Theory question 306
Rivers and River Landforms(a) What is an alluvial fan? (b) With the aid of a diagram, describe the characteristics and mode of formation of an alluvial fan.
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(a) An alluvial fan is a fan-shaped deposit of sand, gravel and boulders laid down where a stream flows out of a steep mountain valley onto a flat plain.
(b) Diagram: a mountain front, a stream leaving a narrow valley and a fan-shaped spread of deposits on the plain. Characteristics: fan or cone shaped; apex at the mouth of the valley; coarse material near the apex, finer towards the edge; streams divide into many channels; common in arid and semi-arid areas. Formation: a fast stream carries a heavy load down a steep slope; on reaching the gentle plain its gradient and speed drop suddenly; it deposits its load; channels become blocked and the stream shifts from side to side, spreading deposits in a fan; repeated deposition builds the fan up.
Stress the sudden drop in speed.