Dams can store rain water or water directly from the river itself. Then, in case of a Drought, the dam will still have a relatively constant supply of water.
Controls flooding & provides recreational activities such as boating fishing and swimming, if the lake is not being used for drinking water (Dam Society).
Simple design makes for inexpensive repairs and maintenance costs (Dam Society).
Produce inexpensive (after completion) and clean power.
Renewable energy source, because the water is not destroyed by passing through the dam.
If needed, dams can be shut down instantly, where thermal plants take hours, and nuclear plants can take days! (Dam Society).
Very few breakdowns.
Hydroelectric power production require flooding of entire valleys and scenic areas.
Disrupts natural seasonal changes in he river, and ecosystems can be destroyed.
Ends flooding that help to clean out the silt in rivers, causing them to clog (Energy Laboratory).
The silt that usually flows down to the Beaches and Estuaries is block by the dam.
Studies show that the plant decay caused downstream of major dams produces as many greenhouse gasses as more conventional methods of producing electricity.
Dams are expensive to build, and due to drought may become useless, or produce much less power than originally planned.
A dam being build in Quebec will end up flooding an area as large as Switzerland (Energy Laboratory).
Dams can break in a massive flash flood.