Why do river floodplains often develop fertile soils?
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Answer: Floods leave behind sediment containing minerals and organic matter
Their flat surface automatically removes excess water — Flat ground does not automatically drain well, and floodplains can remain waterlogged. Drainage affects growing conditions, but flatness alone does not add nutrients to soil.
Wind deposits volcanic ash across every river valley — Volcanic ash is not deposited across every river valley. It can enrich soils near volcanic regions, but river-borne sediment explains why many non-volcanic floodplains are fertile.
Floods leave behind sediment containing minerals and organic matter ✓ — Floods can spread sediment containing minerals and organic matter over the floodplain. Repeated deposits renew the soil, although fertility still varies with the sediment and local conditions.