River Processes Lab

The river gets faster, not slower

The Severn is the longest river in the United Kingdom at about 354 km, rising near 610 m up on Plynlimon in mid Wales and reaching the sea in the Bristol Channel. The Thames runs about 346 km from the Cotswolds to the estuary. Drawn as a side view from source to mouth, the bed of either forms a long profile: steep at the top, flattening out towards the sea, a shape geographers call concave. Now the question that catches people out. Where does the water move fastest? Almost everyone says the headwaters, because that is where it looks fastest: white water, boulders, noise. The answer is the lower course. A river in its upper course is shallow and narrow and its bed is strewn with rocks, so an enormous share of its energy is spent overcoming friction against the bed and banks. Downstream the channel is deeper, wider and far smoother, so much less energy is wasted, and average velocity rises even though the gradient has fallen. Discharge, the volume passing a point each second and measured in cumecs, rises downstream too as tributaries join. That single fact reorganises the whole topic. It explains why the upper course is dominated by cutting downwards while the lower course does more sideways work and more depositing; why the load gets smaller and rounder the further you go; and why the cross profile, the shape of the valley seen end-on, changes from a narrow steep-sided notch to something wide and almost flat. This module covers the processes that do all that, and where along the river each one takes over.

Five terms, and the pair everyone confuses

The third and fourth are the ones examiners see swapped most often. Learn them as a contrast, not as two separate definitions.

Order the journey from hill to sea

These six stages describe what happens to the river and its load along the long profile. Put them in order from source to mouth.

• Water gathers on high ground and begins to flow down a steep slope in a narrow shallow channel
• Most of its energy goes into cutting downwards, carving a narrow steep-sided valley
• Tributaries join, so discharge grows and the channel deepens and widens
• With a smoother, larger channel the river wastes less energy on friction and begins to erode sideways as well as down
• The load is worn steadily smaller and rounder as the pieces knock against each other on the way
• Across a broad, almost flat valley the river drops the finest material and builds up the land on either side

Which process is being described?

A geographer measures pebbles at ten points down a river and finds that they get smaller and rounder with every site further downstream. Which process is she observing, and why?

• Attrition, because the fragments collide repeatedly as the current shifts them along, and every impact takes a little more off their corners. What is being worn down here is the cargo, not the channel, and that is precisely what separates this process from abrasion.
• Abrasion, because the load is being scraped along the bed of the river.
• Solution, because the rock is dissolving into the water as it travels.
• Hydraulic action, because the force of the water is breaking the pebbles apart.

The three courses, side by side

The specification note asks for everything to be tied explicitly to a position along the long profile, so learn the processes in these three boxes rather than as free-floating lists. Read the rows across and the pattern does the work for you.

Label the long profile

The side view runs from high ground on the left down to the sea on the right, and the three shapes above it are end-on views of the channel at three points along the way. Tap a label, then tap the pin it belongs to. Two of the labels are real river terms for things this diagram does not show.

Match each observation to its process

Two precision points and a command word

Almost all the marks lost on this element go to two confusions, and both are fixable in a sentence. The first is abrasion against attrition. They sound alike and they involve the same material, so decide what is being worn away before you write. If the answer is the bed and the banks, it is abrasion and the river is damaging the landscape. If the answer is the load, it is attrition and the landscape is doing nothing at all. A reliable way to prove you know the difference is to state the victim in the same sentence as the name. The second is velocity downstream. The intuitive answer, that a river slows as its gradient eases, is wrong and examiners meet it constantly. Say plainly that average velocity increases towards the mouth, and give the reason: a larger, deeper and smoother channel loses proportionally far less energy to friction than a shallow rocky one, so more of the available energy goes into moving water rather than fighting the bed. Get that right and the rest of the topic becomes consistent, because it is also why the lower course can carry so much fine material and why it deposits when it finally does slow. On the command word: "describe the changes in the cross profile" wants what the shape looks like at each stage and nothing more. "Explain why the cross profile changes" wants the processes that produced each shape. And whenever a question names a course, tie every point you make to that position on the long profile, because the specification note for this element asks for exactly that link and a floating list of processes will not get it.

