The 100' x 100' would be a hair under a half acre. You are correct in saying that it'll be surface fed. If you hit an underground spring, be sure to cover it up and pack it good n tight. Water will flow both ways in a spring, so you could end up losing all the water in the pond.

It's not as easy as digging a hole in the ground and having it fill up with water. You have to design and build/install a primary and secondary (emergency) overflow, etc.

Do some reading on this site on the different problems that people are having with their ponds. Go to the Pond Boss Book Library and at least order PERFECT POND...WANT ONE? That'll get you started. It will be a lot cheaper if you plan everything out and do it right the first time, rather than have to drain it and re-do.

It looks like you have some good soil for holding water.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).