Instead of a riser you could use a straight pipe that angles from the water level to the bottom of the dam. Another option is to have a flat pipe under the top of the dam with an elbow on the backside with the rest of the pipe angled down the back of the dam.

Here is what brettski did for his pond. Scroll down a little bit to look at his pictures.

http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=5741&fpart=3