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A delta is a fan-shaped alluvial tract formed at the mouth of a river, when tidal or other currents can no longer remove the deposits. As more material is deposited, it becomes easier for the river to divide and flow to each side, each new branch forms new banks, which divide and subdivide further. These branches are often called distributary channels or distributaries. As the deposits accumulate, the delta grows outwards the shape of a fan or triangle, which becomes covered with a network of channels; the apex of the triangle is the head of the delta.

Deltas can be created not only where a river flows into the ocean but also where a river enters a lake, or at the confluence of two rivers.

Colorado River entering Lake Mead.
Colorado River entering Lake Mead.
Earth Science Slides by John S. Shelton
Colorado River entering Lake Mead.  Delta  Delta of Flathead River. 

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