How do floodplains develop fertile soils

Info Floodplains Floodplains. Floodplains are the low-lying areas of land where floodwater periodically spreads when a river or stream overtops it banks. Riparian vegetation along streambanks and in the floodplain reduces the velocity of floodwaters, lessening the erosive force of the flood and capturing nutrient-laden sediment. Floodplain soils absorb water during the wet season, then slowly releases moisture to plants and into the stream.

This buffers the effect of peak runoff and keeps plants growing and streams flowing longer. Streambank vegetation also helps cool surface water temperatures, and creates important habitat for fish, waterfowl, and other wildlife species.

Flood waters bring moisture and a surge of new nutrients into the floodplain. A terrestrial ecosystem is a land-based community of organisms and the interactions of biotic and abiotic components in a given area. Examples of terrestrial ecosystems include the tundra, taigas, temperate deciduous forests, tropical rainforests, grasslands, and deserts. The type of terrestrial ecosystem found in a particular place is dependent on the temperature range, the average amount of precipitation received, the soil type, and amount of light it receives.

Use these resources to spark student curiosity in terrestrial ecosystems and discover how different abiotic and biotic factors determine the plants and animals found in a particular place.

A flood happens when water overflows or soaks land that is normally dry. Floods can be destructive to humans and the natural environment, but they also help to drive biodiversity and are essential to the functioning of many ecosystems.

When rivers flood, sediment gets deposited on the floodplain. This sediment is called overbank deposits. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.

Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. A floodplain or floodplain is a generally flat area of land next to a river or stream. It stretches from the bank s 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. Beyond the floodway is the flood fringe. The flood fringe extends from the outer banks of the floodway to the bluff line s 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 square kilometers 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 lake s that document the meandering river and its tributaries over time. Features such as oxbow lakes and seasonal wetlands are often a part of floodplains created through erosion and deposition.

A typical aggradation environment is a wide, shallow, braided river. Braided rivers often include river delta s, 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 breach es, 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.

The layered sediments of many floodplains can create important aquifer s. Clay, sand, and gravel filter water as it seep s downward. Water purification systems often take advantage of this natural phenomenon in a process called bank filtration.

In bank filtration, water is deliberately filter ed through the banks or floodplain of a river or lake. Nearby wells then collect the filtered water, which is then ready for more intense purification processes. The sedimentary patterns of floodplains often provide scientists with evidence of past geologic activity. Thick layers of sand may indicate flash flood ing, for instance, while thin, evenly spaced layers of silt may indicate more moderate and predictable flood patterns.

One of the most important geologic features of a floodplain is its fluvial terraces. Fluvial terrace s are step-shaped areas of land that flank the banks of a river or stream. Fluvial terraces mark the older, higher-elevation paths of the stream, before erosion and aggredation created the current mainstem of the stream or river. Fluvial terraces can mark the bluff lines—outer edges—of a floodplain. There are two major types of fluvial terraces: fill terraces and cut terraces.

Fill terrace s are formed as a valley or gorge is filled with alluvium. Alluvium can aggregate as a river meanders and overflows its banks, or it can be deposited by a glacier.

While fill terraces are associated with aggredation, cut terrace s are associated with erosion. Cut terraces are often formed below fill terraces, as water erodes sediment. Of course, floodplains are ideal sites for agriculture — they are flat, water is accessible, and — at least prior to modification of the system by levees or dams — the soils are among the most fertile on Earth due to recurring flooding that deposits nutrient-rich fine-grained sediments.

Likewise, most major modern agricultural production is localized to floodplains - including the Central Valley of California, the Susquehanna River Valley, the upper Tigris-Euphrates basin, the Nile Valley, and the floodplains of the Mississippi and Missouri Rivers.

Prevention of flooding through the combination of dams which control river discharge and levees which artificially channelize flow and shunt it downstream so that it cannot spill onto the floodplain is a strategy to limit the loss of crops and property, and allow development in otherwise flood-prone areas. This approach, while generally effective in limiting short-term losses, affects soil fertility, groundwater systems, and the health of downstream waterways in the longer-term.

For example, flood prevention eliminates a major source of recharge to aquifers in valley-fill sediments that lie below the floodplain.

Recurring floods also serve to flush salts that accumulate naturally in soils due to evaporation and transpiration i.