A river in a sediment transfer or deposition zone generally has a foodplain, a flat valley bottom constructed fo loose or unconsolidated alluvial sediment deposited when the river floods or shifts its channel. Geomorphologically, a floodplain is an area flooded by the river with a predictable though by no means regular - frequency; a five year floodplain, for example, is that part of the valley which will be inundated by a flood of a magnitude occurring, on average, once every five years. Ecologically, however, the floodplain is better restricted to the zone close to the river which receives regular, seasonal flooding. All rivers experience floods but whereas low order stream, which are strongly influenced by local precipitation, have an irregular flood pattern with numerous peaks, larger rivers, fed mainly by their tributaries, are affected by seasonal weather patterns rather than single rainfall events. This difference is of fundamental ecological importance, as organisms in low order streams are adapted to surviving the rigours of flood events,