OK. I'm trying to visualize this. Here is what I understand:
your pipe is at about a 22 degree angle through the dam, and
you need to reduce the pipe height more, and
dirt in the dam is eroding around the pipe - presumably due to the pipe end's proximaty to the dam.
Can you cut the pipe down however much more you need, then put a 22 1/2 degree PVC elbow in it, with an extension to take it further out in the water so dirt won't erode from around it as bad?
If so, it may be possible to run that extension out a ways, plug the end with a flat piece, and then use a drill to cut lots of holes in the section extending out into the water. Depends upon length of the section and the number / size of holes you cut, but you'd restrict the size fish going through at the cost of reducing your outflow rates some. I guess your higher pipe would be the first backup if it were too much restriction... I don't know. Just thoughts.