Unit 1.3B_4 River Landforms

Now you know a bout the processes that enable the river to change What does the river look like?

A typical long profile: 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 (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: upper middle lower                                                                                                                                                                                                          

Upper Course Characteristics: Deep narrow valley (V shaped) Interlocking spurs Fast flowing river (erosion predominant) Pot holes in the river bed Rapids and Waterfalls The main process at work is the downward or vertical erosion of the stream bed. This cuts into the landscape deepening the valley sides.                                                                                       

In the Upper Course You see V-shaped valleys, interlocking spurs, waterfalls, gorges and rapids. This because the erosion takes place downwards only - Vertical erosion This mainly by abrasion and hydraulic action, making valley deeper Most of the transport is by traction and saltation. As the main work is vertical erosion, the water winds it way around obstacles, weaving from side to side – the point on the valley side ahead is worn away, increasing the cut into the valley side – hense V-shaped valley with interlocking spur

Potholes: 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.                       

RAPIDS

In summary                                                                    

Fast flowing water has more energy River cliff forms Slip-off slope

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

As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and narrower.

Oxbow lakes Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander. The water now takes its shortest route rather than flowing around the bend. Deposition gradually seals off the old meander bend forming a new straighter river channel. Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake. Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up, the feature left behind is known as a meander scar.

Levees (thanks to Mark Bailey)

What are they? Often several metres higher than the river’s floodplain, they are high natural embankments of silt situated close to the river channel. Where are they found? They are most commonly found in the lower course of a river where there is a floodplain, along rivers that flood periodically, have a slow flow and carry large amounts of load.

How are they formed? The river before the flood at regular height. The river whilst flooding

There is an increase in friction between the water leaving the channel and the floodplain when a river overflows. The water is shallower on the river banks and valley floor, thus this is why deposition of load occurs. After repeated flooding causing the deposition of silt onto the banks, this eventually settles which over time builds up to form a levee. The coarser material is deposited first, and is positioned closer to the river. After repeated flooding, the river bed and levees are raised due to the constant process of the deposition of silt. Over time, this eventually causes the river to rise above level of flood plain. This is called an ‘aggraded bed’.

The end of the river Eventually the river flows into a lake or a sea or an ocean. If the water it flows into has little current, any remaining silt is dropped as it enters and there is no current to remove it and a delta forms. Mississippi Delta

But in the UK … There are plenty of currents to wash away any silt that is dropped and so our rivers have mouths instead. I google-mapped this one. Where is it?

Unit 1.6 UK River Case History River Tees

High Force This is where it is

This is the River Tees drainage Basin What is the green line on the map called?

The source of Tees He is Matthew – could not find a picture without him! See the beginnings of the V-shaped valley and interlocking spurs

The source of River Tees This is a cross section What you do is get a strip of paper and mark each occasion a contour line crosses and label it Then on graph paper make a scale for the contour lines up the side The distances are marked off along the bottom and plot the result

So now for the words! The River Tees is located in north-east England. Its source area is high in the Pennines in the west and the river flows eastwards into the North Sea. The source of the River Tees lies on Cross Fell in the Pennines, 893m above sea level, where rainfall is over 2000mm a year. Run-off is high because of the impermeable rocks and the steep slopes. The valley cross-section is steep sided and V-shaped and the long profile has a steep gradient. The river occupies the whole of the valley floor. The river is turbulent and clear, although often stained brown by the peat (semi-decomposed organic matter - too wet/acidic/cold for bacteria to break down) which covers much of the moorlands.

Given what you know of the river, what can you tell about the rate of flow through the drainage basin?

While still in the hills, the valley has been dammed to produce water for the people in Teesdale to drink This is Cow green Reservoir High Force Besides providing water for the Tees valley, what other advantage might having a reservoir on the Tees be?

High Force The River Tywi near its junction with the River Doethie below Twm Sion Cati's cave near Rhandirmwyn

About High Force The river bed is rocky and there are many rapids and a waterfall at High Force It is the highest waterfall in England, with a very deep plunge pool at its base , The cap rock is made of a very resistant rock called whinstone. Below the whinstone there are bands or sandstone and shales as well as some very chin coal seams These rocks are less resistant and erode more easily creating an overhang in the cap rock. Over many thousands of years the waterfall at High Force has retreated upstream, creating an impressive gorge of recession.

How High Force was formed

The lower course Moving downstream the valley begins to widen and the river starts to meander . There are more bridging points and larger villages and towns, such as Yarm. Nearing the river mouth the river meanders in large loops across its flat tood plain (it is 30km as the crow flies from Darlington to Teesmouth but the river travels 75km) It used to be longer but several of the meanders were cut off in the nineteenth century to shorten the journey for boats navigating the river up to Stockton and Yarm. At Teesmouth the nver flows into an estuary where there are huge areas of mud flats such as Seal Sands. These are important wildlife areas for migratory birds and seals.

The lower course The lower course is much more urbanized and industrialized than the upper sections, with large towns such as Stockton and Middlesbrough and the vase port of Teesside. Industries such as chemicals, ships, steel-making and engineering developed during the Industrial Revolution. Today, Teesside is a major centre for the ICl petrochemicals industry based ac Billingham and Wilton, and is well placed to receive oil and gas from the fields in the North Sea. Shipbuilding has been replaced by oil platform construction and there is a huge modern integrated steelworks (raw materials are converted to finished goods in one factory) at Redcar. The heavy industries (bulky/large raw materials and finished products have taken advantage of the flat and relatively cheap expanses of mud flats in the estuary. The land is easily reclaimed and there is a nearby labour supply and good transport: networks. The port also provides a sheltered harbour for the import of raw materials and the export of finished goods.

Further on down the river meanders its way across the plain Eventually the river will cut through – what is that called?

The sea comes up the river at the mouth The sea comes up the river at the mouth. At low tide the edges are exposed. That leaves mudflats at the sides of the river. The land here is mainly used for industry, like docks and factories that need to import or export heavy goods by sea.

Estuaries The River Tees has an estuary. This is a wide, deep mouth. Estuaries are really useful for shipping, so they usually have ports and factories along them.