Floodplains – equally important as main river channel
Floodplains are an integral part of the riverine systems. It is hydrologically connected to the main river channel but was historically neglected. Much of the world floodplains have been reduced or disconnected.
A floodplain is a broad strip of land built up by sedimentation on either side of a stream channel. During floods, flood plains may be covered with water carrying suspended silt and clay. When the floodwaters recede, these fine-grained sediments are left behind as a horizontal deposit on the flood plain.
The floodplain is also continually being eroded, especially in lowland, meandering rivers. The floodplain often represents the mosaic of the side-arms and dead-arms (oxbow lakes), created by the river dynamics.
Some flood plains are constructed almost entirely of horizontal layers of fine-grained sediment, interrupted here and there by coarse-grained channel deposits. Other flood plains are dominated by meanders shifting back and forth over the valley floor and leaving sandy point bar deposits on the inside of curves. Such a river will deposit a characteristic fining-upward sequence of sediments: coarse channel deposits are gradually covered by medium-grained point bar deposits, which in turn are overlain by fine-grained flood deposits.
As a flooding river spreads over a flood plain, it slows down. The velocity of the water is abruptly decreased by friction as the water leaves the deep channel and moves in a thin sheet over the flat valley floor. The sudden decrease in velocity of the water causes the river to deposit most of its sediment near the main channel, with progressively less sediment deposited away from the channel. A series of floods may build up natural levees —low ridges of flood-deposited sediment that form on either side of a stream channel and thin away from the channel. This is the reason why some tributaries do not merge directly but instead flow almost parallel for some length of the course.
The sediment near the river is coarsest, often sand and silt, while the finer clay is carried farther from the river into the flat, lowland area.
Very often, the floodplains are biologically even more productive than the main river channel. The backwaters in the floodplains are important hatcheries for fishes and other animals.
Much of the world floodplains have been reduced or disconnected, and the main focus was on the main river channel. Today, the active, functional floodplain is equally essential as a riverbed. This is especially true in food defense, as floodplains store vast amounts of the water during flooding (integral flood management). Flood defense is just one of the many benefits of the floodplains, so-called ecosystem services.