Lake Manasarovar v.s. Lake Rakshastal: fresh-water v.s. salt-water In Tibetan Plateau, about 4600m elevation, If Lake Manasarovar and Lake Rakshastal ("lake of the demon" ) used to be the same lake, but due to the tectonic activity now they are separated to two by small hills, how come Lake Manasarovar is a fresh-water lake while Lake Rakshastal is a salt-water lake?

Before they were separated, was the overall old lake a fresh-water lake or a salt-water lake?



Is that just because Lake Rakshastal is in the Endorheic basin and Lake Manasarovar is not within the Endorheic basin?

Also, Lake Manasarovar overflows into lake Rakshastal which is a salt-water endorheic lake. These lakes used to be part of the Sutlej basin and were separated due to tectonic activity.
You pretty much answered the question yourself.  Lake Rakshastal, along with most of the lakes along the Tibetan plateau is, indeed an endorheic lake.  The important meaning of that term is that there is no outflow from that basin.  Water evaporates, but as with any evaporative process, the minerals in the water stay behind and become more concentrated.  Since Lake Manasrovar flows into Lake Rakshastal, the water that leaves takes the minerals in contains with it, leading to a much lesser concentration left behind as new water flows in, and, most importantly, out of that lake.