Which of the following describes the phenomenon when a river cannot transport all the sediment it carries?

The phenomenon being described occurs when a river is unable to transport all the sediment it carries, leading to the sediment settling out of the water and becoming deposited on the riverbed, banks, or floodplain. This process is known as deposition.

Deposition happens when the energy of the river decreases, which often occurs in situations such as a reduction in water flow velocity, a change in channel shape, or the river entering a standing body of water like a lake or ocean. When the flow of the river can no longer support the weight of the sediment, particularly larger particles, these materials will begin to settle. This can create various landforms, such as river deltas and point bars, which are crucial to the river's ecosystem and geography.

Other terms, like scour, refer to the removal of sediment from the riverbed or banks due to the force of moving water, while transport limitation refers to conditions when the transport capacity is reduced, and flow separation describes where the flow of water diverges from the main channel, neither of which specifically addresses the settling of sediment that occurs during deposition.