In plains during floods, the river water comes out of its channel and spreads over a vast stretches of land. During this process it deposites its sediments too. in this way a smooth plain comes into existence, which is called flood plain.
A flood plain (or floodplain) is a generally flat area of land next to a river or stream. It stretches from the banks of the river to the outer edges of the valley.
A flood plain consists of two parts. The first is the main channel of the river itself, called the floodway. Floodways can sometimes be seasonal, meaning the channel is dry for part of the year. The floodway of the Todd River in Australia’s Northern Territory, for instance, is an ephemeral stream, meaning its channel can be dry for months at a time.
Beyond the floodway is the flood fringe. The flood fringe extends from the outer banks of the floodway to the bluff lines of a river valley. Bluff lines, also called valley walls, mark the area where the valley floor begins to rise into bluffs. The flood fringe of the seasonal Todd River extends the flood plain to 445 square kilometers (170 square miles).
Some flood plains are extraordinarily wide. The Barotse flood plain of the Zambezi River, for example, is a vast wetland stretching thousands of kilometers through Angola, Zambia, and Botswana. The Barotse flood plain includes the sandy Kalahari basin, which is waterlogged during the rainy season and an extension of the nearby Kalahari Desert during the dry season.
Some rivers have very narrow flood plains. In fact, some rivers, or parts of rivers, seem to have no flood plain at all. These rivers usually have a steep stream gradient—a very deep, fast-moving channel. Ngonye Falls, Zambia, marks a remote stretch of the Zambezi River where the flood plain is extremely narrow. As the Zambezi leaves the wide flood plain of the sandy Kalahari, it enters a narrow basalt channel as fast-moving whitewater rapids.
Geology of a Flood Plain
There are two major processes involved in the natural development of flood plains: erosion and aggradation. The erosion of a flood plain describes the process in which earth is worn away by the movement of a floodway. Aggradation (or alluviation) of a flood plain describes the process in which earthen material increases as the floodway deposits sediment.
A river erodes a flood plain as it meanders, or curves from side to side. The massive lowland flood plain of the Amazon River, for instance, is carved with hundreds of oxbow lakes that document the meandering river and its tributaries over time. Oxbow lakes are formed when a meander, or bend, in the river is cut off from the river’s mainstem. Features such as oxbow lakes and seasonal wetlands are often a part of flood plains created through erosion and deposition.
A meandering stream can contribute to a flood plain’s aggradation, or build-up in land elevation, as well as its erosion. A typical aggradation environment is a wide, shallow, braided river. Braided rivers often include river deltas, where the main floodway is separated into discrete channels and tiny islands. The process of subsidence, in which the elevation of a delta may sink due to sea level rise or human activity, often offsets aggradation in the flood plains in these areas. The huge aggradation of sediment around the Nile delta, for instance, is subsiding due to the rising level of the Mediterranean Sea.
The alluvium, or sediment, of a flood plain varies. Its coarseness and composition depend on the surrounding landscape and the velocity of the currents that created the flood plain. Some flood plains are mostly fine-grained silt, while others are sandy.