A brief overview of the processes behind the formation of a river’s floodplain.
Floodplains are an example of a river landform. A famous example of a floodplain can be identified along the middle course of the Mississippi River. A floodplain is a wide, flat, low-lying area of land found on both sides of a river channel. A floodplain forms as a result of deposition during flooding over a long period of time.
River transportation is an essential process in the formation of a floodplain. At this stage, the river will carry a large load. The load is carried primarily by solution and suspension, though also by saltation and traction.
- Solution: Dissolved rock minerals are carried in solution in the water.
- Suspension: Light sand and silt particles are carried in the flow. This accounts for most of the load.
Flooding is most likely to occur after heavy rainfall or melting of snow. When the water level of the river reaches the level of its banks, this is known as “full-bank discharge”. At this time, river deposition will the main process. The depth of the flooded water will be shallow, leading to friction with the ground surface. This will reduce the speed and energy of the flow. As the floodwater loses energy, the capacity and competence of the flood-water is reduced, leading to deposition. The bed-load, the heaviest part of the load, will be dropped first and nearest to the channel. Ridges of deposition will form beside the river. These are known as levees.
The suspended load is carried further and deposited over a wider area. This deposited material is known as alluvium and consists primarily of sand and silt. Over millenia, thick alluvial soils may build up. This is known as aggradation and these soils tend to be very fertile.
The slopes of the river valley border the edge of the floodplain. These slopes are known as the “bluff line”.