Actually lake mead in game is radiation free, maybe not that realistic but fallout 3 purified the Delaware river by building a giant water purifier in the Jefferson monument, which happens to be at the river's delta, which flows into the ocean. How exactly purified water goes upstream remains a mystery.
I know Lake Powell should be full of sediment in ~500 years. So the extra ~250 years of time would still leave it fairly full of sediment, functionally raising the lake bed. This means even with less water there could be a more apparent height to the reservoir.
I mean the immense population die-off and the god-blessed destruction of that affront to God and all things holy, Phoenix, would surely reduce demand for water 200 years after a nuclear war
