Landform Detective: Rivers

A waterfall is a hole moving upstream

The River Tees rises on Cross Fell in the north Pennines at around 893 m and runs roughly 137 km east to the North Sea at Middlesbrough. About a third of the way down it reaches High Force, where the water drops some 21 m in a single fall. What makes High Force worth knowing is not its height but its geology: the river is running over the Whin Sill, a sheet of hard dolerite, and immediately beneath that cap lies softer rock. That arrangement is the whole explanation. Water attacks the softer rock beneath far faster than the resistant shelf above it, hollowing out a plunge pool and undercutting the cap until the unsupported ledge breaks off. The fall then stands in a slightly more upstream position than it did before, and begins again. Do that for long enough and the waterfall travels backwards up its own valley, leaving behind a steep-sided trench where it has been. That trench is a gorge, and the gorge below High Force is the distance the waterfall has already retreated. This is how to think about every landform in this topic. A meander is not a bend, it is a bend that is still moving. An ox-bow lake is not a crescent of water, it is the piece of river that got cut off. A floodplain is not flat ground beside a river, it is material the river put there. AQA asks you to place each one on the long profile, upper, middle or lower course, and to tell it as a sequence of events. This module does both.

Five river landform terms

Each one is defined by what happened rather than by what it looks like, because that is the form the exam question takes.

Sequence the ox-bow lake

These six stages turn a bend in a river into a crescent of standing water. Put them in order.

• Faster flow on the outside of a bend undercuts the bank, while material settles on the inside
• Erosion on the outsides of two adjacent bends brings them closer together
• Only a narrow neck of land is left separating the two limbs of the loop
• During a flood the river has the energy to cut straight across that neck
• The shorter, steeper route carries the flow, and deposition seals the ends of the old loop
• The abandoned loop is left as a crescent of standing water, which slowly silts up

Why does a waterfall move upstream?

The gorge below High Force runs downstream from the fall itself. Which answer explains why?

• The fall sits where a resistant cap of rock lies over something softer. Falling water and the debris it carries scour a plunge pool into the weaker rock beneath, which undercuts the ledge above until the unsupported overhang collapses. The fall then occupies a position slightly further upstream and the same thing begins again. Over a long period the waterfall works its way back up the valley, and the steep-sided trench it leaves behind is the gorge, so the gorge marks the distance the fall has already travelled.
• The river cut the gorge first, and the waterfall formed at the top of it afterwards.
• The rock downstream of the fall happens to be softer, so the valley there is deeper.
• Material deposited at the foot of the fall has built up the valley sides.

Landforms by course

The specification note asks for every landform to be tied explicitly to its position on the long profile, so this is organised by course rather than by type. The reason each group appears where it does is the balance between cutting downwards and cutting sideways.

Where will the river break through?

The image shows a river from above on a flat valley floor, flowing in the direction of the arrow. Tap the place where the river is most likely to cut a new course during the next large flood.

Match each feature to what made it

Place it, then sequence it

Two requirements sit behind almost every mark on this element, and the specification note states the first of them outright. Place the landform on the long profile. Say whether you are in the upper, middle or lower course before you explain anything, because the reason a landform exists is almost always the balance of erosion in that stretch. Waterfalls and interlocking spurs belong where the river is still cutting downwards; meanders and ox-bow lakes belong where it has begun working sideways; floodplains and levees belong where deposition dominates. A formation explained without that context has left out its cause. Then tell it as a sequence. Formation questions are marked on ordered stages, each one following from the last, and an answer that describes the finished shape earns almost nothing. Build from the weakness or the imbalance, follow the change, reach the moment something gives way, and finish with what is left behind. The joints are where the marks are, so use words that show causation: because, this continues until, eventually, which leaves. One habit that lifts an answer quickly. Say what the landform is evidence of. A gorge shows where a waterfall used to be. An ox-bow shows the course the river abandoned. A levee shows how many times the river has flooded. Writing that sentence proves you are reading the landscape rather than reciting a diagram, and it costs one line. On command words: "describe" wants the appearance, which is the one occasion description is correct. "Explain the formation" wants the ordered sequence. "Suggest how" asks you to apply that sequence to an unfamiliar map or photograph, which is exactly what the skills questions do.

