I was writing something up on technology and water supply and decided that this was not the right thing to say.
It is the right thing to say to those people who want to replace "nature" water with "manmade" water from desalination, reclamation, etc.
Here you go:
In California, about 45% of water goes to the environment, 45% to agriculture and 10% to cities. If we let ALL the "human" water go to the environment, then we'd need to generate about 40 million acre-feet (one acre-foot is 326,000 gallons or 1.234 megaliters).
The cost of desalinating 40MAF would be about $40 billion. My $1,000/AF estimate is too high if we assume that new technology will lower the price but too low if we consider the carbon footprint of current energy sources (or cost of nuclear waste) or the impact of such a massive increase in demand for energy necessary to run the desalination plants. I am also ignoring capital costs, planning costs, etc.
Note that the average California household (about 3 people) pays about $300-400 per year for water service. The average farmer now pays about $20/AF for water. Since those costs are the cost of constructing and operating the water system -- NOT the cost of water -- people would still pay those costs.
So, they would now pay an additional $1,200 per person (there are 34 million Californians) to generate 40MAF. Twenty percent of that price ($240/person per year) would be paid through household bills, and eighty percent ($960/person per year) would be paid through higher food bills (California exports a lot of produce, but we also import it, so I am going to ignore trade -- especially since someone, somewhere will have to pay this cost.)
Bottom Line: Yes, you can save the environment, but there's a price to pay.