Both deltas and alluvial fans are types of depositional land forms formed by flowing rivers. Alluvial fans are formed at foothills where streams flowing from higher level break into foot slope plains of low gradient whereas deltas are formed near mouth of streams where it meets seas or stagnant water bodies.

The two land forms also differ in terms of characteristics of their deposits. Alluvial fans are generally composed of coarse sediments which are not distinguishingly stratified whereas deltas are generally formed by fine deposits.

Factors responsible for formation of alluvial fans:

1. Stream is flowing from higher elevation with steep gradient for greater sediment carrying capacity.
2. Path of the stream is easily erodible.
3. Further, shape of alluvial fan depends on the climate such as in humid areas they show normally low cones with gentle slope from head to toe while in arid and semi-arid climate, they appear as high cones with steep slopes.

Factors responsible for formation of deltas:

1. Load carrying capacity of rivers: If the load is not carried far into the sea, it spreads and accumulates as a low lying cone forming deltas.
2. Composition of sediments carried by rivers: Coarser material settles and finer fraction like silts and clays are carried down into the sea.
3. Continental shelf: Broader and less steep continental shelf aids delta formation.

Economic significance of alluvial fans:

• Porous and permeable fan deposits are the primary source of groundwater, which is used for irrigation and for water supply, especially in arid and semi-arid climates.
• Wet fans tend to concentrate heavy mineral particles in placer deposits that help in economical extraction of elements.

Economic significance of deltas:

• Deltas form one of the most fertile agriculture areas thus supporting the livelihood of millions and also the diversified flora and fauna              
• These hold well sorted sand and gravel which is quarried.
• These become centres of tourism and recreation owing to their natural diversity and richness.