Many years ago, civilizations learned to build dams, to prevent flooding and drought.
Try to build a large scale dam in California today.
In CA and the US many dams have been built and all the good locations for dams already have dams.
If you build a dam in a lesser location it will have a higher ratio of surface to volume and have a high evaporation loss, require much more land, have higher construction costs, have higher risk of failure, and go dry very quickly because they have low inflow rates.
Having said that, there is room for 2 more dams on the Colorado river which would benefit CA, but both dams would be in the Grand Canyon. In the 50s and 60s the Bureau of Reclamation tried to build one of these dams in the Canyon which was called Bridge Canyon Dam, but public opinion killed it.