Trees bind soil and prevent soil erosion and mudslides. 

California needs regular water supply to help preserve and grow green forests. Right now they have dry patches of barren lands - all contribute to erosion and mudslides.

And regular supply of water could be available to California if they were not diverting massive freshwater towards the Pacific. 

So, they are causing a huge environmental disaster by not letting the freshwater from the north fertilize the plains of California. Because these measures have chain effects.