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.

Fema flood zone definitions