RIVERS BEGIN AT THEIRiSOURCE, high above sea

level, and flow downhilltowards their mouth, usually thesea. For ease of study we dividethe river into three sections: theupper, middle and lowercourses. In each of thesesections typical landforms canbe seen a result of the processesat work. This unit investigatesthese landforms and processesalong the River Tees in north-east England.

Figure 1 shows the drainagebasin of the River Tees. Thesource lies high up in thePennines close to Cross Fell(893 metres above sea level).The river flows east to its mouthin the North Sea. Along its waythere are many tributaries. Theyjoin the river at a confluenceand so the river gets wider anddeeper downstream. Changes inthe river downstream are also aresult of changes in theprocesses at work – erosion,transport and deposition.

The upper course of theRiver TeesIn the upper course the riverflows over hard, impermeablerocks. The valley has steep sidesforming a V-shape. The riverchannel is shallow and rockyand the river is turbulent andclear. In the upper course thereis the famous High Forcewaterfall (Figure 2) and gorge aswell as rapids and potholes.Erosion downwards is the mainwork of the river in the uppercourse.

Series 16 Summer issue Unit 330 River Landforms: the River Tees © 2005 Nelson Thornes GeoActive OnlineThis page may be photocopied for use within the purchasing institution only. Page 1 of 4

330

by Ann BowenRIVER LANDFORMS: THE RIVER TEES

GeoActiveOnline

GeoActiveOnline

CrossFell

BarnardCastle

Darlington

Yarm

Middlesbrough

High ForceCow GreenReservoir

NorthSea

0 20 km

R. Tees

301+ 61–300 0–60

Key

N

Height (metres)

Figure 1: The drainage basin of the River Tees

Figure 2: The waterfall at High Force

There are four main processes oferosion in the upper course:• Hydraulic power – the force of the

water on the bed and banks of theriver.

• Corrasion – the sand and gravelcarried by the river grinds away atthe bed and banks of the river.

• Corrosion – some soluble mineralssuch as calcium carbonate inlimestone dissolve in the riverwater.

• Attrition – the stones, pebbles andother materials (load) carried by theriver rub against each other makingthe particles smaller and morerounded as they move downstream.

The waterfall and gorge at HighForceHigh Force is the tallest waterfallin England at 21 metres high,though this is prettydisappointing when you compareit with some of the world’sgreatest – Angel Falls inVenezuela are the world’s tallestat 979 metres.

At High Force, the waterfall andits gorge of recession have beenformed over millions of years. OnFigure 2 notice the high head ofwater, the deep plunge pool atthe base and how the rock typechanges. At the top of thewaterfall there is a very hard rock

called Whinsill, which is the caprock. Below this lie softer rockssuch as limestone, sandstone andshale. At the edge of the plungepool are large boulders and rocks.At High Force the hard Whinsilllies on top of softer limestone andsandstone. The river flows overthe harder rocks and erodes thesofter rocks below more quickly.Over a long time the softer rock isundercut and the hard cap rockoverhangs. Eventually the hardrock collapses under the force ofgravity. The great power of thewater erodes the plunge pool. Therocks resulting from the collapseof the cap rock and from theerosion of the plunge pool aremoved to the edge. As the caprock collapses the waterfallretreats upstream. This is repeatedagain and again causing a steep-sided gorge of recession to beformed. Gradually chips of rockare also broken off the top of thewaterfall and the waterfall reducesin height.

Potholes and rapids at Low ForceFigure 3 shows a pothole in theriver bed at Low Force. Heresmall pebbles have becometrapped in hollows in the rock.The river’s flow has caused thepebbles to swirl around in thehollow making it deeper anddeeper. The pebbles grind andscrape at the bedrock, eroding thehollow by corrasion. At Low

Force there are also rapids formedby smaller outcrops of Whinsill.The less resistant limestone hasbeen eroded away leaving therapids.

The middle course of theRiver TeesAs the River Tees flowsdownstream the gradient becomesless steep. The river begins toerode sideways (lateral erosion)rather than downwards and theriver begins to deposit sand andgravel. The lateral erosion meansthe river gets wider, the rivervalley gets wider and meandersbegin to form.

MeandersA meander is a bend in the river(Figure 4). On the outside bend ofthe river the water is deeper andflows more quickly. The force ofthe water (hydraulic power) andcorrasion erode the outside bendto form a river cliff. On the insidebend the water is shallower andflows more slowly. Here there isdeposition of sand and gravelforming a slip-off slope.

As the meanders get wider theyerode away the valley floor,creating a wider valley in a broadU-shape. The meanders alsomigrate or move downstream.This too broadens the flood plain,creating a line of river bluffs at theedge of the flood plain.

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Figure 3: A pothole at Low Force inUpper Teesdale

Plan view Cross-section along X–Y

XX YY

Outsidebend

Insidebend

Water level

Deepest water andfastest current

Slip-offslope

Deposition

DepositionErosionZone of strongest flow

Erosion

Rivercliff

Key

Figure 4: Plan view and cross-section of a meander

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The lower course of theRiver TeesClose to Yarm the River Tees hasformed very large meanders.Some of these meanders have ledto the formation of ox-bow lakes(Figure 5) and flooding hascaused levées to form (Figure 6).Levées are high banks of siltalong the banks of a river. Theyare formed where a river flowsslowly, carries a large load of siltand floods on occasions. Whenthe river floods the coarsermaterial is deposited first close tothe river channel. The finermaterial is deposited furtheraway. Over many years thiscoarse material builds up to formthe levées on the river bank. Intimes of low flow when the riverhas little water and is flowingslowly it deposits material on theriver bed, building it up.Gradually the river bed increasesuntil it is higher than the landaround. Then flooding isdisastrous as the water cannotflow back into the river. Thelateral erosion by the meandersand the occasional floods buildup a wide, flat flood plain oneither side of the river. Here thevalley is a broad U-shape withquite gentle sides.

