Found the Esshup wisdom..

Trees on dam:

General rule of thumb is 6" and less diameter cut down, treat stumps so they won't re-grow. Larger than 6" diameter leave and pray that they don't die so the roots stay alive. Dead decaying roots leave pathways for water to go thru the dam.

Ref:
http://forums.pondboss.com/ubbthreads.ph...true#Post378635