When you approach Shasta Dam (Shasta Dam Blvd., 530/275-4463) by water, it looks like, well, a big concrete dam. (Also, you’re not allowed terribly close.) To learn what the pipes and structures actually do, you must get to the dam by car and take a tour of the facility.
Even if hydro-engineering isn’t your thing, it’s worth your time to check out the dam, for the views of both lake and Mount Shasta  if nothing else. But don’t bother bringing a camera inside; for security reasons, no photography is allowed inside the dam.
The tour lasts one hour and explains both the construction and the current function of the second-biggest dam in the United States.