California[1] has quite a unique climate; mostly wet winter / dry summer. Water is a huge constraint.
Water used to be distributed much differently. Tulare lake[2], for example, was a massive water feature in the southern San Joaquin valley fed from Sierra snowmelt. Over the years, through damming and enormous engineering efforts such as the Central Valley Project[3] these lower-level lakes and streams dried up and water was reserved upstream, where it could be fed into a series of aqueducts for agricultural use over the dry season.
Water in Calfornia is allocated in three different buckets[4]: environmental, agricultural, and urban. The environmental bucket includes any water that's let run off into streams and rivers to the ocean and accounts for 50% of the water that falls in the state. Agricultural use makes up 40%, and urban makes up 10%. Urban use is lowering each year and the full 10% hasn't ever been used.
The challenge is that California is a naturally dry place. Using 50% of the water that falls on the state puts it in a dangerous position of not being able to sustain critical ecological systems and replenish groundwater. On the other hand, the central valley is a hugely productive agricultural area, producing a majority of the food we eat in the U.S. and a large share of the nuts produced worldwide. Reducing that 40% share of water used would come at a huge cost.
Drought is natural, millions of people living in drought-prone areas is not.
Fewer people or desalination seem like the most viable solutions to reduce the impact on the native environment. Of course, solutions that involve reworking our relationship with water in a much more fundamental way would be even more sustainable, but they go against convention[5] and as a result seem less likely.
Some related links:
Last updated Tue Jul 26 2022 in Berkeley, CA
3: https://en.wikipedia.org/wiki/Central_Valley_Project
4: https://www.ppic.org/publication/water-use-in-california/
6: https://www.latimes.com/opinion/story/2021-05-06/editorial-there-is-no-drought
7: https://news.ycombinator.com/item?id=31234190