Fluvial terraces mark the older, higher-elevation paths of the stream, before erosion and aggradation created the current mainstem of the stream or river. Fluvial terraces are step-shaped areas of land that flank the banks of a river or stream.

One of the most important geologic features of a floodplain is its fluvial terraces. Thick layers of sand may indicate flash flooding, for instance, while thin, evenly spaced layers of silt may indicate more moderate and predictable flood patterns. The sedimentary patterns of floodplains often provide scientists with evidence of past geologic activity. Nearby wells then collect the filtered water, which is then ready for more intense purification processes. In bank filtration, water is deliberately filtered through the banks or floodplain of a river or lake. Water purification systems often take advantage of this natural phenomenon in a process called bank filtration. Clay, sand, and gravel filter water as it seeps downward. The layered sediments of many floodplains can create important aquifers. The thickest layer of sediment is nearest the breach, while the thinnest and youngest sediments are fanned out. The formation of a crevasse splay is very similar to the formation of an alluvial fan. The deposit of alluvium created as a river or stream breaks, or breaches, its bank is called a crevasse splay. Some floodplains are mostly fine- grained silt, while others are sandy. Its coarseness and composition depend on the surrounding landscape and the velocity of the currents that created the floodplain. The alluvium, or sediment, of a floodplain varies. The huge aggradation of sediment around the Nile Delta, for instance, is subsiding due to the rising level of the Mediterranean Sea. 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. Braided rivers often include river deltas, where the main floodway is separated into discrete channels and tiny islands. A typical aggradation environment is a wide, shallow, braided river. Features such as oxbow lakes and seasonal wetlands are often a part of floodplains created through erosion and deposition.Ī meandering stream can contribute to a floodplain’s aggradation, or build-up in land elevation, as well as its erosion. Oxbow lakes are formed when a meander, or bend, in the river is cut off from the river’s mainstem. 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. Aggradation (or alluviation) of a floodplain describes the process in which earthen material increases as the floodway deposits sediment.Ī river erodes a floodplain as it meanders, or curves from side to side. The erosion of a floodplain describes the process in which earth is worn away by the movement of a floodway. There are two major processes involved in the natural development of floodplains: erosion and aggradation. As the Zambezi leaves the wide floodplain of the sandy Kalahari, it enters a narrow basalt channel as fast-moving whitewater rapids. Ngonye Falls, Zambia, marks a remote stretch of the Zambezi River where the floodplain is extremely narrow. These rivers usually have a steep stream gradient-a very deep, fast-moving channel. In fact, some rivers, or parts of rivers, seem to have no floodplain at all. Some rivers have very narrow floodplains. 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. The Barotse floodplain of the Zambezi River, for example, is a vast wetland stretching thousands of kilometers through Angola, Zambia, and Botswana. Some floodplains are extraordinarily wide. The flood fringe of the seasonal Todd River extends the floodplain to 445 square kilometers (170 square miles). Bluff lines, also called valley walls, mark the area where the valley floor begins to rise into bluffs. The flood fringe extends from the outer banks of the floodway to the bluff lines of a river valley. 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.īeyond the floodway is the flood fringe. Floodways can sometimes be seasonal, meaning the channel is dry for part of the year. The first is the main channel of the river itself, called the floodway.

It stretches from the banks of the river to the outer edges of the valley.Ī floodplain consists of two parts. A floodplain (or floodplain) is a generally flat area of land next to a river or stream.

Meander definition geography