No doubt it was the Russians, same as with blowing up the Nordstream pipeline and shelling the Zaporozhia nuclear plant. There was absolutely no shelling of the dam from the Ukrainian side and numerous threats to blow it up when the Russians held both sides of the river. It's in the Russian's best interest to deny water supply to Crimea, cooling to the Zaporozhia nuclear plant, and to flood the left bank (which is much lower than the right one) to clear most of prepared defensive structures and mine fields. /s