What does 'siltation' refer to in a river context?

Siltation refers to the accumulation of sediment, such as sand, silt, and clay, in a river or water body. This process can occur naturally over time due to erosion from the surrounding land and the transport of these materials by the river's flow. When sediment builds up in a river, it can significantly reduce the waterway's capacity, potentially leading to shallower areas that affect navigation and the overall health of the aquatic environment. This reduction in navigability can pose challenges for transportation and can also increase the risk of flooding, as the river may struggle to channel water effectively.

The other options relate to different processes that do not accurately describe siltation. The increase in flow speed does not characterize the accumulation of sediment but rather a change in the river's dynamics. The seasonal migration of fish pertains to biological patterns rather than sediment deposition. Lastly, the buildup of organic matter on riverbanks, while relevant to river ecosystems, does not specifically describe the sedimentation process itself.