Plot hole: After Superman has reversed time the Hoover Dam reverts to its previous undamaged state as it should, but Jimmy Olsen should then have been replaced back on the dam. Instead he interrupts the potential kiss between Lois and Superman and complains of being abandoned by Superman in the desert.
Superman (1978)
Ending / spoiler
Directed by: Richard Donner
Starring: Gene Hackman, Marlon Brando, Ned Beatty, Christopher Reeve, Margot Kidder
Superman stops Lex Luthor's plan to send the west coast into the sea by repairing the San Andreas fault, he also puts the Golden Gate bridge back together and stops a town near the Hoover Dam from being destroyed. But Lois Lane gets buried alive by rocks and sand and dies. Superman finds her, goes around the Earth, and ends up going into the past before she was buried. Superman then picks up Lex Luthor and Otis and takes them to the state penitentary. The movie ends with him flying into space with John Williams' brilliant main title playing. [To Credits]
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Suggested correction: Superman is moving at speeds fast enough to turn back time. He can spend 5 seconds to yank Jimmy away from the top of a dam he knows is going to be destroyed just to be safe in case he can't actually stop it all from happening again.
Lex Luthor: We all have our little faults. Mine's in California.
Trivia: To get the muscular build required to play Superman Christopher Reeve went on a training regime supervised by David Prowse who played Darth Vader in the first Star Wars trilogy.
Question: Why, in the 3 hour TV version of the movie, does Luthor waste time on that elaborate trap if he already knows, from reading Lois' interview, that bullets, fire, and cold won't stop Superman?
Answer: He wanted to see for himself if the stories were true. Some reporters tend to exaggerate the facts and if he had any other weaknesses. He couldn't be sure the kryptonite would work.
While I don't exactly disagree that Lex wanted to see for himself how invincible Superman is, I don't think that's the main reason why he did it. The bullets and fire were simply a charade to make it look like this was his security system. Keep in mind, he wanted Superman to enter his lair because the real trap was the Kryptonite that he had in the lead box.