Answer: The beach becomes narrower and the cliffs behind it are subjected to a higher rate of erosion.
Step-by-step explanation:
Building a dam on a river that flows to the sea makes the coastal beach narrow, and the cliffs behind it become subject to a higher rate of erosion.
Due to the damming of inflowing rivers, the sediment supply to the coastal shore becomes limited, making the coast on the opposite side of the dam more vulnerable to coastal erosion. Dams accumulate a significant volume of sand, reducing the amount of sand accessible to beaches, which narrow as a result.