How might building a dam on a river that flows to the sea affect a coastal beach? How might building a dam on a river that flows to the sea affect a coastal beach? The cliffs behind the beach experience lower rates of erosion. The beach becomes narrower and the cliffs behind it are subjected to a higher rate of erosion. The cliffs behind the beach have a lower probability of experiencing mass movement. The beach becomes wider, as there is less water input from the river to erode the beach's sand. The coast becomes emergent, as less water is driven to the beach by the river once the river is dammed.