I am sure you have done reading on this site about the fact that underground springs can be more of a problem than a blessing. When the water pressure on the bottom of the pond is greater than the pressure of the incoming spring it can reverse and take water from the basin.

I highly recommend that you buy the book "Perfect Pond, Want One" from Pond Boss to take all items into consideration. As far as a higher dam or a deeper hole it is generally less expensive to build up the dam than to remove material from the pond to deepen it.

"I love living. I have some problems with my life, but living is the best thing they've come up with so far." � Neil Simon,