If you can sleeve or somehow fix the pond end, and get the riser back above the water surface, I'd look into having a plumber that has a camera come out and look at the pipe coming thru the dam. If that rusts thru you could lose the whole pond. Capping the "past the dam end" leaves the water and pressure in the pipe.

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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).