Delta

Definition:

A delta is a low, watery land formed at the mouth of a river.

It is formed from the silt, sand and small rocks that flow downstream in the river and are deposited in the delta.

A delta is often (but not always) shaped like a triangle.

Source: NY Geography Glossary 

More:
Nile River Delta Nile River Delta

Source: NASA/MISR Wikipedia 

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