Floodplain

A 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 floodplain 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 floodplain to 445 square kilometers (170 square miles).

Some floodplains are extraordinarily wide. The Barotse floodplain of the Zambezi River, for example, is a vast wetland stretching thousands of kilometers through Angola, Zambia, and Botswana. The Barotse floodplain 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 floodplains. In fact, some rivers, or parts of rivers, seem to have no floodplain 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 floodplain is extremely narrow. As the Zambezi leaves the wide floodplain of the sandy Kalahari, it enters a narrow basalt channel as fast-moving whitewater rapids.

There are two major processes involved in the natural development of floodplains: erosion and aggradation. The erosion of a floodplain describes the process in which earth is worn away by the movement of a floodway. Aggradation (or alluviation) of a floodplain describes the process in which earthen material increases as the floodway deposits sediment.

A river erodes a floodplain as it meanders, or curves from side to side. The massive lowland floodplain 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 floodplains created through erosion and deposition.

A meandering stream can contribute to a floodplain’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 floodplains 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 floodplain varies. Its coarseness and composition depend on the surrounding landscape and the velocity of the currents that created the floodplain. Some floodplains are mostly fine-grained silt, while others are sandy.

The deposit of alluvium created as a river or stream breaks, or breaches, its bank is called a crevasse splay. The formation of a crevasse splay is very similar to the formation of an alluvial fan. The thickest layer of sediment is nearest the breach, while the thinnest and youngest sediments are fanned out.

National Geographic