The Floodplain
The floodplain consists of two main components:

• An area of floodable land that is submerged in the flood and dry in the low water season.

• A number of water bodies some of which retain water throughout the year and some of which dry out.

During the floods, water rises in the channel until it reaches a level beyond which it overflows the riverbank and begins to spread across the low-lying floodplain on both sides of the river. This level is known as bankfull discharge.

When water first crosses the bank, the speed of its current diminishes rapidly and it deposits the coarser sediments to form a raised area known as a levee. Levees usually flank the main channel, but may be present on the banks of channels feeding the floodplain water bodies. Levees provide areas that are relatively suitable for human settlement and many villages are found on them because they are less prone to flooding. Levees are often heightened artificially to lessen risks of flooding, and to retain the main channels of the river water within the channel even during floods.

Overbank flows across the floodplain form localised currents that erode the plain, building new channels and lakes. In other places eddies and local slowing of currents deposit sediments to form areas of higher ground and levees. Differences in altitude over the floodplain are not great and the whole area gives an impression of being flat. Nevertheless the few centimetres of difference between one area of plain and another can make a great difference to flooding patterns, habitats for aquatic organisms and vegetation types.

At any one time, the form of the floodplain results from interactions between the channel forming process of the main river and the erosion-deposition processes on the floodplain. Under natural conditions, these create a ratio of water bodies, channels and floodable land that remains relatively constant over time although individual water bodies disappear through silting and new ones take their place. Under modified conditions, the rate of disappearance of old bodies and appearance of new ones is greatly influenced by the amount of silt carried by the river.

Water bodies formed from channels are usually relatively deep and, initially, the same width as the channel from which they were formed. Water bodies formed from surface scour are usually shallow but more extensive.

In their original state many floodplains were forested. Others were covered with grassland that formed extensive floating mats during the floods. Some of the grassland floodplains were used to graze cattle and were maintained by seasonal burning. More floodplains of both types recently have been cleared and developed for various types of agriculture. In the Mekong basin most floodplains now serve for rice culture, with artificial separations between fields and extensive ditching that has largely replaced the original structure of the plain.

Habitats of the floodplain are very varied, despite the apparent flatness of the environment. During the wet season the roots and submerged stems of the masses of floating vegetation that grow across the plain are covered with mats of epiphytes. Young fish and other aquatic animals feed on this as well as find shelter among the roots. Young of other species are found in the shallow water at the edge of the advancing flood where there is an abundance of small animals (crustacea and insects) that serve as their food. Adult fish use the floating vegetation or the shallow water at the edge of the floodplain as breeding sites. They also feed on the larger invertebrate life in the vegetation as well as on seed falling from the flowering heads of the grass. Other aquatic organisms use the flooded forest for similar services with the addition of the food provided by falling fruits and seeds.

During the dry season, the different types of pool form different types of living space depending on their size and depth. The riparian zone of floodplain lakes is usually fringed by emergent vegetation. The shallows are filled with floating leaved plants and the clearer areas attract rooted, submersed aquatics.

Many pools dry out killing all the fish that have taken refuge there. Others retain water throughout the year. Larger lagoons may retain oxygen throughout the year but smaller lagoons may become completely devoid of oxygen.

All these different characters attract different species of fish and other aquatic animals accounting for the huge diversity of species.

Normally floodplains are relatively narrow following the river for much of its course. However, in certain areas special features arise. Extra large plains may be flooded where geographical conditions are right. In these the river tends to form several main channels and such areas are known as internal deltas.

In other areas, large lakes may persist - these are usually the remnants of marine incursions or old freshwater lakes that are silting up but have not yet become completely filled.

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