One can distinguish two types of floodplains. The hydro-logical floodplain is the land adjacent to the baseflow channel residing below bank-full elevation. It is inundated about two years out of three. Not every stream corridor has a hydrological floodplain. The topographic floodplain is the land adjacent to the channel, including the hydrological floodplain, that is flooded by a peak flow of a given frequency (e.g., the 100-year flood - the flood flow that is equaled or exceeded once every 100 years on average -defines the 100-year floodplain). Higher flood-peak flow return periods define wider topographic floodplains. These two types of floodplains are shown in Figure 3.

Floodplains provide temporary storage space for flood-waters and sediment. This lengthens the lag time of a flood. This lag time is the time between the middle of the rainfall event and the runoff peak. If a stream's or river's capacity for moving water and sediment is diminished, or if the sediment loads become too great to transport, the valley floor will begin to fill. River training methods are often adopted for constraining the lateral movement of alluvial rivers.

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