1.) You are correct about water reaching its own level.

2.) You are PROBABLY correct about the water level of the spring supporting your pond at full normal pool - even if you have a leaking spring in the basin of your pond.

3.) I typed that long explanation above for a low-odds, worst case scenario. If the permeable layer that is the water source for your spring is discontinuous, then the water level of a possible sand/gravel layer at the site of your pond COULD be much lower than the water level (potentiometric surface) of your spring up the hillside.


** Many springs are due to permeable layers in the bedrock that are capable of transmitting flows of water, usually combined with elevation changes. However, unconsolidated sediments can also create the conditions for springs. Typically via a continuous layer of sand or gravel that is constrained above and below by compacted clay. (Shale is a rock layer. Compacted clay is not.)


I think (low value, long-distance internet advice) that you are probably good to go on your pond project. I was just giving you some advice on what to look for during the excavation phase, so you can fix an observed problem before back-filling and compacting, and then finding out your pond leaks at an unacceptable rate.

Finally, if you do get a backhoe or small excavator on your property to pipe in your spring "bathtub", then you might consider a small slot pit in the approximate location of your proposed core trench. If you dig it when the surface ground is not mushy from recent rains, then observing the overnight fill up of your pit might give you some more information about the likely natural water level at the site of your pond.