Why do reservoirs fill faster than designed?

Erosion upstream delivers a heavy sediment load that consumes storage quickly.

How do suspension and bed load differ?

Suspended particles stay in the water and make it cloudy, while bed load rolls or bounces along the channel floor.

S5 Geography Rivers, Paper 2: transport and deposition

2026-11-02

Paper 2 works from gradient and discharge to middle course features, the three transport modes, catchment runoff and annual basin runoff, then explains silting at reservoir heads.

Focus　Gradient is fall over length, discharge is cross section times velocity, fixed area means discharge and velocity rise together, and a lower slope turns erosion into deposition.

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 250 km with a fall of 1500 m. What is its average gradient in metres per kilometre?
FoundationWhat features usually mark the middle course of a river?
FoundationName the three ways a river transports its load.
FoundationWhy does a delta form at a river mouth?
IntermediateA channel is 15 m wide and 1.8 m deep with a flow of 0.9 m per second. What is the discharge?
IntermediateA catchment of 40 square kilometres gets 60 mm of rain that all runs off. What is the runoff volume?
IntermediateA basin of 250 square kilometres has 2000 mm of rain a year and a runoff coefficient of 0.4. What is the annual runoff volume?
ChallengeWhat happens to erosion and deposition when a river leaves the hills for a plain, and why do reservoirs silt up at the head?
ChallengeDischarge rises from 20 to 80 cubic metres per second while the cross section stays the same. By what factor does velocity rise?
Error redoError: a student says erosion is strongest in the lower course so the valley is deepest there. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	6 m per km	Gradient is fall over length, and 1500 divided by 250 is 6.
2	A wide valley with meanders, dominated by lateral erosion and transport	With a gentler slope the river cuts down less and erodes sideways, so the valley widens and meanders develop.
3	Solution, suspension and bed load	Dissolved load travels in solution, fine sediment stays suspended, and sand and gravel roll or bounce along the bed.
4	The flow slows sharply, competence falls and sediment is deposited at the mouth	On reaching a lake or the sea the flow slows sharply, so the load it can no longer carry is dropped at the mouth.
5	24.3 cubic metres per second	The cross section is 15 times 1.8, which is 27 square metres, and times 0.9 gives 24.3.
6	2 400 000 cubic metres	Forty square kilometres is forty million square metres, and times 0.06 m gives 2.4 million cubic metres.
7	200 000 000 cubic metres	Two hundred and fifty square kilometres is 250 million square metres, times 2 m gives 500 million, and 0.4 of that is 200 million.
8	Velocity and competence fall so erosion gives way to deposition, and the backed up water slows further so sediment settles at the reservoir head	Slope drives velocity, so the plain brings deposition, and a reservoir slows the inflow further so coarse sand settles at its head.
9	Four times	Discharge is area times velocity, so with area fixed the two rise together and 80 over 20 is 4.
10	The lower course is dominated by lateral erosion and deposition, giving a wide shallow valley, while downcutting is deepest upstream	A steep upper course cuts down into a narrow deep valley, while the flat lower course has little power to erode and mainly deposits.

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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