wow, your description matches my situation very well LF ! Also in the NW, built on a slope at about 900' elevation. Great surface flow and seepage through the clay during the winter. pond is just under 1/4 acre.
however, once the flow stops there is maybe a week of stasis and then the dropping begins. Like the water table plugs the drain and fills...then unplugs when it drops.
Bentonite worked to keep the deepest section filled last summer...now the rest has been packed with bentonite and we haven't seen the results yet.
The other thing we did was french drain the uphill side of the pond to get rid of the pressure variation. (i.e. break the suction when the water table goes down) The captured water is funneled into the pond through a backflow valve.
I don't see the small water vents and air bubbling that I've seen in the past two winters so the trench may have helped that. however, without the bentonite, the clay is just too porous and it was packed by a sheep's foot. You may find the same situation. (if you're going to drain and pack it anyway, might as well add the bentonite.)
best of luck !
Brad
