Answer:
a barrier of concrete, earth, etc, built across a river to create a body of water for a hydroelectric power station, domestic water supply, etc. a reservoir of water created by such a barrier.
a barrier constructed to hold back water and raise its level, forming a reservoir used to generate electricity or as a water supply.
a wall built across a river that stops the river's flow and collects the water, especially to make a reservoir (= an artificial lake) that provides water for an area: