Gate 2011 1.9

1.9 The depositional feature that forms where a stream emerges from a mountainous region onto a plain is called

(A) alluvial fan

(B) natural levee. 

(C) delta

(D) point bar

Alluvial Fans. The alluvial material which flows down from

mountains accumulates at foothills where the stream enters a plain. The deposition occurs due to abrupt change in the gradient of the river valley. Such deposits spread out in the shape of flat fans and are called “alluvial fans”. Usually, the coarse material is dropped near the base of the slope while finer material is carried further out on the plain. Alluvial fans form many adjacent streams along a mountain that may merge to form a long wedge of sediment called “alluvial aprons”.

Deltas. These are submerged equivalents of alluvial fans. “Deltas” are deposits built at the mouths of streams. The deltas are usually triangular in shape with their apex pointed upstream. When a stream enters an ocean or lake, the currents of the flowing water dissipate quickly. This results in the deposition of the series of sedimentary layers which make up the delta. The material of most deltas is well sorted and many deltas are uniformly graded. The structure of a delta deposit is shown

Point Bars. In meandering rivers, sediment deposits occur as point bars. The “point bars” are the crescent-shaped deposits which occur at inside bends of a river channel.

Natural Levees. “Natural levees” are the low ridges which are formed on both sides of a river channel by the accumulation of sediment. They tend to confine the flow of river water into its channel between flood stages. The natural levees occur in rivers which have broad flood plains. During floods, the river overflows its bank and its velocity decreases rapidly. As a result most of the coarse sediment is deposited along the area bordering the river channel and finer sediments are deposited more widely over the flood plain. In this way, successive floods build up ridges on both sides of a river channel, which are called “natural levees.