the bed of the river @ flotation leaves and twigs are carried on the surface of the river © Mostly cay and sit © Mostly sand © mostly gravel and cobbles Bank calving Slope Figure 10 Types of transport in aver Deposition Deposition occurs as a river slows down and it loses its energy. Typically, this occurs as a river floods actoss a floodplain or enters the sea, or behind a dam. It is also more likely during low flow conditions (such as in a drought) than during high flow (flood) conditions ~ as long as the river is carrying sediment The larger, heavier particles are deposited first, the smaller, lighter ones later. Features of deposition include deltas, levées, slip-off slopes (point bars), ‘oxbow lakes, braided channels and floodplains. Activities 4a Briefly describe the four main ways in which rivers erode. b Suggest how they will vary with (velocity of water (i) rock type and i) pH of water. 2.2 What are the main types of transport? 'b_ How might the type and quantity of the rive’ load vary between flood conditions and low flow canditions? The long profile A number of processes, such as weathering and mass, movement, interact to create variations in eross- and long profiles (Figure 11). Irregularities, or knick- points, may be due to: © geological structure, for example hard rocks erode slowly, which can result in the formation of waterfalls and rapids «variations in the load, for example when a tributary with a coarse load may lead to a steepening of the gradient of the main valley @ sea level changes ~ a relative fallin sea level will lead to renewed downcutting which enables the river to erode former floodplains and form new terraces and knick-points,River processes cross-section Upper valley | Middle valley Lower valley 8888 ‘Altitude (rn) for cross-section Kick point Middle Height Figure 11 Long and cossprofies Rivers tend to achieve a condition of equilibrium, or grade, and erode the irregularities. There is a balance between erosion and deposition in which a river adjusts to its capacity and the amount of work being, done. The main adjustments are in channel gradient, leading toa smooth concave profil. Cross-profiles ‘The cross-profile of the upper part of a river is often described as V-shaped (Figure 11). Rivers in their upper course typically have a steep gradient and a narrow valley. The rivers are shallow and fast flowing. There is normally much friction with large boulders, and much energy is used to ‘overcome friction. The processes likely to occur are vertical erosion, weathering on the slopes, mass movement and transport. Features likely to be found lude waterfalls, rapids, potholes, gorges and interlocking spurs. In the middle course of the river, the valley is still ‘V-shaped but is less steep. Slopes are more gentle. A floodplain is beginning to form and meanders are visible. Processes in the middle course include erosion (both vertical and lateral), meandering, transport, and some deposition on the inner bends of the meanders. In contrast, in the lower course the cross-profile is much flatter. Processes include erosion (on the outer banks), transport and deposition (especially on the inner bends and on the floodplain). Characteristic features include levées, oxbow lakes, floodplains, deltas and terraces Features of erosion Localised erosion by hydraulic action and abrasion, lly by large pieces of debris, may lead to ormation of potholes (Figure 12). These are typically seen in the upper course of a river when the load is larger and more rugged. Waterfalls frequently ‘ur on horizontally bedded rocks (Figure 13). ‘The soft rock is undercut by hydraulic action and abrasion. The weight of the water and the lack of support cause the waterfall to collapse and retreat. Over thousands of years the waterfall may retreat enough to form a gorge of recession (Figure 14). Where there are small outcrops of hard and soft rock, rapids may develop rather than a waterfall Potholes formed by abrasion Figure 12 Formation of potholes Figure 13 Formation of waterfalls2.2 RIVERS Resistant rock ayer leaving a gorge of recession downstream. The ew ton Niagara Gorge is 11 km long due to the retreat of above Niagara Falls (Figures 15 and 16). Niagara Falls Course — Waterfall Former position of waterfall Section Figure 14 Formation of a gorge of recession Gorges and waterfalls Gorge development is common, for example where the local rocks are very resistant to weathering but susceptible to the more powerful river erosion Figure 15 Nagar Fas onthe USICanaan border Similarly, in arid areas where the water necessary for ‘weathering is scarce, gorges are formed by episodes of fluvial erosion. A rapid acceleration in downcutting is also associated when a river is rejuvenated, again creating a gorge-like landscape. Gorges may also be formed as a result of, Most of the world’s great waterfalls are the result of the undercutting of resistant cap rocks, and the retreat or recession that follows. ‘The Niagara river flows for about 50km between Lake Erie and Lake Ontario. In that distance it falls just 108 metres, giving an average gradient of 1:500. @ antecedent drainage pattern, for example the Rhine However, most of the descent occuts in the 1.5 km gorge above Niagara Falls (13 metres) and at the Falls « collapse of underground caverns in carboniferous themselves (58 metres). The Niagara river flows in a limestone areas, for example the River Axe at 2km wide channel just 1 km above the Falls, and then Wookcy Hole, UK into a narrow 400 metre wide gorge, 75 metres deep © retreat of waterfalls, for example Niagara Falls and 11 km long. Within the gorge the river falls a (Figure 15). further 30 metres. ‘The course of the Niagara river was established about 12000 years ago when water from Lake Erie began to spill northwards into Lake Ontario. In doing so, it passed over the highly resistant dolomitic (limestone) escarpment. Over the last 12000 years the Falls have retreated 11 km, giving an average Plunge flow occurs where the river spills over a sudden change in gradient, undercutting rocks by hydraulic impact and abrasion, thereby creating a waterfall, There are many reasons for this sudden change in gradient along the river: @ a band of resistant strata, such as the resistant rate of retreat of about I metre /year. Water velocity limestones at Niagara Falls accelerates over the Falls, and decreases at the base of © a plateau edge, such as Livingstone Falls on the the Falls. Hydraulic action and abrasion have caused ‘Congo river in D. R. Congo the development of a large plunge pool at the base of @ a hanging valley, such as at Glencoyne, Cumbria in the Falls, while the fine spray and eddies in the river the UK help to remove some of the softer rock underneath the #@ coastal cliff, such as ar Kimmeridge Bay, Dorset (UK). resistant dolomite. As the sofier rocks are removed, the dolomite is left unsupported and the weight of the water causes the dolomite to collapse. Hence the waterfall retreats forming a gonge of recession, The undercutting at the base of the waterfall creates a precarious overhang which will ultimately collapse. “Thus a waterfall may appear to migrate upstream, ©