Many deltas are shaped like a triangle. The dirt that is washed down a river is deposited at the mouth of the river. It continues to build up over the years.

This is the most famous delta in the world. It is located at the mouth of the Nile River.

Related Terms
coast - a delta is often created on a coastline
mouth - a delta is formed at the mouth of a river
channel - large deltas often have many water channels going from the river to the large body of water - usually an ocean
marsh - sometimes located along a delta
estuary - where the water from the river and water from the ocean meet (for example)