The main process in the lower course is deposition. Water from other tributaries contributes to the main river therefore increasing discharge, but the river’s energy is usually overpowered by the increased sediments. This section is also where the river meets with the lake, sea or ocean.
Levees
Levees are raised banks created from the accumulation of dumped sediments by a river. Large sediments are usually deposited nearer the river while small sediments such as silt are deposited further into the floodplain. Levees are useful as they prevent water from overspilling out of the river (flooding). Humans can build their own levees to prevent flooding.
Braids/Braiding
Braids are formed when a river splits into several distinct channels separated by islands called eyots or bars. When the load overpowers the river’s energy to be carried any further, the river has few options but to drop some of the load. Braiding usually occurs when a river carry a lot of sediments
On fairly steep land
Alternating river discharge.
Erodible banks.
Sediments accumulate behind vegetation. Sometimes braids can be mixed up with meandering rivers
Tanana River in central and eastern Alaska, photo by James St.JohnDeltas
These are true lower course landforms. Deltas occur on the intersection of a river and the sea or ocean or any large water body. For deltas to form:
- the river must carry a large load
- river velocity and gradient must be low so that sediment is deposited thoroughly
- tides must be low or else sediments are washed away
- fresh and salt water so that clay sediments flocculate and settle down at the bottom
When sediment is deposited it forms three layers; the bottom set is composed of fine silt and clay which are deposited further into the sea and lie horizontally; the second layer is called the foreset which consist of coarse sediments with a sloping profile; the top layer is called the topset wwhich include more coarse particles that also lie horizontally.
3 Main Type of Deltas
Bird’s Foot
As the name suggest, these deltas resemble the foot of a bird. Bird’s foot deltas extend and deposit their load further into the sea because the river’s energy is more powerful than the sea tides and waves (rare). A good example is the Mississippi Birds Foot Delta in USA.
Cuspate Delta
These form when ocean or sea waves hit head on with the river redistributing sediments evenly, Ebro in Spain is a good example.
Arcuate
Arcuate deltas are the most commonest types of deltas and mostly controlled by tides. They contain extensive distributaries that form a fan appearance. Niger Delta in Nigeria and Ganges in India and Bangladesh are good examples.
Delta’s are advantageous as they provide very fertile lands for agriculture. The Ganges Delta is mostly used for farming purposes. However, land mismanagement often pose a threat to deltas. Infrastructure such as dams and HEP and tempering with river flows often prevents the formation of deltas. Dams collects sediments behind them and reduce sediment supply for delta formation.
