A stream erodes a large amount of sediments and debris of the rocks and earth surface. The separation of sediments from the surface is called Weathering. The movement of these sediments from their origin to low lying areas is called erosion. Both the processes of weathering and erosion are collectively called Denudation. As a result of all these three works, the sediments are weathered, eroded, and then deposited at some other place. The process of deposition is the final stage of denudation. The deposited materials are found in different forms. The different forms of depositional material are called Depositional Landform.
(1). Alluvial Fans: The alluvial material which flows down from the mountains accumulates at foot hills where the stream enters a plain. The deposition occurs due to abrupt change in the gradient of 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 from many adjacent streams along a mountain may merge to form a log wedge of sediment called alluvial aprons.
(2). Flood Plains: During floods a river overflows its overflows its bank and submerges the adjacent low-lying areas where deposition of alluvial material takes place. A wide belt of alluvial plain formed in this way on either side of a stream, is called “Flood Plains”. Its name is appropriate, because the flood plain gets submerged only when a river overflows its bank at flood stages.
(3). 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 built up ridges on both sides of the river channel, which are called “natural levees”. The natural levees of the lower Mississippi river rise 6 meters above the valley flood.
The area behind the levees is poorly drained as water can not flow up the levees to join the river. The marshes thus formed, are called “back swamp”. A tributary stream often has to flow parallel to the main stream until it can breach the levee. Such streams are called “yazoo tributaries”.
(4). 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.
(5). Deltas: Deltas are deposits built at the mouths of the 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 in the following image.
It consists of three sets of beds: (i) bottom-set beds, (ii) fore-set beds, and (iii) top-set beds.
- Bottom-set Beds: The thin horizontal beds which overlie the ocean bottom are called “bottom-set beds”. They are mainly composed of fine grained sediments, such as silts and clays.
- Fore-set Beds: Forest beds begin to form prior to the accumulation of bottom-set beds. These beds are composed of coarse sediment which is dropped almost immediately when a river enters a lake or ocean. The fore-set beds appear similar to cross bedding and their angle of slope varies from 12 degree to 32 degree depending on the grain size of the material.
- Top-set Beds: Fore-set beds are covered by thin nearly horizontal top-set beds. These beds occupy the upper surface of the delta. They are composed of a mixture of coarse and fine materials.
Major rivers, such as Mississippi, Danube River, Ebro River, Godavari River, Yellow River, Amazon, Ganges River, Danube, Indus River form deltas thousands of square kilometers in area. On large deltas the main channel of the river divides to form several smaller branches, called “distributaries”. They discharge water in various paths to the sea.