River flood plains: Some observations on their formation

On many small rivers and most great rivers, the flood plain consists of channel and overbank deposits. The proportion of the latter is generally very small.

Frequency studies indicate that the flood plains of many streams of different sizes flowing in diverse physiographic and climatic regions are subject to flooding about once a year.

The uniform frequency of flooding of the flood-plain surface and the small amount of deposition observed in great floods (average 0.07 foot) support the conclusion that overbank deposition contributes only a minor part of the material constituting the flood plain. The relatively high velocities (1 to 4 fps) which can occur in overbank flows and the reduction in sediment concentration which often accompanies large floods may also help account for this. Although lateral migration of channels is important in controlling the elevation of the flood plain, rates of migration are extremely variable and alone cannot account for the uniform relation the flood-plain surface bears to the channel.

Detailed studies of flood plains in Maryland and in North Carolina indicate that it is difficult to differentiate between channel and overbank deposits in a stratigraphic section alone.

Because deposition on the flood plain does not continue indefinitely, the flood-plain surface can only be transformed into a terrace surface by some tectonic or climatic change which alters the regimen of the river and causes it to entrench itself below its established bed and associated flood plain. A terrace, then, is distinguished from a flood plain by the frequency with which each is overflowed.