If all else fails, you could use a collar or coupling to glue on another length of 12 inch PVC in order to extend the outlet to the level that you want the pond water to be. The result would be an unsightly passive system and the riser would probably have to be braced against breakage. You could paint it green or brown and tell people to use their imaginations as to what it might represent.
Alternatively, you could cut off the outlet at ground level and install a valve, but that would require you to manually allow or disallow water flow, and certainly not as great a flow as you had hoped for. Yes, I know, but just overlook that preposition business. You could cut off the outlet below ground level and trench in additional pipe to get the outflow away from the dam, installing the valve anywhere along the discharge line.
