Pipe is a lateral with a slight slope. At the crown of the dam it is probably 3 1/2 to 4 ft deep.
Good, not too deep.
If the bentonite (fingers crossed) doesn't work, then you are probably going to have to do some excavating repairs next time your water level is down.
Where frost heaving is a problem, most people run a deeper lateral line and then use a stand pipe as a riser. The ice can't pull up very readily on the exterior of a smooth, vertical pipe.
One option is to dig back past your mortar collar and cut your pipe. Install a 22.5 or 45 ell, a straight section going downwards, another ell (same as previous), a straight section going horizontal, a 90 ell, and then your riser. The top of the riser will still set your pond level. (The advantage is that you could easily change the water level by just temporarily installing a pipe collar and small section of pipe above the existing top.) Bury the first ell back under your dam and rebuild your slope.
The solution above should help with your frost-heaving problem.
However, it will be more prone to plugging! You will definitely need an effective trash rack designed to screen out your most typical debris.
Another option is to dig back into your dam a fair distance and install a nice anti-seep collar. Tamp well and re-pack your clay around your drain pipe from the collar all of the way back to the lake inlet before you bury.
Hopefully, someone has some more elegant solutions.
