The correct option is They are formed when the load carried by rivers is dumped and spread into the sea..
Explanation
Deltas are prominent depositional landforms developed at the mouth of a river where it enters a standing body of water, such as a sea or a lake. The sudden decrease in river velocity leads to the deposition of the sediment load.
Option Analysis:
- They are formed when the load carried by rivers is dumped and spread into the sea. is Correct: Deltas are formed when the sediment load carried by rivers is dumped and spread into the sea. If this load is not removed by currents or tides, it accumulates to form a delta.
- Unlike alluvial fans, they are not stratified. is Incorrect: Deltaic deposits are characterized by being very well sorted with clear stratification. This distinguishes them from alluvial fans, which are often composed of coarse, poorly sorted, and unstratified material.
- As the delta grows, the river distributaries decrease in length. is Incorrect: As a delta grows and progrades (extends) into the sea, the river distributaries must increase in length to reach the new coastline.
- They are exclusively formed by wind action in coastal areas. is Incorrect: Deltas are formed by fluvial (river) action. Wind action is responsible for aeolian landforms, such as sand dunes, not river deltas.
Key Takeaway: Deltas are stratified, well-sorted depositional features formed by rivers at their mouths, distinct from the poorly sorted deposits of alluvial fans.