Well it's sometimes better to give people drinking water than to have old settlments.
https://insamer.com/en/the-middle-eastern-water-crisis-and-t...
https://insamer.com/en/the-middle-eastern-water-crisis-and-t...
But an emerging principle in infrastructure design is that demand grows to saturate supply, and that's not usually sustainable.
This is true for traffic on the interstate (people avoiding rush hour traffic, until a lane is added) or for water availability (the existence of Las Vegas).
Engineers are uncomfortable with the concept, as it tends to veer into public policy. In the case of flood risk management, us policy is catching up with engineering, and FEMA has been offering block grants for relocation of flood prone communities for some time now.
Sometimes the right answer is to help people move to an area that can sustain them.
And I'm all for improving civic development and people's standard of living, fully knowing that it's at some expense to nature. That still meets my definition of practical sustainability.
That said...
The Hoover Dam does have a design life and an expiration date. From an engineering perspective, that's sustainable as long as we have a replacement or mitigation plan figured out and in place by that date.
As for right now, there are real present day environmental costs, up and down river, and throughout the desert. The low flowrate of the lower Colorado certainly is not sustainable, although it's also certainly not 100% due to the Hoover dam. And Lake Mead is filling up much quicker than expected. Some people see that the status quo can't last forever, and say that's not sustainable.
It's not much different than New Orleans relationship with levees, although I suspect few people have thought of it that way.
Either way, to think that Vegas could survive without the Hoover Dam is flat out wrong. I'm not saying tear it down and move. I think it was the right decision to build the dam when they did. But if it weren't built yet, would it be the right decision to build it today? I doubt it, at least designed as is.
My assumption was that after the dam is filled the flow rate can be regulated very close to pre dam levels of at least 95% flow rate factoring in any losses due to evaporation.
All the water being siphoned out to supply Vegas and irrigation comes from somewhere. And doesn't make it downstream. Plus all the water retained by Urban build out (ponds, vegetation, pools) that normally would flow straight into a river and head downstream.
Over burden pressure from the lake forces water into the ground. The temperature of the water in the lake rises, causing chemical changes and making evaporation downstream more likely. Sediment drops out of the water column, allowing light to penetrate deeper than it normally would.
All of that is before you even get into the flow rate variance. Because winter floods don't "flush out" the canyon, there are problems with sedimentation downstream, which causes water to slow and evaporate more...
And this is just hydrology/hydraulics, before you touch any of the biological / ecological aspects.
It gets complicated, quickly.
Even if you assume it's only 5% evaporation reduction and exclude what's pumped out, there are at least what? 15-20 major dams that feed the Colorado? Even if you assume 3% times 15 dams, that's almost half the flow of the river, before you even pump anything out.
On the other hand, it's extremely hot there, and not really an enjoyable place to live unless you like being inside most of the year. On top of that, all the asphalt and concrete causes a "heat island" effect, making the city even hotter than the surrounding desert, and unlike the open desert, it never cools off at night (all that concrete and asphalt constitutes thermal mass). And since people don't like living in 100F+ conditions continuously, a lot of energy is used running air conditioners to try to make the place habitable.