If you did a hole 3', 5' or 10' deep next to the pond does it fill up with water?

Is it a spring (a true spring that has water flowing out of it) or just ground water that is seeping out of the ground to fill the pond?

If it's a ground water pond, you will be at the mercy of the water level in the ground, which you are seeing now. If it is a true "spring" with water flow, hopefully the water enters the pond at a higher elevation than the current water level.

If it is a ground water pond, or if the spring is inside the pond below the current water level, anytime you have water pressure in the pond that is greater than the water pressure in the spring or ground water, the water will flow out of the pond. There is no one way valve on them.......

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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).