most of the load carried by rivers is deposited into the oceans, seas and lakes into which the rivers empty.
Sometimes the load is carried far away into the mouth of the river before it sinks to the bottom.
Deltas are formed when the load instead sinks at the bottom of the mouth of the river.
When this happens layers of sediment collect and pile up to form a gently sloping platform.
With time the platform extends to the surface to form a delta.
Conditions necessary for the formation of a delta.
The river must have a large load.
The velocity of the river must be low enough to allow its load to be deposited in the river’s mouth.
The river’s load must be deposited faster than it can be removed by the action of tides and currents.
The chances for the formation of a delta are greatly enhanced when clay particles are part of a river’s load resulting in them coagulating and thickening as they mix and react with seawater and settle at the bed in a process called flocculation.
The Congo River has a large velocity at the point at which it meets the sea and thus has no delta as most of its load is carried off into the sea.
The River Niger has low velocity at its mouth resulting in the formation of an extensive delta.
Stages in the formation of a delta.
Deposition in the river’s mouth results in the river forming several distributaries.
The delta begins to form when the initial sediment collects at the bottom near the river’s mouth.
As depositions continues layer upon layer a slow platform results.
Deposition on the banks of the distributaries forms levees.
The area between the distributaries may result in the formation of lagoons.
The lagoons begin filling with sediment which causes further division into distributaries and to the formation of smaller distributaries.
The delta starts to take a more solid appearance although it may still be swampy and usually covered with water loving vegetation.
Further in-filling of the lagoons plus the growth of vegetation results in the older parts of the delta coming to stand above water level forming dry land.
Continued development of a delta can lead to it merging with the flood plain and forming deltaic plains
Types of deltas
There are four types of deltas viz:
Arcuate, Bird’s foot,Estuarine and Cuspate Deltas.