From your pics, it looks like what you have dug has no place where the water is flowing out of the pool. Is this correct? Even if the water is coming out of the ground at 50 degrees it will be entering a pool that is stagnant warm water during the summer. I would think a better case scenario would be the cold spring water flowing in and replacing the warmer water which exits out to a drain. Is there a place you could siphon water to a lower area that would let you find out how quickly the the water is replaced in your hole, starting with a cheap garden hose to test it.
In my area, there are many places that I can dig and it fills up with water instantly(not from run-off). I think of it as a ground water supply, not so much a spring. The water comes in, but is not replaced with new, fresh, cold water. These get very warm during the summer because the volume of water is not being changed. We do have underground rivers, and that would be a different story.
You say that you have trout already during the winter in another pond. How about taking one of the trout and throwing it into this new pool at spring time. If you get it to live thru the summer you have some possibilities of expanding the amount of trout or the size of the pool,etc, etc. If it don't work, nothing really lost.
