Going to move all the "displacement" when rain does finally come or just drink the rocks?
Because the water is drawn from the bottom of the middle of the lake...?
And technically you could have a large (several) inflatable bladders anchors that were variable to raise and lower.
I am not saying it's a free solution but it's probably cheaper than building new reservoirs that still need rain and still have the exact same issues when lake levels drop.
I don't think the people commenting that I haven't thought it through thought about it themselves at all.
Displacing water pushes lake level up, win-win. You don't get more mass but you get more volume. Same amount of water but you can still use it. I'm certain it could be implemented but at what cost who knows. I still think it's very clever and a pretty simple/basic solution. Certainly viable on a smaller scale more easily... you'd just have to have the weight to sing a suitably durable bladder of air to displace the water. I was just wondering if it had ever been done or experimented with on any scale before in the sense I was describing it.