Sunday I went out to do my usual Sunday yard chores, and, before turning the corner into the yard, I heard that ominous sound of the waterfall splashing from too high a distance.
Yup, the water level was way low - about 15%.
What happened was, something (or was it just freak motion of the water itself??) had dug an indentation in the gravel in my bog filter, near the edge. Since there wasn't much gravel covering the incoming water, in this spot, the water was gushing up like a mini-fountain. It was enough that it was splashing over the side - not much, but enough that overnight, it had drained the pond quite a bit.
I had no idea water would come gushing up like that. I thought it would just simply flow downhill toward the waterfall.
Whatever did that, I hope it never comes back!