What is the long river profile?

The gradient is less steep than in the upper course. The valley gets wider and flatter. Erosion is more lateral (or sideways) than vertical. The river has surplus energy which it uses to transport material and erode. Meanders are formed. Facts about the middle course.

Processes at work From the source to mouth of a river three processes are taking place River Erosion River Transport River Deposition River Erosion River Transport River Deposition

Traction Heavy rocks and boulders are rolled along the river bed. Happens most in times of flood, when the current is strongest.

Saltation. Small stones and pebbles are ‘bounced’ along the river bed. Saltation can take place when the river flow is less than that needed for ‘Traction’ to take place.

Suspension Very small particles of sand or clay that are ‘suspended’ in the water. These particles will ‘settle’ if kept in a jar of water overnight and the water will look clear.

Solution. Takes place when material is dissolved in the water, it is invisible and does not colour the water. Occurs often in limestone landscapes where the water if very acidic. Some pollutants like weedkiller are also held in solution in the water

Methods of transport Copy this picture into your books and add the title above and labels for each type of transport Traction Saltation Suspension Solution Shows the rate of flow needed

River Valley Development A 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 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

Which stage? How do you know?

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?

Middle-course features Meanders In 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 the outside of the bend where water flow has most energy. On the inside of the bend, where the river flow is slower, material is deposited.

Watch the video clip of the middle course… Describe what the middle course of the river is like. How is it different to the upper course?

Meander = a bend in a river

Meander formation esources/displayframe. php?src=309/consulta nts_resources%2F_file s%2Fmeander4.swf

Copy the image:

Add these labels to your diagram…

This is a cross section of a meander bend. Sketch the diagram and mark on the following: slip off slope river cliff area of deposition undercutting fastest flow

Deep, fast water and erosion on the outside of the bend Slow, shallow water and deposition on the inside of the bend

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Extension… Describe and explain (and use diagrams? ) how this meander will turn into an ox bow lake.

Plenary Add all of the information learnt today about the middle course to your profile of the river….