If you could be sure of no trace chems that might be harmful - resrvois might be a heck of a use for an abandoned mine site. Resorts could surround it, and lots of economic activeties result. But would it be safe?

Fishing, camping, lakefront resorts, all with sunny weather, and moderate temperatures in the middle of winter elsewhere.

Assuming of course that any water diverted in didn't just SLURP right down into the ground.

There is an old legend of an underground river that empties into the Baja Gulf, far beneath the sands. Clive Cussler briefly talked to that idea in his book "Inca Gold"

It makes me wonder what an orbital scan by satellite or shuttle would show us? A similar scan showed evidence of irrigation and river channels beneath the Sahara that originated in the Nile region and emptied into the atlantic ocean. It points to what is now desert in Africa having once been highly fertile areas.