The correct option is It is formed when streams flowing from higher levels break into foot slope plains of low gradient..
Explanation
Alluvial fans are depositional landforms formed by running water (fluvial processes). They typically develop where a high-gradient stream leaves a confined canyon or mountain valley and enters a flat plain or a valley floor with a significantly lower gradient.
Option Analysis
- It is formed when streams flowing from higher levels break into foot slope plains of low gradient. is Correct: An alluvial fan is formed when streams flowing from higher levels (mountains) break into foot slope plains of low gradient. The sudden decrease in slope causes the stream to lose kinetic energy and velocity. Consequently, the stream can no longer carry its sediment load, leading to the deposition of coarse materials (boulders, gravel) in a cone or fan shape spreading outwards from the mountain front.
- It is a depositional feature formed by the vertical erosion of a stream into its own floodplain. is Incorrect: Vertical erosion of a stream into its own floodplain typically results in the formation of River Terraces or Incised Meanders. These are erosional features marking former valley floor levels.
- It is a linear ridge of coarse deposits found along the banks of large rivers. is Incorrect: A linear ridge of coarse deposits found along the banks of large rivers describes Natural Levees. These are formed during flooding when water spills over the banks and velocity drops, depositing coarser sediments immediately adjacent to the channel.
- It is a depression formed at the base of waterfalls due to the impact of water. is Incorrect: A depression formed at the base of waterfalls due to the impact of water and rotation of boulders is known as a Plunge Pool. This is an erosional feature.
Key Takeaway
Alluvial Fans are cone-shaped depositional features formed due to a sudden break in slope (high to low gradient), causing streams to dump their coarse sediment load at the foothills.