So here it is. My understanding is that they're draining the lake to clear out some of the sludge and trash and restore the crumbling retaining walls around it. There was talk about installing a liner in it, too, to prevent excessive water loss, but I think they decided that was too expensive.
It's a lovely, large lake, and I'll be happy when it's all tidied up and full of water again. Miss Sophie, too, who likes to swim there.