(a) An estuary is a deep fan shaped sunken mouth of a river valley whereas a delta is a depositional alluvial landmass roughly irregular in shape formed at the mouth of the river.
(b) In an estuary the river has a single entry into the sea whereas in a delta the river has multiple entries into the sea.
(c) River which forms estuaries is tidal. The tides sweep away the sediments and keep the mouth of the river clear of sediments but the rivers which form a delta, have a large alluvial plain.
(d) The sea adjacent to the coast in an estuary, is deep or the gradient is steep, but, the sea adjacent to the coast is shallow or the gradient of the river is very low then the river divides into channels (distributaries) forming a delta, e.g., river Narmada and Tapi form estuaries. River Ganga, Brahmaputra form the largest delta of the world called Sundarbans, river Nile also forms a delta.
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