They have dude, even the 2nd line in one place.
The first line is 17Km away from the line of contact. UAF hasn't even reached the first line of defence yet, they are just 3Km in.
You can't attack across a wide silt bed regardless of whether there's water on it. Occam's Razor says the simplest explanation is right. Russian was in control of the dam, only it had access, it was easiest for it to blow the dam. It did it to divert Ukrainian resources at the moment the counter-offensive began. Your shallower water argument would require an actual assault from that direction to back it up, there is none, therefore you are unequivocally wrong.
The enemy attacks what the other side controls.
How come they've lost so many aircraft then? Why are they using expensive cruise missiles to attack Ukrainian cities instead of bombs. They only have air parity over the battlefront, they poke a bit and the run.
Considering the scale of the war and pre-existing UAF ADGE, they haven't lost a lot.