An alluvial fan is a fan-shaped deposit of sediment that forms when a stream or river carrying sediment flows onto a flatter plain. The term 'alluvial fan' is derived from the word DELTA, which is a five-letter word used to describe the triangular shape of the sediment deposit. Alluvial fans are typically found in arid and semi-arid regions, where the sediment is deposited due to a decrease in the velocity of the water.