The mouth of the River TeesThe mouth of the River Tees is anestuary. An estuary is a river valley ina lowland area that has beenflooded. During the Ice Age vastamounts of water were stored assnow and ice. This caused sea levelsto be much lower than they arenow. At the end of the Ice Age themelting of the snow and ice causedsea levels to rise and so the lowerparts of river valleys were flooded toform these wide estuaries. The RiverTees has a very wide estuary withmudflats and sandbanks. Theestuary has been a magnet forindustry with iron and steel,engineering and chemical works allalong its length. However, parts ofthe estuary are very important sitesfor wildlife, such as seals, andmigratory birds. Some areas are Sitesof Special Scientific Interest (SSSIs),and are carefully managed for theunique ecosystems they support, egSeal Sands.

River basin managementThe drainage basin of the River Teeshas been managed for over acentury. The management has hadseveral aims:• to reduce flooding

• to improve water supply

• to improve water quality

• to improve navigation

• to provide more opportunities forrecreation.

A variety of strategies have beenused, including the building ofreservoirs such as Cow Green andGrassholme. In the 19th century,‘cut-offs’ were built near Stockton tostraighten the river for navigation,and recent flood protection schemeshave been built at Yarm. Today thereis a huge watersports complex at theTees barrage.

A meanderIn the lower course, meanders are large.Corrasion and hydraulic power erodethe outer bends and deposition builds upthe slip-off slopes.

The neck narrowsThe erosion continues and the neck ofthe meander narrows.Eventually, usually in times of flood, theneck is broken through.

The ox-bow lake is formedThe river now follows a straight path. Themeander is sealed off by deposition to formthe ox-bow lake. The lake may dry up toform a meander scar.

Meanderneck

Necknarrows

Ox-bowlake

Key

Erosion

Deposition

Marsh

Fastest flow

Figure 5: Formation of an ox-bow lake

River in flood

River at low flow

After repeated floods

Coarse materialFine sand

River channel

River aboveflood plain

Deposition onriver bed

Floodplain

Levée

Figure 6: Formation of levées

1 Tops and tails – match the wordor term with the correct definition:

2 Complete a table like the onebegun in Figure 7 to show themain landforms and processesalong the River Tees. Try to includeactual names of places andlandforms.

3 Waterfalls and gorges(a) Make a copy of Figure 2.(b) Label your sketch with themain characteristics of High Force.Add a title.(c) Match the numbers on Figure 8with the following sentences. Re-write the sentences in the correctorder to explain how waterfallsand gorges are formed. A The overhang collapses under gravity.

B The waterfall retreats upstream.

C Splashback from the waterfall erodesby hydraulic power.

D A steep-sided gorge of recession isformed.

E The softer rocks are undercut.

F The hard cap rock overhangs.

4 Meanders and levéesAccess the Ordnance Survey Get-a-Map website (http://getamap.ordnancesurvey.co.uk). Search forthe extract for Yarm and downloadand print a copy of the map.(a) On your map add the followinglabels in the most appropriateplaces:Meander Slip-off slopeRiver cliff LevéesFlood plain(b) Add an arrow to show theriver’s direction of flow.You could repeat this for maps ofUpper Teesdale and the rivermouth – practise recognising thelandforms and other features of theriver and its valley.(c) At what height above sea level isthe river at Yarm?(d) Approximately how wide is thevalley floor?(e) Copy and complete thefollowing sentences to explain theformation of ox-bow lakes:

In a meander, erosion by hydraulicpower and c____________ on theoutside bend leads to the formationof a ________ __________. On theinside bend deposition builds up a_______ _______. Over time theerosion on the outside bend causesthe _______ of the meander tonarrow. Eventually, usually in timesof ________, the neck is brokenthrough. The river follows astraight path. The old meander issealed off by ___________ to forman ox-bow lake. Eventually the ox-bow lake dries up and a meanderscar is formed.(f) Copy the diagrams in Figure 6to show the formation of levées.Add notes alongside each one toexplain how levées are formed.

5 (a) What is an estuary? In whichpart of the river is it found?(b) Explain how an estuary isformed.(c) Why are estuaries good sites forsettlement and industry?

6 Produce an A4 leafletexplaining the journey of a riverfrom source to mouth suitablefor a pupil in Year 6 at primaryschool.

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ActivitiesConfluence Diagram of a river

from its source to its mouth

Cross-section Where a river starts

Drainage basin Where a river flows into the sea

Long profile Diagram of a river from bank to bank

Mouth Where two rivers meet

Source The imaginary line around a drainage basin

Tributary The area of land that a river drains

Watershed A smaller river flowing into a large river

Upper course Middle course Lower course

Long profile

Cross-section

Valley features

Gradient

River and channel features/landforms

Processes at work

Figure 7: Landforms and processes along the River Tees

Cap rock(resistantWhinsill)

Softersandstoneandlimestone

Plungepool

1

2

3

4

5

6Formerposition ofwaterfall

Figure 8: The formation of HighForce and its gorge

Source

Pennines Yarm MouthNorthSea