Hi, I figured out the cause of the hippo, at last. It is heavy rain. Initially, until my first couple of posts here, I had kept the water level at the recommended weir door level, which is quite low in the pond. When it rained, there were about 6 inches of excess depth available before the pond would overflow. I think it maybe happened only once during that period, and barely. Now I keep the water level about two or three inches below the overflow point, since the skimming action of the skimmer is less important to me than having sufficient water depth for the winter. I've noticed that there is still plenty for me to clean out of the skimmer basket and filter mat even though the water level is comfortably above the top of the weir door, so it's still helping with the filtering process anyway. But the added water depth has made overflowing more likely to an extent I hadn't anticipated.
We have been getting some pretty heavy rain today after a long day of soft, ground-saturating rain yesterday. The pond is overflowing. The low point of the pond is where I put the skimmer, and that is also where I cut the liner so that it is right at the surface of the pond rim. (Before I trimmed it, the liner was totally covering the skimmer opening making maintenance too difficult.) I have watched the water flowing like a stream over this rim and under the liner. The hippo in turn has been inflating like a balloon right before my eyes. I started the siphon trick that morewater showed me (thanks again; it's a lifesaver), so hopefully things will be drained by the time our next freeze strikes on Thursday. Interestingly, I don't believe we've even had an overnight freeze since the last one a few weeks ago. By coincidence, I ordered 15 feet of that venting tubing that Meyer recommended, just last night. It was only about $40, and if it works that will be a bargain. It will be going under the liner when I raise the skimmer after the winter ends for sure.
Thank you again, very much, everyone for guidance into diagnosing problems and providing fixes. I've got the de-icer in, ready for the next freeze. Now that I know where the hippo causing leak is coming from, I think I'm going to keep the waterfall running a bit longer. The next freeze will only last for two or three days, and then we'll be back to the upper 40s again for at least a week. The waterfall is definitely not a factor in the current hippo for sure. Since the waterfall is the reason I built this thing in the first place, I'd like to keep it going until I know we have a sustained period of ice coming. I will change course, however, if events so dictate. Hopefully they won't.
Thanks again.