what is an area of land roughly triangular in shape which flows where a river enters a lake
delta. a roughly triangular area of land formed at the mouth of a river which reaches the sea in two or more branches. the delta of the Nile.elÂ·ta. A usually triangular mass of sediment, especially silt and sand, deposited at the mouth of a river. Deltas form when a river flows into a body of standing water, such as a sea or lake.