Why does silt often accumulate at the mouths of rivers?

Silt accumulates at the mouths of rivers primarily because of the relationship between water velocity and sediment transport. As a river flows from its source toward the sea, its velocity typically decreases, which reduces its capacity to carry suspended materials.1

When a river reaches a standing body of water, such as an ocean or lake, its velocity drops significantly. Because silt is easily transported by moving currents but settles in still water, the reduction in flow causes these fine particles to drop out of suspension and accumulate. This process is central to the formation of deltas, where sediment is deposited faster than marine processes can remove it.234

In the context of land reclamation, this natural accumulation is sometimes leveraged; engineers may divert sediment-laden streams into diked-off coastal areas to raise the land level, which facilitates subsequent drainage and development efforts.5