Why does the bed below a dam incise?

Clear water carries no sediment, so the spare energy of the flow scours the bed instead.

Do oxbow lakes last forever?

No, they silt up and colonise with plants until they become marsh or meadow.

S5 Geography Rivers, Paper 3: base level and oxbow lakes

2026-11-02

Paper 3 starts from base level and sediment sorting, then calculates discharge, catchment runoff and velocity at a narrowing, and explains how a dam affects the bed and the delta.

Focus　Gradient is fall over length, discharge is cross section times velocity, a fixed discharge means a narrower section flows faster, and base level limits downcutting.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Geography: Rivers
Level	S5

02　Questions (10 items)

FoundationA river runs 300 km with a fall of 450 m. What is its average gradient in metres per kilometre?
FoundationWhat is base level, and why can a river no longer cut down once it reaches it?
FoundationWhy is a river bed full of boulders upstream but fine sand downstream?
FoundationHow does an oxbow lake form?
IntermediateA channel is 22 m wide and 1.5 m deep with a velocity of 1.2 m per second. What is the discharge?
IntermediateA catchment of 25 square kilometres receives 80 mm of rain that all runs off. What is the runoff volume?
IntermediateA basin of 120 square kilometres has 1800 mm of rain a year and a runoff coefficient of 0.5. What is the annual runoff volume?
ChallengeA channel narrows from 30 m to 15 m while discharge stays at 45 cubic metres per second and the depth there is 1.5 m. What is the velocity?
ChallengeWhat does an upstream reservoir do to the lower course of a river?
Error redoError: a student says river velocity depends only on slope. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	1.5 m per km	Gradient is fall over length, and 450 divided by 300 is 1.5.
2	The surface of the water body it enters, usually sea level, and with no remaining fall the river stops downcutting	Erosion needs water driven by gravity, and once the channel reaches base level no fall is left to use.
3	Fast upper water carries coarse material that drops out as the flow slows, leaving fine sand to travel furthest	A stone the fast upper course can move settles where the flow slows, so particles grow finer downstream.
4	Erosion narrows the neck of a meander until a flood cuts it through, and the abandoned loop silts up into a lake	Meanders grow until only a narrow neck remains, which a flood cuts through and leaves the old loop abandoned.
5	39.6 cubic metres per second	The cross section is 22 times 1.5, which is 33 square metres, and times 1.2 gives 39.6.
6	2 000 000 cubic metres	Twenty five square kilometres is twenty five million square metres, and times 0.08 m gives two million cubic metres.
7	108 000 000 cubic metres	One hundred and twenty square kilometres is 120 million square metres, times 1.8 m gives 216 million, and half of that is 108 million.
8	2 m per second	Velocity is discharge over cross section, and 45 divided by 22.5 is 2.
9	Discharge falls and becomes steady, sediment supply is cut so the bed incises, and the delta at the mouth shrinks	A dam stores flood water and sediment, so the lower course keeps a steady flow but loses the sand that builds its bed and delta.
10	Velocity depends on slope, cross section, bed roughness and discharge together	At the same slope a narrower or smoother channel flows faster, so slope alone does not set velocity.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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