The geographical definition of a floodplain is pretty simple: it is generally a flat area surrounding a river or stream. However, a few components of a floodplain are necessary to understand.
Parts of a floodplain
Firstly, there is the floodway, which is usually what we consider the river itself. It is the main channel where the river flows. However, sometimes it can be a seasonal river, which means there are times in the year when there is no water in the floodway, but a channel of some description remains.
There is then the flood fringe, the land between the banks of the floodway and the valley wall or anywhere the valley land starts to rise.
The width of a flood fringe can vary according to the river, but it can be surprisingly wide. For example, the Mekong River in Vietnam, known for the size of floods it causes every year, can increase its size by up to 43%. On the other hand, flood fringes can also be surprisingly narrow, so the river then tends to reach fast speeds that can be used as Whitewater rapids.
How do floodplains form?
Floodplains develop in two common ways: erosion and deposition (also known as aggradation). When rivers start to meander, curving from side to side, the water erodes the banks of the river and creates a wide flat area around the sides.
It floods its banks when the river floods because of heavy rainfall or ice melting upstream. As a result, the water that moves out of the floodway and into the fringes loses the force carrying much of the sediment acquired from erosion upstream and deposits it on the surrounding land.
The portions of the river that have been consistently flooded will naturally start to build a bank up again. Likewise, as sediment is deposited over time, most of it is deposited where the land elevates out of the river bed and creates a bank where one had previously eroded.
These deposits flanking the river are called fluvial terraces and can tell geologists a lot about where floodplains used to be before the banks built up. Analysis of the rock and sediment deposited allows geologists to examine thousands of years of natural history.
The benefits of floodplains
Plastic sediment deposition in floodplains can be the source of major fertility. This sediment is usually built up of alluvium, or silt, considered some of the richest soil, containing nutrients like potash, phosphoric acid, and lime.
This kind of soil promotes many biodiversitieses and provides the perfect landscape to farm. For instance, it is believed that the Amazon’s biodiversity can be attributed largely to the fact that over 250,000 km2 are covered with water by rivers that overflow yearly. These rivers transport rich sediment from the Andes Mountains to the forest and are partially responsible for the vast number of species living in the Amazon Rainforest.
But nature is not the only beneficiary of floodplains. Plenty of communities subsist on flooding, whole economies even! For example, in Vietnam, the Mekong River brings in approximately 8-10 billion US dollars because of the farming the floodplains facilitate.
The disadvantages of floodplains
Of course, by their very geographical definition, floodplains are destructive. The unpredictability of flooding makes them dangerous land, and the many communities living on floodplains are vulnerable to natural disasters.
For Cambodia, this has led to several deaths and the mass destruction of homes. In addition, the nation has been ranked as one of the most vulnerable countries at risk from the effects of climate change because of increased rainfall that causes rivers to flood. Not only that, but the filling of lakes and floodplains for urban developments has meant the rain has less space to occupy.
