They did not cut the line to Crimea, that is on the east bank and can be supplied without the dam apparently, and they damaged the dam just enough to engulf and drown the islands but not flooding the RF controlled territory. If you ask me, the only thing that does not match is them not shelling the boats that were evacuating the Ukrainian soldiers.
I don't know what they planned, but seems that things didn't went as planned. Some of RF controlled cites and villages like Nova Kahovka or Oleshky become almost completely flooded. Also, floodfing broke first defense line of Russia. And this information was posted by Russian sources.
Also, Russians telling that dam was destroyed by Himars. But you must be really dumb to believe that's possible to destroy such dam and make so much damage with Himars. You only can do it from inside.
That's why Russians are now saying that they did not blow it up because they made a mistake. They drowned their own people who were stationed along the banks of the river and damaged the power and water supplies to their own, occupied land. Land that they were supposed to protect from nazis. Now they're going to blame the Ukrainians just like they did with that downed Malaysian flight.
And here's how the protection is going. Russians will come to liberate you with tanks, rape your women, kill your dogs, murder men in cold blood, steal your cars and washing machines, drink all your alcohol and then blow up a dam or a nuclear power plant and destroy what's left. If I were to find myself under the protection of RF, I'd pack my bags and run West.