Complete the river process facts

The longest river in the UK, the Severn, runs about _____ km from high ground in mid Wales to the Bristol Channel, and its long profile is steep near the source and gentle near the mouth. Despite that gentler slope, average velocity _____ downstream, because a wide deep channel with a smooth bed wastes much less of its power fighting its own sides than a narrow rocky one does. In the upper course the river cuts mainly _____, producing a narrow steep-sided cross profile. Two processes are easily confused: abrasion wears away the _____, while attrition wears away the load itself, which is why pebbles get smaller and rounder with distance. The volume of water passing a point each second is called _____, and it is measured in cumecs.

Spot the three accurate river claims

Select the THREE statements that are true.

• Average velocity is higher in the lower course than in the upper course, because the channel there is deeper, wider and smoother, so proportionally less of the river's energy is spent overcoming friction.
• Abrasion and attrition both involve the load, but abrasion wears away the bed and banks while attrition wears away the load itself, which is why material becomes smaller and rounder downstream.
• Fine sand, at around 0.2 mm across, needs a lower velocity to be picked up than any other particle size, while clay needs more than its tiny size suggests because its particles cling together.
• A river slows down as it moves downstream because the gradient becomes gentler.
• Rivers only ever erode downwards, which is why valleys get deeper.
• A river deposits material only when it reaches the sea.

A 6-mark explanation, marked

Explain how the cross profile of a river valley changes from its upper course to its lower course. [6 marks] In the upper course the river is small, shallow and flowing over a bed strewn with large angular boulders, so a high proportion of its energy is used simply overcoming friction and what remains is directed downwards. Hydraulic action forces water into cracks in the bed and banks and prises material away, while abrasion grinds the channel using the coarse load the river is dragging along, and because this erosion is almost entirely vertical the valley is cut as a narrow notch with steep sides. Moving downstream, tributaries join and discharge increases, so the channel becomes deeper and wider and its bed smoother, which means proportionally less energy is lost to friction and average velocity rises even though the gradient has eased. This allows the river to swing across its valley and erode laterally as well as vertically, so the cross profile widens and the valley sides become gentler. In the lower course the bed is close to sea level, so there is little scope for further downcutting and the dominant processes are lateral erosion and deposition. The river meanders across a broad valley, eroding the outer banks of its bends and laying down fine sand, silt and clay when it slows or floods, which builds a wide flat floodplain on either side. The result is a cross profile that changes from a narrow steep-sided notch to a very broad and almost level valley floor, because the direction of the river's erosive energy shifts from downwards to sideways as friction falls and the channel grows. What earned the marks: each of the three courses tied explicitly to its position on the long profile; named processes rather than general wearing away; the crucial link between falling friction, rising velocity and the switch from vertical to lateral erosion; and a closing sentence that states the pattern the whole answer has demonstrated.

Find the two statements about the load

Five statements about river processes. Two of them describe something happening to the material the river is carrying, rather than to the channel or to the water. Tap those TWO.

• Pieces knock against one another as they travel, losing their corners until what began as a jagged block is a rounded pebble.
• Sand and gravel are scraped along the bed, grinding away at the rock of the channel floor.
• Small stones are carried in a series of hops, lifted by the current and dropping back before being lifted again.
• Water is forced into a crack in the bank and the trapped air is compressed until a lump is dislodged.
• Slightly acidic water reacts with limestone in the bed and dissolves it chemically.

Build the velocity sentence

Assemble the sentence that most answers get backwards.

Four sites, one river

You are surveying a river from source to mouth. At each site, work out what you are looking at.

• Site one, high on the hillside. The channel is barely two metres across and very shallow, the bed is a jumble of angular boulders, and the valley sides rise steeply on both sides.
• Site two, several kilometres downstream. You measure a hundred pebbles and find their average size has halved and their edges are visibly rounder than at site one.
• Site three, on a broad flat valley floor. The river swings in wide bends, the water is brown with fine material, and the ground stretches away almost level on both sides.
• Site four. Your flow meter reads a higher average velocity here, near the mouth, than anywhere upstream, and a colleague insists the instrument must be faulty because the land is almost flat.

Explain the processes

Explain how the processes of erosion and transportation change along the long profile of a river. [9 marks]

• The correct term each time, not general wearing away
• Tying processes to the course is what the specification note requires
• What causes the shift, not just that it happens
• Average velocity increases downstream, for the friction reason: the intuitive answer is wrong
• What is worn away in abrasion and in attrition