Seems last year I saw hardware cloth that had a plastic coating, that might
work to make a large cage over the drain?

I'm still looking around and have seen some interesting grated drain
covers (much like landscape catch basin grates) which would be a huge
improvement over what I have. I also noticed that some vortex tanks
can be buried, and it sounds like the right sized unit is 4' deep.
Pretty pricy, but it makes me think that there still may be a way to
use a settlement tank of some sort. I need to figure out "pond level"
in the location where I might be able to put a tank. Offhand, I don't
know if that would be ground level, below ground or above ground level
(which I doubt).

What do you do in the winter, though? Our frost layer is about 2'
down, so the top water would definitely freeze up if left to its own.

I drain my barrels in the lily pond and toss a low watt heater in with an
air stone. In the koi ponds I drain one bio-chamber and run the rest with a
slower pump circulating through the ponds, but not running the main
waterfall. In the ground with a covering over the top seems to keep them
ice free. I'm in Zone 7 though.

The barrels you're talking about, are they those plastic ones or
something else? Where do you find them? Dave

Plastic. Used to hold juice, car wash soap or that blue stuff for toilets.
I'm not sure what my first 4 held, as we got them clean from a salvage
yard. The lily pond ones held the blue stuff for chemical toilets. If from
a car wash you'd sure want to power wash them I would think. ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us