Thanks once again for all the information and well informed suggestions. I may have to adapt them a bit to my specific situation, but once it is drier, I am going to try and cap the springs so I can release outflow into the pond and prevent inflow from the pond.
From my own limited past experience, ( .. as in having dug a lot of holes in the past 30 years here for fruit trees, foundations, drainage trenches, outhouses etc ) the water seems to be traveling downhill in numerous veins through the clay, and if I follow it back up the mountain, it is more likely to break up into many small seeps. ( Think of tiny underground rivers running to the ocean which tend to get larger as you go downhill and smaller heading uphill )
The clay here usually seems to have many small veins of water running through it. This is why I was surprised there wasn't a hundred tiny seeps coming into the pond basin at this time of year. Perhaps this lack of porosity is why there seems to be an unusual amount of water coming in from one spot from on top of the clay.
My pond is shaped like a coma, with the tail going up the hill a bit to connect with where I noticed incoming water. The tail is only 2 feet deep at the tip and as it goes down to the main round body of the pond it gradually gets deeper to be about 6 feet at most.
The main source of incoming water for most the year is right at the place the topsoil meets the clay at the tip of the tail of the coma. I've already dug there over the years trying to follow back the water as it dried up in the summer, which is why my pond is shaped like a coma in the first place. Today I went out and dug some more and once I uncovered it I see it is also a very energetic seep or "spring" up about 2 feet from the water level when the pond is full, though in the summer this seems to dry up or go a lot deeper.
I'm thinking my situation might be like a hose that is siphoning water downhill that has some small leaks on the way. Except in this situation it is probably several different "hoses" or water veins involved, each with it's own unique properties of water source, capacity for flow and drainage.
I'm afraid that if I go digging after water any further than I have, I may break into another vein which has better drainage down slope and be in a situation of loosing even more water than I am now.
Rather than messing with something that may not be broken, I think I would rather try to fix my leak in stages, starting with the most likely problem and if that doesn't work going on to the next.
The spring I think may be a leak is located right at the very back tip of the coma, about 2 feet into the clay below where the topsoil ends. Once it is drier, it would be relatively easy to create a small concrete box with an outlet pipe and to compact a couple feet of clay ( maybe including some bentonite ) over that spring, and see if that fixes the problem.
The good spring (at this time of year anyways) is about half way down the tail of the coma and about 3 feet below the surface of the ground level. It probably has even better water pressure than my very unprofessional plastic pipes in a pile of goo revealed, as I doubt 8 inches of wet goop created a complete seal. As I don't think that spring is my most likely leak it seems to make sense to leave it be for now and first try and plug up / box in other more likely leaks.
I am most suspicious of the spring at the tip of the coma being a substantial leak because late last summer it quickly filled it's bowl if I bailed it out, but it usually wouldn't over flow suggesting it had only marginal water pressure. The other spring I just tested for water pressure is a foot and a half below this and it always over flowed the same amount ( about the same as it is now ) Muddying the water with clay in the spring that wouldn't overflow that is located above did not result in the water coming out the nearby good spring looking at all cloudy, which makes me think they are separate water veins with different qualities.
There is also a seep located a couple feet below the good spring that began running as soon as the pond was dug in September. The pond filled with 3 feet of water from this and the other spring before the fall rains, so I am assuming this seep has at least 3 feet of pressure behind it in the driest part of the year. I suspect this seep is connected with the same water vein that feeds the good spring, but if the pond still leaks after I seal up that quirky spring, my next guess would be to try and seal up that seep. At least that is in an area where someone with an excavator can get in to compact the clay .
The idea of 4 by 4 feet of compacted clay over both springs would probably work very well, but it would also more than fill in the tail of the coma where the springs are located.
If I can't hold a satisfactory amount of water using a couple feet of compacted clay I may have to do this and just pipe the water into the pond from the covered springs, but if I don't have to do this I would prefer not to. As the bigger and more professional solution would require all the smaller attempts at containment to be done anyways, it isn't like I would be wasting my time by doing something stupid I'd have to redo later. ( something I excel at BTW )
But wanting to approach this in stages is why it's important to have an idea of the most likely source of the problem. I am loosing about an inch every 5 hours from roughly a 20 by 20 foot circle when the pond is full, but this slows down to only an inch a day when the pond goes down 8 inches from full and looses water pressure. I really don't think the dam is leaking in a substantial way as that much water would leave some trace of itself. At this time of year it is impossible to see where the water level stops going down as our frequent heavy rains never give it a chance to really fall to it's lowest level. So far, I haven't noticed any particular level. What I do notice is the fuller the pond is the faster it looses water.
This is why I was wondering if I am right in assuming that since everything is saturated at this time of year and I only saw water coming in 3 places inside the clay basin, that the general clay basin is probably not the source of loosing that much water, and it is most probably the springs or seeps with the least water pressure?
