I always speculated that it should be possible to substitute the Wolga inflow to the Caspian sea via water from the Don river near Wolgograd? Certainly politically and technically complex. But given the long Russian history in in geo-engineering, certainly not a new idea?
The Don River does not transport enough water. In order to fully balance a Caspian Sea Level drop of 18 m in 80 years, you would need a water volume input of about 2600 m3/s, which is 3 times the Don River water volume transport (and only if the Don River catchment basin does not get drier in the future)!