I don't see any logical reason to blow up the dam (which may be the error in my thinking since we are talking about Russia, but alas).
If they want to cut Ukraine off from the generated electricity then wouldn't it be smarter and less bothersome to just cut the electrical power lines from the power plant? The plant is on the left bank of the river in Russian held territory, so even after retreating from the right bank it would be out of reach for Ukraine. And since it is a stationary target Russia could destroy the plant itself at any time.
Also I don't see how it would prevent Ukrainian troops from pursuing the Russians over the river because how should they even do that in the first place? The bridges are in ruins, pontoon bridges would be easy targets and if Russia has at least a tiny residual of common sense left they will take everything that can swim to their side of the river, so Ukraine would be unable to cross it. Ukraine would be left with small inflatable boats which are only useful for small commando forces but not to pursue an army.
It would even be counterproductive since it would lower the water level of the reservoir making a potential river crossing more easy for Ukraine in that area (albeit, as said, very unlikely in my opinion).
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