Benefits:

Costs:

The creation of a reservoir requires the clearing of the nearby land, displacing farmers and homes.
Dams are very expensive to build.
Soils downstream, particularly in floodplains, can become less fertile. This affects agricultural production.
Dams can interfere with migrating species.
Algae collected behind the dam can deoxygenate the water in the reservoir.
The weight of the reservoir on the land can trigger an earthquake.
Reservoirs often flood areas of rare or endangered species.