What are hydroelectric dams made of?

Related questions

• Why do hydroelectric dams raise the water behind them high as possible?
• What is the name of the dam in Paraguay that is home to the world's largest hydroelectric plant?
• Why is there limited use of tidal energy?
• Why do hydroelectric dams have limited lifetimes?
• How do hydroelectric dams affect river systems?
• How do hydroelectric dams affect the ecosystem?
• How do hydroelectric dams affect the environment?
• How do hydroelectric dams negatively affect the environment?
• How can hydroelectric dams be used as a renewable resource?
• How long do hydroelectric dams last?