A. Decrease; reduces
The dams decrease the amount of sediment carried downstream, which reduces downstream soil replenishment.
Step-by-step explanation:
Dams are a huge cause of human influenced environmental threat to the natural ecology. Dams play a major role in controlling the water flow, sediment load, and rate of sedimentation of the water flowing in the rivers which supply the dam.
The flow of water through restricted channels of a dam will filter or reduce the sediments present in the downflow water. this results in piling of sediment load in the upper regions while starvation of sediments in the downstream lower regions. This results in the lower regions downstream with poorly-enriched or sediment-starved habitat which cannot support a large number of species. Thus dams reduces the soil replenishment downstream.
Maintaining a fertile downstream is essential for maintaining ecological biodiversity, protecting delta regions, formation of fertile lands with fertile soil like alluvial soil, coastal wetlands etc.