During the winter I demolished my old watercourse, which always leaked, and built a new header pool. Last week, after all the ice had gone, I started running the pump every day and turning it off at night. All was well, with no leaks or problems of any sort.

On Monday morning I realised that I'd left the pump on overnight. Well, that shouldn't have been a problem, except that the pond was nearly empty...! After emergency measures to keep the fish alive, I set about trying to find out what the cause was. With the pump off, the pond level didn't drop at all, nor did the header pool level. I excavated as much as I could of the hose from the pump to the header pool and ran the pump. No leaks there, and none from the little watercourse down to the pond. And no change in levels at all.

Then I noticed a large adult frog in the header pool, and that reminded me of something. A few days earlier, I'd seen a frog in the header pool right in front of the outlet, swimming gently to maintain his position in the slight current... I'm now as sure as I can be that a frog took up position in front of the outlet, which is a concealed circular pipe, and blocked it completely. The header pool would then have overflowed. From the quantity of water involved, I think froggy stayed there for half an hour to an hour, then moved on and let things get back to normal.

I'd intended to put a strainer around the outlet to prevent loose floating plant material blocking it. Needless to say, that strainer became a big priority, and I've now done it in such a way as to keep not just plant matter, but froggy too, safely away from the outlet.

Today I've had the pump running all day, and the levels have remained constant. So I'm quite sure it really was froggy who drained my pond for me!