When a river enters a sea or lake, sometimes a delta is formed. As the water enters the sea, the velocity slows down, until it is not possible to transport any sediment. Flocculation will also happen due to the salt in the water, and smaller particles join together into larger clumps. Over time, these will build up on the sea floor, until they rise up above the sea - this is a delta.
As smaller sediment is deposited further from the channel, and larger closer to the channel, this forms the semicircular shape of the delta. The river may also split into distributaries as it flows through the delta, because the main channel will periodically be blocked by excess sediment, and its flow will spill over and create a new channel.
Types of deltaEdit
Deltas form differently, depending on the strength of the tide and the amount of sediment it is carrying.
The land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect. - like the Nile delta.
With much more sediment, the channel is built out further than the arcuate deltas. There are also weaker currents. An example of a bird's foot delta is the Mississippi delta.
A cuspate delta juts out into the sea in an arrow shape.