Complete the river landform facts

High Force on the River Tees drops about _____ m over a shelf of hard dolerite lying above softer rock. Falling water scours a _____ pool into the weaker rock beneath, undercutting the ledge until it collapses, so the fall retreats upstream and leaves a steep-sided _____ behind it. Further down the river, the fastest flow on the outside of a bend cuts the bank back while material settles on the inside, so the whole bend migrates sideways. When erosion narrows the _____ between two adjacent bends far enough, a flood cuts straight across and the abandoned loop is sealed off as an ox-bow lake. In the lower course, water spilling from the channel slows at once and drops its heaviest load nearest the bank, raising a _____ along it.

Spot the three accurate river landform claims

Select the THREE statements that are true.

• A gorge downstream of a waterfall is a record of where that waterfall used to be, because the trench was cut as the fall worked its way back up the valley.
• A meander migrates across its valley because erosion and deposition happen on opposite banks at the same time, undercutting one side while building up the other.
• Levees are highest immediately beside the channel because a river spilling over its banks loses velocity at once and drops its coarsest material first, carrying only fine silt further out.
• A gorge forms first and a waterfall then develops at the upstream end of it.
• Ox-bow lakes are characteristic of the upper course, where the river has the most energy.
• Describing the finished shape of a landform is sufficient to answer a question about how it formed.

A 6-mark formation answer, marked

Explain the formation of an ox-bow lake. [6 marks] Ox-bow lakes form in the middle and lower courses of a river, where the channel is large enough and smooth enough for the river to erode sideways as well as downwards. Water flowing round a bend moves fastest on the outside, so it undercuts that bank into a steep river cliff, while on the inside the current is slower and drops the material it was carrying as a shelving slip-off slope. Because erosion and deposition are happening on opposite banks at the same time, the bend does not simply deepen but migrates across the valley floor. Where two adjacent bends are both migrating, the strip of land separating them is attacked from both sides and grows progressively narrower, until only a thin neck remains. During a flood the river is carrying far more water and has the energy to take the shorter, steeper route straight across that neck rather than travelling the long way round. Once it does so, almost all the flow follows the new channel, so velocity in the old loop falls away and the material the river is carrying is deposited at the two points where the loop meets the new course, sealing it off at both ends. What is left is a crescent of standing water separated from the river, which over time silts up, is colonised by vegetation and eventually disappears, leaving a curved scar on the floodplain that marks a course the river no longer takes. What earned the marks: the landform placed on the long profile in the opening sentence; ordered stages each caused by the last rather than listed; the crucial point that erosion and deposition act simultaneously on opposite banks, which is what makes the bend move; the flood identified as the trigger; and a closing observation about what the feature is evidence of.

Find the two stages with causes

Five sentences from an answer about waterfalls. Three of them only describe. Tap the TWO that give a stage of the process together with its cause.

• Water scours a hollow into the weaker rock beneath, so the resistant ledge above is left with progressively less supporting it.
• The fall at High Force is about twenty-one metres from top to bottom.
• Eventually the unsupported overhang can no longer carry its own weight and breaks away, leaving the fall standing slightly further upstream than before.
• The water is loud enough to be heard from some distance away, and the rock around it is permanently wet.
• Below the fall the valley has steep rocky sides.

Build the meander sentence

Assemble a sentence that explains why a bend moves rather than merely deepens.

Four features on an unfamiliar map

You are handed a map and a photograph of a river you have never seen. Read each feature.

• Near the head of the valley, the contour lines show ridges projecting alternately from each side, and the river winds between them in a narrow steep-sided valley.
• On the flat ground beside the lower river there is a curved crescent of water with no visible connection to the channel, and a faint curved mark in the fields continuing its line.
• A cross-section through the lower valley shows the ground slightly higher immediately beside the channel than a hundred metres away from it, with coarser material near the river and fine silt further out.
• A waterfall on the map has a steep-sided rocky valley running downstream from it, and the geology shows a band of hard rock lying over softer rock at the fall itself.

Explain a formation

Explain the formation of a waterfall and gorge, and explain why the landforms found in the lower course of a river are so different. [9 marks]

• A resistant band over softer rock makes a waterfall possible
• Plunge pool, undercutting, collapse, retreat upstream, gorge
• Reading the landscape, not describing it
• Downward erosion giving way to sideways erosion, and the dominance of deposition
• Mechanisms, not a list