3The River System – the drainage basin. A river system is an open system. An open system has inputs, processes and outputs. So unlike water in the hydrological cycle, where there is an endless cycle of rain, rivers, sea and evaporation, water in the river system enters the system, and eventually leaves it again.A river system operates inside a drainage basin (or river basin). A drainage basin is the area of land drained by a river and its tributaries. Any water falling inside the drainage basin will find its way into the main river channel (unless it is taken out of the system - for example by being piped to a factory). The boundary of a drainage basin is called the watershed. The diagram below shows a drainage basin. Make sure you know all the key terms used when talking about river systems.The River System – the drainage basin.
4Processes at workFrom the source to mouth of a river three processes are taking place River Erosion River Transport River DepositionRiver ErosionRiver TransportRiver Deposition
5River Erosion Processes: Attrition Processes:Corrasion, (abrasion) is the grinding of rock fragments carried by the river against the bed and banksof the river. This action causes the channel to widen and deepen. This grinding is most powerful in floodtime when large fragments of rock are carried along in the river bed. AttritionAttrition is the collision of rock fragments in the water against one another.The rock particles are broken into smaller pieces and become smoother thelonger the process continues.River Erosion
6River Transport Solution Solution (corrosion) is the process by which river water reacts chemically with soluble minerals in the rocks and dissolves them. An additional process is also at work caused by the force of the water itself known as Hydraulic action. In this rocks are dragged away from the bed and banks by the force of the running water. When water from a fast moving stream enters minute cracks in a rock, the pressure exerted weakens and eventually breaks up the rockRiver Transport A river carries or transports eroded materials such as mud, sand, boulders and dissolved materials on its journey. These materials are known as its load. The load is carried along by three processes.tractionsuspensionsolutionTractionIs where boulders or stones are rolled along the stream bed by the force of thewater
7River Deposition Suspension Fine particles such as clay, silt and fine sand are carried along in the river.SolutionDissolved materials containing minerals like calcium and sodium are carried in the water and are invisible to the naked eye.River Deposition A river drops its load when the speed or volume of the river decreases. The load, which it carries, is deposited. The heavier material is deposited first and the finer material carried further. Rivers reduce their speed when they enter flat land, enters a lake or the sea or reach an arid area. The volume can be reduced during a dry season or when the river passes over more porous landscapes e.g. sand or limestone.
11A typical long profile: River Valley DevelopmentA river has a life cycle from source to mouth. The processes at work cause changes to the river valley and river features in the landscape that it passes over.Traditionally this cycle has been described in 3 stages, the Upper (Torrent) section, the Middle (Valley) section and the Lower (Flood Plain) section. The terminology's Youthful, Mature and Old have also been used to describe the sections.The stages are not clearly defined but merge from one to the other and rivers show individual characteristics. There are river features that can be defined within a broad pattern.A typical long profile:uppermiddlelower
172. How do you think the channel shape changes here? 1. How much energy do you think the river has here? How do you think the river uses its energy here?2. How do you think the channel shape changes here?3.In which direction is the river eroding here? How do you know?4. Do you think the river is at its fastest here? What do you think happens to velocity as you travel downstream?5. What do you think the rivers load is like here? Why? Where does its load come from?6. What processes of erosion do you think may be happening here?7. How do you think the river transports here?8. Do you think deposition occurs here? If yes when?9. What landforms do you find in the upper stage?
18Upper Course Characteristics: Deep narrow valley (V shaped) Interlocking spursFast flowing river (erosion predominant)Pot holes in the river bedRapids and WaterfallsThe main process at work is the downwardor vertical erosion of the stream bed. Thiscuts into the landscape deepening the valleysides.
19Interlocking SpursAs the water twists around obstacles such as rocks and boulders the erosion forces undercut outward bends and a snaking pattern is produced. From lower down the interlocking of the spurs of land between the bends blocks the view up the valley. V-shaped valley with interlocking spurs
20Click on the link below to see this animated. Interlocking Spurs
22Potholes:Potholes are formed by corrasion. Pebbles carried by the river are swirled around on the riverbed. This action erodes the rock on the riverbed forming potholes. Over time, they may widen and join with other potholes to form larger potholes, and the whole riverbed is deepened.
24Rapids:Rapids are stretches of fast-flowing water tumbling over a rocky-shallow riverbed. Different resistance among rocks cause rapids. They are formed when the water goes from one hard rock that resists the water's erosion to a softer rock that is easier eroded.
26Waterfalls:Waterfalls occur when a band of hard rock lies across the river with softer rock downstreamwhich is more rapidly eroded. At first rapids would form but then develop into a waterfall as thesofter rock erodes further. Variations may be caused by the rock structure.(1) Rock layer is horizontal(2) Rock layer dips upstream
27Click on the link below to see this animated. (3) Rock layer is verticalClick on the link below to see this animated.
32Middle-course features MeandersIn the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster:As the river erodes laterally, to the right side then the left side, it forms large bends, then horseshoe-like loops called meanders.The formation of meanders is due to both deposition and erosion.The force of the water erodes and undercuts the river bank on theoutside of the bend where water flow has most energy.On the inside of the bend, where the river flow is slower, material is deposited.
33Watch video of meander formation! What happens at a meander?Watch video of meander formation!