River Flooding and Floodplain Geography

A river floodplain is the flat, low-lying land bordering a channel that forms from repeated deposition of sediment during overbank flows, the natural process by which a river periodically exceeds the capacity of its channel and spreads across adjacent land. Geomorphologists define the "bankfull discharge" as the flow at which a channel is completely filled, a threshold that, for many temperate rivers, recurs on average every one to two years according to the classic work of Luna Leopold and M. Gordon Wolman in their 1957 U.S. Geological Survey study of river channel patterns. Floodplains are built from two distinct sediment facies: coarser channel-lag and point-bar deposits within the active channel belt, and finer overbank or "vertical accretion" deposits — silts and clays — laid down as floodwater slows upon spreading beyond the banks. This same fertility explains why floodplains have attracted dense human settlement since the earliest agricultural civilizations: the Nile, Tigris-Euphrates, Indus, and Yellow River valleys all owe their agricultural productivity to millennia of nutrient-rich alluvial deposition, a benefit that came bundled with recurring flood hazard from the outset of settled agriculture.