The creation of freshwater flood wetlands
is enhanced by the sediment-carrying
properties of rivers. When a river floods and spreads across its floodplain, the
reduced water velocity lowers its sediment-carrying abilities and it deposits
material that it is carrying. Coarse materials, requiring the most energy to keep in
suspension, tend to be dropped first, close to the edge of the channel, creating a
barrier, or levee, which effectively raises the height of the channel rim.
The effects of an embankment or levee
are two-fold: it reduces the number of times
a river will spill over onto its floodplain, increasing the seasonal nature of flooding,
but at the same time it impedes drainage, so that the wetted area will remain
waterlogged for a long period after the flood has subsided.
