Engineer a Dam - use this lesson plan from TryEngineering to give students a hands-on opportunity to create a dam of their own that will hold back water. Great lesson in engineering and teamwork.
The Association of State Dam Safety Officials has a site full of activities for teachers and kids. Great ideas for building a model dam, a great PowerPoint on different types of dams, information on safety, games, and more.
TeachEngineering has a whole list of dam related lessons for you to help students understand the value, the impact, the safety and the purposes for dam construction. You are going to find something here that will aid your unit on dams.
Check out American Experience: The Hoover Dam from PBS. Learn about one of the great dams of the United States, how and why it was built, who built it, and a timeline of events.
The Bonneville Power Administration offers curricular materials relating to dams, hydropower, rivers, fish, etc. Browse their collection.