CA is in a wet period. And dams dont just trap water but they regulate the flow so as to prevent flooding. Before the Shasta lake dam was installed the Sac used to have devastating floods. There is a train trestle about a mile long in Redding that was built to go over the flood plane. Now it has housing complex's under it, thanks to the water saving Shasta dam
A few years before the painted crack someone dropped a banner down the front of the dam which appeared as a top to bottom crack. I don't recall who did it, Earth First or Greenpeace, but the news report said it scared the crap out of a worker driving up the road that morning.