PROS:

• 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.
• Producing Power.
• 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. 

   both pictures courtesy of: wnfm.com         Hoover Dam generators                                                                                          Lake Mead

CONS:

• 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.