A flood plain is a type of geological feature that results when a river periodically overflows its banks due to rainfall, snow melt or other factors. Floodplains are initially formed due to the meandering course of a river gradually. Floodplains were critical to the survival of human civilization in antiquity because of their role in promoting agriculture, such as the annual flooding of the Nile River delta in Egypt. Flood plains contain other geological features such as oxbow lakes, point bars and natural levees due to the erosion and deposition of alluvium, or sediment.
Meanders and Floodplains
A meander occurs when a river alternates its direction of flow due to the downward slope of a valley. Because valleys are V-shaped, this creates an alternating course for the river as it flows toward the ocean or sea. As the meander approaches the ocean, the valley flattens out and the course of the river widens. When the water overflows, it carries layers of sediment and gravel that create a floodplain.
An oxbow lake is a crescent-shaped lake that results from the meandering course of a river along a floodplain. According to Enchanted Gardens Wetlands Restoration, the defining factor in the formation of an oxbow lake is erosion. Water flows more quickly on the inside edge of a bend than it does on the outside edge, eroding the two adjacent banks on either end of the meander over time and diverting the water flow along a straighter path. The cut-off portion of the river becomes an oxbow lake. Oxbow lakes eventually become wetland due to the deposit of sediment and lack of water flow.
Point bars consist of alluvium that has been swept or rolled into place by secondary water flow at the bottom of the river. According to MIT, secondary water flow results from a pressure differential created by differing velocities of primary water flow along a curved path. The pressure causes gravel and silt to roll or be swept into place, creating a gentle slope that matches the riverbank's elevation.
Natural levees form when a river periodically floods its bank and deposits coarse alluvium such as gravel onto the banks in progressively higher stages when the river spreads and slows down its flow. If the river is not flooding, alluvial deposits can settle on the riverbed, thus raising the river level. Natural levees act as raised boundaries against rising water levels.
- Floodplain image by Bailey from Fotolia.com