River Processes Lesson Aims To learn what is erosion, transportation and deposition. To learn which processes carry out erosion, transportation and deposition in river systems. Within rivers a number of different processes occur which enable the river to alter the landscape. These processes can be grouped under three different headings. Erosion is where parts of the land are worn away. The parts that are worn away will then be transported somewhere else where they can be deposited as new land. A) Write definitions for the terms erosion, transportation & deposition. Erosion happens through four main processes. Hydraulic action is the force of moving water. The pressure of moving water and the friction of water on particles, forces loose particles to move from the banks and bed of a river. This is like aiming a hosepipe at a pile of sand. The faster the water comes out of the tap then the more that the sand is moved about. Abrasion (also called corrasion) is where the rocks and sand carried by a river rub against the banks and bed. A flooded river may carry rocks weighing more than a ton in weight. Imagine the amount of erosion a rock weighing over a ton and travelling at twenty miles an hour will achieve. 12
Because of the power that abrasion can generate in times of flood it is usually the biggest agent of erosion in rivers. Attrition is where the particles of sand and rock carried by a river are smashed into smaller pieces. As a result particles normally become smaller as a river moves downstream. Solution (or corrosion) is where rocks are chemically dissolved. Some rocks such as limestone dissolve easily but almost all rocks are soluble to some extent, especially in hot climates. B) Explain the terms hydraulic action, abrasion, attrition & solution as agents of erosion. C) Use the slide/picture to help you to draw an annotated sketch, which explains the different methods of river transport. Transportation is the movement of eroded material by the river. The material moved by the river is called the load. The load may be rolled along the bed this process is called traction. Some material bounces along hitting the bed and then bounces back into the water column. This bouncing is called saltation. The material moved by traction & saltation is called the bedload. Some particles are small enough to be held above the bed by the force of flowing water. This process is called suspension and the material carried in suspension is called the suspended load. The water in solution carries rocks that have been dissolved. This process is called solution and the load carried is called the dissolved load. 13
Deposition is where the material transported by a river is dropped to form new land. Material such as sand forms beaches and sand bars. Clay particles form mud -flats and rocks and pebbles may form banks and small rapids in a river called riffles. Deposition occurs when the water slows down and can no longer move particles. This is called settling. D) Explain how deposition occurs. Where a river flows fastest erosion is the most important process. As it slows down transport becomes more important. When the river flows slowly deposition will become the most important process. The velocity (speed) of a river is caused by the gradient (angle) of the riverbed. The steeper the gradient then the faster the river flows. E) Look at your long profile of the river Wye. Label where you think erosion is the most important process, where transportation is the most important and where deposition is the most important 14
Along the course are many different landforms. Some are caused by erosion and some are caused by deposition. Some landforms are caused by a combination of both erosion and deposition. Close to the source the gradient of the bed is steep and erosion is usually the most important process. Geographers call this the upper section (upper course) of a river. As the gradient becomes slower the main process changes to deposition (however erosion and deposition occur, often balancing each other out) this stage is called the middle section (middle course). Close to the mouth the gradient is nearly flat so the river flows very slowly, deposition now becomes the most important process. This stage of a river is called the lower section (lower course). Geographers expect to find certain types of landform in each section (but there are always exceptions). WATERFALLS Occur in places where a layer of hard rock is above a layer of softer rock. The soft rock is eroded away more quickly than the hard rock. So in places where hard rock meets softer rock in a riverbed a vertical drop (called a waterfall) is often found. 15
Lip A) Watch the video clips carefully. Listen to what your teacher tells you and look carefully at the presentation. Draw a detailed annotated diagram of a typical waterfall. Label carefully and explain the importance of the lip, plunge pool, hard rock and soft rock. You will be expected to be able to do this in your assessment! 16
B) Using annotated diagrams to help you write a detailed explanation of how a typical waterfall will form. Use terms such as corrasion in your answer. C) How do waterfalls lead to the formation of gorges? 17
