Welcome to the forum! Finding it and fixing it may be 2 totally different things. It also could be a tree root that has died and decomposed, leaving a void in the dam.

Bentonite may or may not fix it. There have been both success and failures using it.

Maybe go to a local farm store and buy a gallon of pond dye? Pour some near where you see the leak and if the leak changes color, that may narrow down the search a bit.

www.hoosierpondpros.com

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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).