Meaning & use
alluvial fan
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Meaning
nouncountabletechnical
A fan-shaped pile of sand, gravel and silt left on flat land where a fast stream leaves a steep valley and slows down.
Grammar: Countable: several alluvial fans lie along the base of the range.
Patterns
- an alluvial fan at something
- an alluvial fan of something
The stream spread its gravel into an alluvial fan at the foot of the mountains.
Farmers planted orchards on the alluvial fan because the soil drained well.
After the storm, a new alluvial fan of mud and boulders covered part of the road.
Word forms
| plural | alluvial fans |
|---|
Common combinations
- build an alluvial fan
- Over thousands of years the creek has built an alluvial fan across the plain.
- on an alluvial fan
- The town was built on an alluvial fan and floods during heavy rain.
- the apex of an alluvial fan
- The apex of the alluvial fan sits where the canyon opens out.
- a desert alluvial fan
- A desert alluvial fan can form after only one violent flood.
- alluvial fan deposits
- Alluvial fan deposits hold clues about the region's past climate.
Synonyms and how they differ
- debris fan
- This is a close alternative that stresses the rough, mixed material, while alluvial fan points more to water-carried sediment.
- delta
- A delta forms where a river enters a lake or sea, whereas an alluvial fan forms on dry land at the foot of a slope.
Word family
- alluvium noun
- loose soil and sand deposited by flowing water
- alluvial adjective
- made of or relating to material deposited by water
- alluvial plain noun
- a flat area built from river-deposited sediment
Commonly confused: alluvial fan or delta?
alluvial fan vs delta
A delta builds out into standing water such as a sea, while an alluvial fan spreads over land at the base of a mountain.
Practice
Where a steep mountain stream reaches a flat valley floor and drops its gravel, the deposit often forms an ____.
Choose one answer.
An alluvial fan forms on land where a fast stream slows at the base of a slope. A river delta needs the water to enter a sea or lake, so it fails here.
Make it your own
Write a sentence using “alluvial fan”, then check it and compare it with the examples above.