Depositional Landforms – Landform and their Evolution

There are two types of sedimentary landforms: depositional and erosional. Depositional landforms result from layers of sediment being deposited by water, wind or ice.

Erosional features occur when the mechanical actions of glaciers, rivers and waves have removed materials from the earth’s surface, the action having been defined as either “Abrasion” or “Scouring”.

1. Alluvial Fans:

• Alluvial fans are formed when streams flowing from higher levels break into foot slope plains of low gradient.
• Normally very coarse load is carried by streams flowing over mountain slopes. This load becomes too heavy for the streams to be carried over gentler gradients and gets dumped and spread as a broad low to high cone shaped deposit called alluvial fan.

2. Deltas

• Deltas are like alluvial fans but develop at a different location.
• The load carried by the rivers is dumped and spread into the sea. If this load is not carried away far into the sea or distributed along the coast, it spreads and accumulates as a low cone.
• Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear stratification.
• The coarsest materials settle out first and the finer fractions like silts and clays are carried out into the sea. As the delta grows, the river distributaries continue to increase in length and delta continues to build up into the sea.

3. Floodplains, Natural Levees and Point Bars

• Deposition develops a floodplain just as erosion makes valleys.
• Floodplain is a major landform of river deposition. A river bed made of river deposits is the active floodplain. The floodplain above the bank is inactive floodplain.
• Inactive floodplain above the banks basically contain two types of deposits — flood deposits and channel deposits.
• The flood deposits of spilled waters carry relatively finer materials like silt and clay.
• The flood plains in a delta are called delta plains.
• Natural levees – are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of rivers, quite often cut into individual mounds.
• Point bars are also known as meander bars. They are found on the convex side of meanders of large rivers and are sediments deposited in a linear fashion by flowing waters along the bank.
• If there more than one ridge, narrow and elongated depressions are found in between the point bars.
• As the rivers build the point bars on the convex side, the bank on the concave side will erode actively.

4. Meanders

• In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel patterns called meanders develop over flood and delta plains
• Meander is not a landform but is only a type of channel pattern. As meanders grow into deep loops, the same may get cut-off due to erosion at the inflection points and are left as ox-bow lakes.

5. Braided Channels

• When rivers carry coarse material, there can be selective deposition of coarser materials causing formation of a central bar which diverts the flow towards the banks; and this flow increases lateral erosion on the banks.
• These thread-like streams of water rejoin and subdivide repeatedly to give a typical braided pattern.