Learning Outcome Communication to work as part of a group and give a presentation to the class. CHALLENGE Learning Outcome D) To learn how waterfalls and gorges form. Challenge In your group give a presentation on how waterfalls and gorges form. You need to explain why waterfalls happen, what are the main parts of a waterfall and how waterfalls change over time to create a gorge. Product Criteria Rule Criteria o You will have 35 minutes this lesson and 30 minutes next lesson to prepare. o Presentation will be supported by visual material. o All group members must participate. Form Criteria o Must contain images. o Presentations should be colourful o Presentations should be neat. Content Criteria o Why do waterfalls form? o What are the main parts of a waterfall? o How does a waterfall change over time to create a gorge? Process Criteria o Working in groups o Communication discussing o Communication presenting Skill Knowledge interprets information and applies to the challenge. Collaboration working together. Disposition Specific observable behaviours o All group roles are being filled. o Everybody is involved and has a job to do. o During presentations each group member is a quality audience. 18
Waterfalls and Gorges Assessment for Learning On a poster describe and explain the formation of waterfalls and gorges - make sure that you now what WILF is looking for. WILF SAYS Level 2 Level 3 Level 4 Level 5 Level 6 You draw a basic sketch of a waterfall or write a sentence describing what a waterfall is. You give a very simple description of a waterfall labelling its main features or writing a very basic series of sentences. You start to recognise and describe the processes that led to a waterfall. You begin to offer explanations for the processes that lead to the formation of waterfalls and possibly gorges. You give/offer explanations for the formation of waterfalls and gorges. You begin to show that they are not static formations but change over time. Level 7 You give a detailed explanation using a range of appropriate terminology correctly. You describe changes over time such as undercutting and retreat leading to gorges or reduction in height. Level 7+ You can t really access level 8 on this exercise but if WILF feels that your answer is better than level 7 then this would show that you are close to if not achieving level 8 standards. You would show very accurate use of terms such as hydraulic action or abrasion rather than erosion and naming examples, which are a feature of good geography answers could all lead WILF to feel that you are at this level. 19
Meanders In the middle section of a river the gradient decreases so the speed of the water also decreases. This means the river does not have enough power to carry large rocks, which are the main agents of vertical (downward) erosion. Transportation is now the dominant process. However there is still some erosion where the river flow is fastest. Water flow is fastest on the outside of bends. The water is thrown outwards as it goes around a bend just like we are when a car goes around a bend. This means that there is sideways erosion on the outside of bends. This is called lateral erosion. A) Explain why river valleys stop getting deeper in the middle course. B) Explain why river valleys get wider in the middle course. If the water on the outside of the bend flows the fastest then the water on the inside of the bed flows the slowest. This means that deposition occurs. So the outside of a bend is eroded while the inside has stones, sand and gravel deposited. In this way the bend can become shaped like a giant loop. It will have a steep river cliff on the outside where undercutting of the bank by erosion leads to loss of land. The inside will be made of a gentle beach made up of deposited material called a point bar. The actual bend/loop is called a meander. C) Explain the terms river cliff, point bar & meander 20
D) Draw a cross-section of a meander. E) Annotate your diagram to explain where/why erosion and deposition occurs. 21
F) Draw a series of annotated diagrams to explain how meanders and ox-bow lakes are formed. Floodplains The dominant process in the lower course of a river is deposition. The river flows in a wide flat valley, which has been made in the past by lateral erosion. In normal flow the river meanders across its valley and point bars build up on the inside of the meanders. These help to build the wide flat areas of land in the river valley. When the river floods over its channel banks, sediment is deposited over the valley bottom. Over time (many floods) this sediment builds into a thick, flat plain which is often very suitable for farmland. Close to the valley banks the coarsest sediment is deposited first and this can lead to natural raised banks called leveés. This area of new land formed by deposition is called a river flood plain. A) Draw an annotated diagram to explain how a flood plain forms. B) Explain why flood plains are mainly found in the lower courses of rivers. 22