Close, but the Russian had heavily mined **inside** the dam and the power plant last year. They likely also placed more heavy explosive charges as well and wired the lot to a switch of some sort.
It takes tons of explosives to blow a dam, a single mine will not blow it. A bomb in the water will just displace water, doing nothing to the dam. The explosives have to be placed inside the dam to work.
The Russians did not open the gates because they wanted to build up the water pressure on the dam to an extreme, so that when it blew, complete destruction was ensured.
As for the detonation - some heroic Russian clicked a switch to blow the mines or heavy explosives - water pressure set off any remaining mines that had not detonated - those are the explosions in the after-detonation images.
It is instructive to point out here that not only the dam was blown, but the entire hydroelectric power plant - you can see in one image smoke coming from inside what remains.
First: For now, but if you read what Zaluzhny said - it actually makes it easier once the flood waters subside in a few weeks: a 250km front instead of just 70km. By then the river will be at its historic width and easily crossed.
I would agree, however, the big unknown is how long it will take for the river to subside and the mud to dry out enough for heavy armor traffic.
My guess is RU high command thought the counterattack was imminent and decided to blow the dam.
Second: It takes tons of explosives to blow a dam, a single mine will not blow it...The explosives have to be placed inside the dam to work.
Excellent analysis, thanks.