delta

A low-lying area found at the mouth of a river and formed of deposits of alluvium. Deposition occurs as the river's speed, and hence its silt-carrying capacity, is checked when it enters the more tranquil waters of a lake or sea. Furthermore, clay particles flocculate in salt water, become heavier and sink. Post-glacial rises in sea level have increased the slowing effect of the sea and some deltas grow by as much as 200 m a year.

Other, older deltas may be eroded. The morphology of a delta is the result of the interplay of the following factors: the input of sediment from the river, the density and depth of the sea water, waves, and currents, and any tectonic activity in the region. Cuspate deltas have a pointed seaward end. Lobate deltas have a curved seaward end and digitate or bird's foot deltas, like that of the Mississippi, have long ‘fingers’ of alluvium extending into the sea. Inland deltas form in hot, arid areas of inland drainage where water is gradually lost through evaporation.