It's called 'lack of water'. Back then, plenty of snow, which was stored in the Sierra Mountains. When warm weather came, snows melted and ergo the rivers ran high. Able to be stored, and used. Not so much today. The 5 is full of dried up trees, stone fruit, nuts whatever, because the farms west of the 99 don't have enough water allocation. It was very pretty, once upon a time. California would rather spend tens of billions on a train from Bakersfield to Merced (ps, not a single rail of track laid to date), instead of dams to hold that water for sustained release. And older timers than I have told tales of boats that sailed down the middle of the state from San Francisco, on the waterways.