Unfortunately, the St. Francis dam collapsed in 1928, killing approximately 600 people. (In Chinatown, the depressed water engineer Hollis Mulwray is vaguely based on Mulholland post-St. Francis dam disaster.)
So, in the 1930s, Los Angeles went back and pushed a huge amount of dirt in front of the Hollywood version of the dam to keep from losing Hollywood. I hadn't realized how tall the dam is under all the dirt until seeing this photo of the safety project from a 1934 Popular Science:
So, I don't know. Maybe we have good reasons for doing things more slowly now?